Unit 13 Myoglobin and Haemoglobin
UNIT 13
MYOGLOBIN AND HAEMOGLOBIN
Structure
13.1 Introduction Chloride shift (Role of chloride ion in oxygen transport) Expected Learning Outcomes 13.4 Cooperativity phenomenon: 13.2 Structural and functional myoglobin and haemoglobin aspects: myoglobin and haemoglobin Introduction
Structure of myoglobin and Concerted model haemoglobin Sequential model Function of myoglobin and Hill plot (Hill equation and Hill haemoglobin coefficient) Differences: myoglobin and 13.5 Haemoglobin disorders haemoglobin Introduction 13.3 Curves for oxygen binding and dissociation: myoglobin Types and causes and haemoglobin 13.6 Summary Oxygen binding curves 13.7 Terminal question Oxygen dissociation curves 13.8 Answers Influence of factors on oxygen 13.9 Further readings dissociation curves
Bohr effect 13.1 INTRODUCTION
You are aware and have acquire the knowledge about the amino acids, peptides, proteins and also various techniques used for the analysis in the 199
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previous units of this course. You are also aware of primary, secondary, tertiary and quaternary structures of the proteins. Myoglobin and haemoglobin are appropriate and suitable examples of such quaternary structures. Now, the present unit describes the connection between the structural features with the physiological functions of myoglobin and haemoglobin. Expected Learning Outcomes
After studying this unit, you should be able to:
❖ describe the connection between structural and functional characteristics of myoglobin and haemoglobin;
❖ know the oxygen binding curves and influential factors;
❖ draw the Hill plots;
❖ explain the co-operativity phenomena.
13.2 STRUCTURAL AND FUNCTIONAL ASPECTS: MYOGLOBIN AND HAEMOGLOBIN
Haemoglobin and myoglobin are very important proteins regarding the oxygen carrying capacity. A comparative analysis of myoglobin and haemoglobin showed the key characteristics of these two proteins in context of structure and function. These two proteins are evolutionary related and have similarity in structure and physiological functions. Haemoglobin has a 90% of potential oxygen-carrying capacity, whereas, myoglobin has only 7% of oxygen-carrying capacity. These two transport proteins were the first proteins for which the three-dimensional structure was determined by X-ray crystallography. 13.2.1 Structure of Myoglobin and Haemoglobin
Myoglobin
Myoglobin is also denoted as Mb. This is oxygen binding hemoprotein and Rhabdomyolysis: This is the abnormal condition, found in the cardiac and skeletal muscle. This protein has the appearance of in which, muscle tissue red colour. This protein doesn't occur in the blood, but during muscle injury, it is degenerated. So, the is released in the blood stream and this is an abnormal condition, which is content of muscle fiber, known as rhabdomyolysis. including myoglobin, released into the blood. Myoglobin is an enormously compressed molecule with the dimensions of 45 × 35 × 25 Å. This protein is a single polypeptide chain (monomeric) of 153 amino acids (molecular weight =17,000 Da). 70% of the main chain is folded into eight α- helices and rest of the chain produces turns and loops between helices [Figure-13.1]. The folding of the main chain of myoglobin is composite and asymmetrical, which is also observed by most of the other proteins. Such overall arrangement of the polypeptide chain of a protein is considered as its tertiary structure. The distribution of side chain amino acids in protein molecule is covered by the generalized principle. According to this principle, the non- 200 polar or hydrophobic amino acids are found in the interior, whereas, polar or
Unit 13 Myoglobin and Haemoglobin hydrophilic amino acids are projected outwards of a protein molecule. The interior of the myoglobin molecule contains amino acids like leucine, valine, methionine, and phenylalanine. Whereas, aspartate, glutamate, lysine, and arginine are absent from the inside of myoglobin. The two histidine residues (polar residues) are present in the interior part of the molecule. The outside of myoglobin, on the other hand, consists of both polar and nonpolar residues.
The molecule of myoglobin has a capability to bind oxygen. This capability is acquired by the presence of heme, a non-polypeptide prosthetic group consisting of protoporphyrin IX and a central iron atom. This complete structural arrangement is known as protoheme group. This protoheme group is stabilized by histidine residue above (His 64) and below (His 93, Fig.13.1). The two histidine residues located interior part of the molecule is also facilitate the binding of iron and oxygen. ( ( ( A B C ) ) )
Fig. 13.1: (A) Structural model of myoglobin containing eight alpha-helices (A, B, C, D, E, F, G & H), (B) Schematic diagram of myoglobin showing the oxygen-binding site, proximal histidine, distal histidine and heme, (C) Myoglobin consist of a backbone and heme binding domain. The protoheme group is stabilized by histidine residue above (His 64) and below (His 93).
(Source of image: Protein structure and function. web2.aabu.edu.jo) Haemoglobin
Haemogloin is also denoted by as Hb. This is oxygen binding hemoprotein and found in the erythrocytes (red blood cells). This protein also has the appearance of red colour. Molecular weight of haemoglobin is 67,000 Da. The normal concentration of Hb in blood of males is 14-16 g /dl and in females it is 13-15 g/dl.
Haemoglobin is categorized as a conjugated protein because this encloses heme as prosthetic group and globin as the protein part apoprotein. Heme is the common prosthetic group in haemoglobin as well as in myoglobin. Each haemoglobin molecule consists of a tetramer of globin polypeptide chains. Every polypeptide chain (Fig. 13.2) encloses heme in the hemepocket. In this way, one haemoglobin
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Fig. 13.2: (A) Schematic diagram of haemoglobin showing the heme, α chain 1, α chain 2, β chain 1 and β chain 2. (B) Structural of heme (haeme) containing a protoporphyrin IX, with iron at its centre. Protoporphyrin IX consists of four pyrrole rings to which four methyl, two propionyl and two vinyl groups are attached. (Source of image: eGyanKosh, IGNOU)
molecule contains four heme units. The tetrahedral arrangement of haemoglobin subunits presents the tight spherical appearance and each individual polypeptide is folded in such a manner to maximize polar residues being on the exposed surface and non-polar interactions being internal, making this large protein water soluble. The interior surface of the molecule lined with nonpolar groups, form a hydrophobic pocket into which heme is inserted. The polypeptides in haemoglobin are held together by hydrogen bonding, hydrophobic interactions and multiple ionic interactions. All these binding interactions are carried out at the contact points between subunits (Figure-13.2). These subunit interactions are responsible to perform a vital role
in the binding of O2 to Hb. The α-chain consists of 141 amino acid residues in linear sequence. The β, ϒ and δ chains consist of 146 amino acids. In a helical arrangement, the 75% of the amino acids are in α or β-chains. The near sphere is formed by the tertiary folding of each globin chain and gives rise to functionally important characteristics of the molecule. The haemoglobin tetramer contained two identical dimmers (α, β) 1 and (α β) 2 [where the number refers to dimer1 and 2]. The two-polypeptide chains in each dimer is bounded by H-bonding, hydrophobic and ionic interactions. The two dimers can move with respect to each other, being held together by polar bonds. The dimers have additional contacts to give the tetramer. Such arrangement is considered as its quaternary structure (Fig. 13.2).
Forms of haemoglobin
There are various forms of haemoglobin. Human adult haemoglobin (HbA1) contains of two α-chains and two β- chains. The polypeptide chain has 141 and the β polypeptide chain has 146 amino acids. The fetal form (HbF) contains two α-chains (same as in HbA1) and second type of chain (ϒ chain) occurs in the tetramer molecule and differs in amino acid sequence from that 202 of the β chain of adult HbA1. Additional forms appear in the first months of
embryonic stage in which the α- chains are substituted by zeta (z) chains of
Unit 13 Myoglobin and Haemoglobin different amino acid sequence and the ɛ- chains serve as the β-chains. A slight form of adult hemoglobin (HbA2) consists of about 2% of normal adult hemoglobin and contains two α- chains and two chains nominated delta (δ) (Table -13.1).
Table-13.1: Forms of human haemoglobin
Developmental Symbol Chain Designations Stage
Adult HbA1 α2β2
Adult HbA2 α δ 2 2 Fetus HbF α ϒ 2 2
Embryo Hb Gower-1 ᶋ2ε2
Embryo Hb Portland ᶋ2 ϒ2
13.2.2 Function of Myoglobin and Haemoglobin
Till now you have studied about the structure and types of Myoglobin and Hemoglobin. Now let us learn about their biological functions.
Function of myoglobin
Myoglobin is a molecule of oxygen transporter and it performs the following functions:
(a) Myoglobin facilitates respiration in rapidly respiring muscle tissue like cardiac tissue.
(b) The rate of oxygen diffusion from capillaries to tissue is slow because of the low solubility of oxygen and myoglobin increases the solubility of oxygen.
(c) Myoglobin facilitates oxygen diffusion.
(d) The oxygen storage capacity of myoglobin is tenfold greater in Whales and Seals than in land animals.
Function of haemoglobin
Haemoglobin is an important molecule of oxygen transporter (red blood cells) in vivo and this is it performs the following important functions:
(a) It transports oxygen and partially carbon dioxide.
(b) It binds to H+ ions and act as a buffer system.
(c) Oxygen bound to Fe++ (ferrous ion) in heme (haem) and produces the oxy- hemoglobin (HbO2). Four oxygen molecules bounded to one haemoglobin molecule. In this way, it transports oxygen in the blood from the lungs to the rest of the body.
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13.2.3 Differences between Myoglobin and Haemoglobin
Myoglobin and haemoglobin are the oxygen carrier or oxygen transporters molecules. Though they are almost similar in their functions they differ at few points. Haemoglobin is a tetrameric protein, whereas myoglobin is a monomeric protein. Haemoglobin is occupied systematically by all over the body, while myoglobin is situated in muscles tissues only. Haemoglobin is the most vital part to sustain life as it works in transporting oxygen as well carbon dioxide throughout the body. Myoglobin receives the oxygen from the RBC and further carries it to a mitochondrial organelle of muscles cells only. Subsequently, this oxygen is used for cellular respiration to create energy. All the remarkable points are summarized in table-13.2, which differentiate the haemoglobin and myoglobin.
Table: 13.2 Differences between haemoglobin and myoglobin
Characteristic HEMOGLOBIN MYOGLOBIN feature Number of Haemoglobin has 4 chains It contains single polypeptide
polypeptide chains of two different types (α2β2) chains.
Structure A tetramer. A monomer.
Binding molecule Binds CO2, CO, NO, O2 and Binds to O2, tightly and firmly. H+.
Occurrence Systemically all over the Muscles cells. body.
Nature of oxygen Sigmoid binding curve. Hyperbolic curve. binding curve
Abbreviated Hb. Mb.
Functions Haemoglobin is transported Myoglobin supplies oxygen to along with blood to whole muscles only, which is helpful at body and carries oxygen. the ravenous time of oxygen.
Found in Blood (RBC) Muscle
SAQ 1 1. Indicate whether the following statements are true or false:
i) Myoglobin is a monomeric oxygen binding hemoprotein found in heart and skeletal muscle.
ii) Haemoglobin is a tetrameric protein.
iii) The non-protein part of the haemoglobin is globin. 204 iv) Haemoglobin is found in erythrocytes.
Unit 13 Myoglobin and Haemoglobin v) Myoglobin, the oxygen carrier in muscle, is a single polypeptide chain of 153 amino acids.
2. Match the following in column A with those given in column B:
Column A Column B
(1) Myoglobin (a) Tetrameric protein
(2) Haemoglobin (b) Monomeric protein
(3) RBC (c) Myoglobin
(4) Muscle (d) Haemoglobin
13.3 OXYGEN BINDING CURVES AND OXYGEN DISSOCIATION CURVE 13.3.1 Oxygen Binding Curves
Oxygen binding is defined as a capacity of binding of an oxygen molecule to a specific functional protein for either transport or storage in vivo (process that take place in living organisms). An oxygen-binding curve is a plot that shows fractional saturation versus the concentration of oxygen. Partial pressure determined the concentration of oxygen.
(A) Fractional saturation indicates the presence of binding sites that have oxygen. Fractional saturation can range from zero (all sites are empty) to one (all sites are filled). [In box]
(B) Dalton's law of partial pressures states that the total pressure of a mixture of gases is equal to the sum of the partial pressures of the component gases.
Consider an ideal gas mixture of nitrogen (N2), hydrogen (H2) and ammonia (NH3). In this mixture, the total pressure of the gas mixture is P.
P= PN2 + PH2 + PNH3, where: P = total pressure of the gas mixture 205 PN2= partial pressure of nitrogen PH2= partial pressure of hydrogen PNH3= partial pressure of ammonia
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Oxygen binding curves for myoglobin and haemoglobin
A sharp increase is observed with the increase of pO2 levels of oxygen binding curves for myoglobin (Fig. 13.3. A). Oxygen binds with high affinity to 1 Torr = 101325760 myoglobin is shown by the half-saturation of the binding sites (denoted as P ) pascals 50 is at the relatively low value of 2 torr (mm Hg). Comparatively, the oxygen- binding curve for haemoglobin in red blood cells shows various notable features. This curve is completely different from the oxygen binding curve for myoglobin. Such curves are considered to as sigmoid curve due to their S-like
shape (Fig.13.3. B). Oxygen binding curve for haemoglobin (P50 =26 torr) shows that there is a weak binding of oxygen to haemoglobin as compared to myoglobin. There is a specific oxygen binding characteristic shown by sigmoid binding curve for haemoglobin. This curve suggests that the binding of oxygen at one site within the hemoglobin increases the binding of oxygen at the remaining unoccupied sites. (Fig. 13.4)
A B
Fig. 13.3: (A) Oxygen binding curve [fractional saturation vs partial pressure of
oxygen (PO2)] for myoglobin showed the hyperbolic characteristics, (B) Oxygen
binding curve [fractional saturation vs partial pressure of oxygen (PO2)] for haemoglobin showed the sigmoid characteristics.
(Source of image: haemoglobin:https://www.istitutogreppi.edu.it/)
The P50 represents the partial pressure at which myoglobin or haemoglobin is 50 percent saturated with oxygen. This is a conventional measure of haemoglobin affinity for oxygen (Fig. 13.4 (A) and (B).
Fractio A Fraction B n of of molecu les molecule s saturat saturated ed with O with O 2 2
Fig. 13.4: (A) Myoglobin oxygen binding curve represented the 50% occupancy
206 (P50) (B) Haemoglobin oxygen binding curve showed the P50 with sigmoid characteristics. (Source of image: http://www.chembio.uoguelph.ca/educmat/
Unit 13 Myoglobin and Haemoglobin chm356/3560L2.pdf)
13.3.2 Oxygen Dissociation Curve
The oxygen dissociation curve is known by several other names such as oxygen haemoglobin dissociation curve, the oxyhemoglobin dissociation curve. The oxygen dissociation curve is a curve that draws the proportion of hemoglobin in its saturated form on the vertical axis adjacent to the existing oxygen pressure on the horizontal axis. This curve explains the mechanism of binding of oxygen to haemoglobin and its releases. The oxyhemoglobin dissociation curve, especially represents the relation of oxygen saturation (SO2) and partial pressure of oxygen in the blood (PO2). This determined the "hemoglobin affinity for oxygen" and releases oxygen molecules into the fluid that surrounds it (Fig.13.5).
Fig. 13.5: Oxygen dissociation curve representing the full saturation of haemoglobin in the lungs and release of oxygen in the tissue during the respiration. Note that there is a 22% decrease in the percent of oxygen as blood passes from arteries to veins in the tissue. This results in unloading of approximately 5 ml of oxygen per 100 ml of blood. (Source of image: eGyanKosh, IGNOU).
The transportation of oxygen starts in the blood from the lungs. The partial pressure of oxygen is high (approximately 100 torr) in the lungs as compared to the partial pressure of oxygen is much lower (typically, 20 torr) in energetically metabolizing tissues. (Fig. 13.6)
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Fig. 13.6: (A) Cooperativity enhances oxygen delivery by haemoglobin. Because of cooperativity between oxygen and binding sites, haemoglobin delivers more oxygen to tissues as compared to myoglobin. (B) Responding to exercise. The drop in oxygen concentration from 40 torr in resting tissues to 20 torr in exercising tissues corresponds to the steepest part of the observed oxygen- binding curve.
(Source of image: https://www.istitutogreppi.edu.it/sites/default/files/ page/2017/9%20Hemoglobin.pdf)
There is a need to understand the cooperative behaviour of haemoglobin by the sigmoid curve leads to competent oxygen transport Fig. 13.6(A) and (B). Haemoglobin is approximately saturated with oxygen in the lungs. There is a 98% of the oxygen-binding sites are engaged. The saturation level falls to 32% during the movement of haemoglobin to the tissues with the release of oxygen. In this regard, there is a whole of 66% of the potential oxygen-binding sites contribute to oxygen transport. Releasing of oxygen in a cooperative manner is responsible for nearly a more-complete unloading of oxygen in the tissues. As an assumption, if myoglobin would be engaged in oxygen transport, it would be 98% saturated in the lungs, but would remain 91% saturated in the tissues, in this way only 7% of the sites would contribute to oxygen transport. The reason behind this, the tight binding of oxygen to the myoglobin is unable to be useful in oxygen transport. Thus, the cooperative binding and release of oxygen by hemoglobin facilitates it to transport almost 10 times more oxygen as could be transported by myoglobin.
There is a fall of 20 torr of oxygen pressure in muscle from the stage of rest to the stage of the exercise. There is a reduction of oxygen pressure from 100 torr in the lungs to 40 torr in resting muscle and in this regard, the oxygen saturation of haemoglobin is decreased from 98% to 77%. So, there is a release of 21% of the oxygen over a fall of 60 torr in pressure of oxygen. There is a decrease from 40 torr to 20 torr in oxygen pressure; the oxygen saturation is reduced from 77% to 32%, corresponding to an oxygen release of 45% over a fall of 20 torr. Thus, the change in oxygen pressure from rest to exercise corresponds to the steepest part of the oxygen-binding curve, oxygen is efficiently delivered to tissues where it is most needed. 13.3.3 Influence of Factors on Oxygen Dissociation Curves
Binding capacity of oxygen to haemoglobin is influenced by several factors. These factors are responsible for shifting or reshaping the oxygen dissociation curve. The oxygen dissociation curve can be shifted right or left. A right shift signifies the reduced oxygen affinity of haemoglobin and facilitating the more oxygen to be available to the tissues. A left shift signifies the increased oxygen affinity of haemoglobin and responsible the availability of less oxygen to the tissues (Fig. 13.7).
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Fig. 13.7: Three oxyhaemoglobin dissociation curves – normal (P50 = 26.7 mmHg), left-shifted (P50 = 17 mmHg) and right-shifted (P50 = 36 mmHg). The vertical line represents normal oxygen loading tension (PO2 = 100 mmHg). The points on the curves to the left of the line correspond with an oxygen extraction of 5 ml/100 ml blood, assuming a haemoglobin concentration of 15 g/100ml.
(Source of image:https://media.lanecc.edu/users/driscolln/RT127/ Softchalk/Oxygen_transport_softchalk/Oxygen_Transport_Lesson4.html) These factors are given below:
H (1) Temperature, (2) p or hydrogen ion concentration, (3) CO2 (Carbon dioxide) and (4) 2, 3-bisphosphoglycerate (BPG).
(1) Temperature Normal body temperature Temperature is a major factor, which influences the oxygen dissociation curve. is around 98.6 F (37° C). An increase in temperature shifts the curve to the right, whilst a decrease in Hypothermia is a temperature shifts the curve to the left (Fig. 13.8). An increase in the pathological condition, temperature is responsible for lowering the affinity of oxygen to haemoglobin. which arises due to loss of heat faster as compared to In this regard, the concentration of oxygen and haemoglobin is decreased, heat production, causing a whereas oxyhemoglobin is increased. dangerously low body temperature. Hyperthermia is a pathological condition, which arises due to elevated body temperature and failure of thermoregulation that occurs when a body Fig. 13.8: Influence of temperature on oxygen dissociation curve represented produces or absorbs more the curve at two different temperatures (37°C and 40°C). heat than it dissipates. Extreme temperature (Source of image:https://www.slideshare.net/KarthiMurugan1/hypothermia-dhca- elevation then becomes a rcp-acpoxygen-consumptioncooling-rewarming) medical emergency requiring immediate The temperature has remarkable effects in the conditions of hypothermia and treatment to prevent hyperthermia. Hypothermia and hyperthermia are two medical or clinical or disability or death. pathological conditions and also a life threatening condition. Hyperthermia is observed during high fever. (2) pH or hydrogen ion concentration
The oxygen dissociation curve is shifted to the right with a decrease in pH (increase in H+ ion concentration) and shifted to left to increase in pH (decrease in H+ ion concentration) Fig. 13.9. 209
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2-3-BPG also known as Fig. 13.9: Influence of pH on oxygen dissociation curve represented the curve at 2,3-DPG three different pH (7.2, 7.4 and 7.6). (diphosphoglyceric acid) (Source of image:https://www.slideshare.net/KarthiMurugan1/hypothermia-dhca- compound is synthesized rcp-acpoxygen-consumptioncooling-rewarming) from glycolytic Haemoglobin has less affinity for oxygen and it is stabilized in the intermediates. In the deoxygenated form with the reduction in pH. The effect of pH on the oxygen erythrocyte, 2-3-BPG constitutes the affinity of haemoglobin depends in the amino terminals and side chains of two predominant histidine molecules. These are histidine 146 on the β-subunit and histidine 122 phosphorylated compound, on the α-subunit. Lower pH or acidic environment is responsible to facilitate accounting for about two the formation of a bond between the carboxyl group of histidine 146 and a thirds of the red cell lysine residue in the α subunit of the other αβ dimer. In this way, the histidine phosphorus. The molecule becomes a carrier of positive charge and takes part in the formation proportion of 1,3-BPG pathway appears to be of salt bridge with a negatively charged aspartate (94) on the same β-subunit. related largely to cellular This bond is responsible to favor the stabilization of deoxygenated state of ADP and ATP levels, when haemoglobin. Conversely, oxyhaemoglobin is not capable to form this kind of ATP falls and ADP rises, a bond and is not influenced by pH. greater proportion of 1,3- BPG is converted through (3) CO2 (Carbon dioxide) the ATP-producing step. This mechanism serves to Carbon dioxide (CO2) performed its influence on the oxygen dissociation curve assure a supply of ATP to by two ways. According to one way, accumulation of CO2 is responsible for the meet cellular needs. In the production of carbamino compounds. This carbamino is forming the bond with de-oxygenated state, haemoglobin and produced the carbaminohemoglobin. Carbaminohemoglobin hemoglobin A can bind 2,3- stabilizes the deoxygenated state of hemoglobin by formation of ion pairs. In BPG in a molar ratio of 1:1, another way, CO influences intracellular pH due to formation of bicarbonate a reaction leading to 2 reduced oxygen affinity and ion. The formation of a bicarbonate ion will release a proton into the plasma improved oxygen delivery and raising the acidity. In this way, acidic environment is responsible for to tissues. shifting the curve to the right (Fig. 13.10). Conversely, a low CO2 level in the blood is responsible to increase the pH and also perform the shifting of the curve to the left. This situation facilitates most favourable binding conditions
for hemoglobin and O2. This is a physiologically favoured mechanism because hemoglobin is carried out the release of more oxygen as the concentration of carbon dioxide increases (Fig. 13.10).
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Fig. 13.10: Influence of carbon dioxide on oxygen dissociation curve represented the curve at three partial pressures of carbon dioxide [PCO2; 5.6, 7.8 & 9.5].
(Source of image: https://www.slideshare.net/KarthiMurugan1/hypothermia- dhca-rcp-acpoxygen-consumptioncooling-rewarming)
(4) 2, 3-bisphosphoglycerate (BPG)
2, 3-bisphosphoglycerate (2,3-BPG) level influences the oxygen dissociation curve. An increase in the level of BPG shifts the curve to the right, whilst a decrease in 2,3-BPG shifts the curve to the left (Fig. 13.11. A).
Fig. 13.11 (A): Oxygen dissociation curve is influenced by the concentration of 2,3-BPG (Haemoglobin stripped of BPG, normal level of BPG and high level of BPG].
(Source of image: https://www.umassmed.edu/globalassets/office-of- undergraduate-medical-education-media/documents/ protein_structure_notes_2016.pdf)
Fig. 13.11 (B): 2,3-BPG structure
2,3-BPG (Fig. 13.11 B) performed the stimulation for the release of oxygen from erythrocytes by functioning as an allosteric effector or regulator (Refer unit-6 of BBCCT-107) to hemoglobin molecules. Binding of one molecule of BPG with one molecule of deoxyhaemoglobin in the central cavity of tetramer is responsible to carry out the formation of salt bridges. The histidine and 211
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lysine of β-globin chains is the carrier of positive charge and take part to form the salt bridges with the negative charged BPG. In this way, deoxygenated form of haemoglobin is stabilized and facilitates the release of oxygen from hemoglobin in the tissues. There is an observation of shifting of the oxygen dissociation curve to the right. BPG also perform the inhibition of red blood cells from rebinding the oxygen after it is released. There is a significant and vital role of BPG for stable release of molecular oxygen and inhibition of reabsorption. 13.3.4 Bohr Effect
The Bohr effect is a physiological phenomenon first described in 1904 by the Danish physiologist Christian Bohr.
Bohr effect explains the uptake of oxygen in lungs and its release in the tissue + as well as its regulation by the concentration of CO2, H ion (hydrogen ions)
and the partial pressure of oxygen (pO2). So, this phenomenon explains the transportation of oxygen in a regulatory manner. Partial pressure of carbon
dioxide (pCO2) is lower in lungs as compared to tissue. In this regard, haemoglobin has higher affinity for oxygen and it facilitates the formation of oxy-haemoglobin. By following this path, transportation of oxygen is carried
out from lungs to tissue. Similarly, partial pressure of carbon dioxide (pCO2) is
higher in tissue and it facilitates the dissociation of oxy-haemoglobin (HbO2).
In this way, oxygen (O2) is released and carbon dioxide (CO2) is transported from tissue to lungs (Fig. 13.12).
Mechanism ( ( A B ) ) A B
Fig. 13.12 Partial pressure of carbon dioxide (pCO2) is lower in lungs as compared to tissue. In this regard, haemoglobin has higher affinity for oxygen and it facilitates the formation of oxy-haemoglobin. By following this path, transportation of oxygen is carried out from lungs to tissue. Similarly, partial pressure of carbon dioxide (pCO2) is higher in tissue and it facilitates the dissociation of oxy-haemoglobin (HbO2). In this way, oxygen (O2) is released and carbon dioxide (CO2) is transported from tissue to lungs. Source of image: https://faculty.uca.edu/lorii/4320_pdf/transparencies/figs/bohr_effect.pdf.
Fig. 13.12, represented the mechanism of Bohr effect. In capillaries of
metabolizing tissues, CO2 enters in the red blood cells (RBCs) and quickly
converted into carbonic acid (H2CO3) with the catalytic action of enzyme carbonic anhydrase. By covering this route, eighty percent (80%) of carbon dioxide is converted into carbonic acid. This carbonic acid suddenly ionizes to + − + 212 H and HCO3 . These hydrogen ions (H ) are responsible to reduce the oxygen affinity of Hb and facilitate oxygen delivery to the tissues. De-
Unit 13 Myoglobin and Haemoglobin oxyhemoglobin act as a weaker acid as compared to oxy-haemoglobin. Therefore, de-oxyhaemoglobin binds the excess hydrogen ions (H+) and also minimizes the decrease in pH Figure-13.12 (A).
The above-described processes are reversed at the lungs Figure-13.12 (B). Oxygen enters in red blood cells (RBCs) and the resultant binding of oxygen to haemoglobin encourages the liberation of carbon dioxide-bound + − haemoglobin. The hydrogen ion (H ) binds to bicarbonate ion (HCO3 ) and produce the carbonic acid (H2CO3). This carbonic acid easily dissociated and produced the CO2 and water (H2O). The lungs expel the carbon dioxide. In this regard, there is a reduction observed in carbon dioxide and hydrogen ion concentrations. Such reduction is responsible for increasing the affinity of oxygen to haemoglobin. Oxygen binds to the haemoglobin and transported it to tissues from lungs. A combined effect of carbon dioxide and hydrogen ion on the affinity of oxygen to haemoglobin has been called the “classical Bohr effect,” whereas the affinity of oxygen to haemoglobin is affected only by hydrogen ion is called the “Bohr effect”. 13.3.5 Chloride Shift (Role of Chloride ion in Oxygen Transport)
Chloride shift (also known as the Hamburger phenomenon or lineas phenomenon, named after Hartog Jakob Hamburger) is a course of action, which take place in a cardiovascular system. Under this course of action, − − substitute of bicarbonate (HCO3 ) and chloride (Cl ) across the membrane of red blood cells (RBCs).
Mechanism
Carbon dioxide is a by-product of normal metabolism of tissues. It is solubilized in the blood plasma and into red blood cells (RBC) and with the catalytic action of carbonic anhydrase, carbonic acid (H2CO3) is produced. − Spontaneous dissociation of carbonic acid into bicarbonate Ions (HCO3 ) and a hydrogen ion (H+) takes place. Thus, causing a decrease in intracellular pCO2 that is responsible for passive diffusion of more carbon dioxide into the cell.
+ − In general, cell membranes are impermeable to charged ions (H , HCO3 etc). But, red blood cells (RBCs) are capable to exchange bicarbonate ion for chloride ion with the support of the anion exchanger. In this way, rising in intracellular bicarbonate ions leads to bicarbonate export and chloride intake. Such exchange or shift is referred as the "chloride shift" Fig. 13.13 (A). As a result, the chloride ion concentration is lower in systemic venous blood as compared to systemic arterial blood. High concentration of venous carbon dioxide (pCO2) is responsible for enhancing the rate of bicarbonate production in red blood cells (RBCs) and further it is exchanged by the chloride ion.
The reverse process occurs in the pulmonary capillaries of the lungs. In lungs, the partial pressure of oxygen (PO2 ) rises and carbon dioxide (PCO2 ) falls. Consequently, carbon dioxide is released from haemoglobin and binding of oxygen to haemoglobin is started. This incident is responsible to release hydrogen ions from hemoglobin and increases the free hydrogen 213
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ions concentration within the red blood cells. Subsequently, shifting of the
equilibrium towards CO2 and water formation from bicarbonate occurs. Later, reduction in the intracellular level of bicarbonate ions is carried out by the reverses chloride-bicarbonate exchange Fig. 13.13 (B). There is a movement of bicarbonate ion into the cell in exchange for chloride moving out. Thus, chloride shift is a regulatory and supporting process for the oxygen transportation via the haemoglobin.
Fig. 13.13: (A) Chloride shift: exchange of oxygen and carbon dioxide in the tissues (internal respiration), (B) Reverse chloride shift: exchange of carbon dioxide and oxygen in the tissues (external respiration).
(Source of image: http://zimbelman.com/ap2/E4-StudyGuide2.pdf)
SAQ 2
1. Indicate whether the following statements are true or false:
i) Oxygen binding is defined as a capacity of binding of an oxygen molecule to a specific functional protein for either transport or storage in vivo. ( )
ii) Dalton's law of partial pressures states that the total pressure of a mixture of gases is equal to the sum of the partial pressures of the component gases. ( )
iii) The P50 represents the partial pressure at which myoglobin or haemoglobin is 50% percent saturated with oxygen. ( )
iv) The oxygen dissociation curve is known by several other names such as oxygen haemoglobin dissociation curve, the oxyhemoglobin dissociation curve. ( )
v) Temperature does not influence the oxygen dissociation curve. ( )
2. Match the following in column A with those given in column B:
Column A Column B
(1) Normal body temperature (a) BPG
(2) Synthesized from (b) 37o C
214 glycolytic intermediates
Unit 13 Myoglobin and Haemoglobin
(3) Bohr effect (c) Christian Bohr (4) Chloride shift (d) Hamburger phenomenon
13.4 Co-Operativity Phenomenon: Myoglobin and Haemoglobin
Cooperativity is a phenomenon observed by a receptors that have multiple sites. The affinity of the binding sites for a ligand may be increased (positive co-operativity) or decreased (negative co-operativity). Binding of oxygen to the haemoglobin molecule is based on the phenomenon of co- operativity. Co-operativity maximizes the ability of haemoglobin both to load
O2 at the PO2 of the lungs and to deliver O2 at the PO2 of the tissues. Two models are proposed to explain this phenomenon. These are concerted model and sequential model, measurement of the degree of co-operativity is explained by Hill plot.
13.4.1 Concerted Model
The first model was proposed by Jacques Monod, Jeffries Wyman, and Jean- Pierre Changeux in 1965, and is called the MWC model or the concerted model. The concerted model hypothesized that all the subunits of a co- operatively binding protein are functionally identical. Each subunit can exist in two conformational states and simultaneously all the subunits undergo the transition from one conformational state to the other. These two conformational states are in equilibrium. The ligand can bind to either conformation, but binds each with different affinity. Consecutive binding of ligand (a substance/compound that can form bonds with complex biomolecules) molecules to the low-affinity conformational state (which is more stable in the absence of ligand) makes a transition to the high-affinity conformational state more likely. According to this model, no protein has individual subunits in different conformations.
Binding of oxygen to haemoglobin is explained by this model (Fig. 13.14). In view of this model, haemoglobin has two conformational states. One is the tense (T) state and another is the relaxed (R) state. The T state of the hemoglobin is more tense as it is in the de-oxyhemoglobin form, while the R state of the hemoglobin is more relaxed as it is in the oxy-haemoglobin form. T state is controlled by the subunit-subunit interactions, whereas the R state is more flexible due to the ability of oxygen binding. De-oxyhemoglobin and oxy- hemoglobin is differentiated on the basis of the oxygen binding. The de- oxyhemoglobin form doesn't have oxygen and the oxy-hemoglobin form is highly oxygen bounded. The binding of oxygen at one site of the haemoglobin molecule is responsible to enhance the binding affinity in other active sites. Thus, in the concerted model of the hemoglobin, it shows that the one oxygen binding to an active site will increase the probability of other oxygen binding to the other active sites in the hemoglobin. In general, oxygen binding to the haemoglobin molecule is responsible to shift the equilibrium toward the R 215
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state. Ultimately, the R form will be prevalent at high oxygen levels and the T form will be prevalent at lower oxygen levels. Allosteric effectors of haemoglobin are carried out their function by shifting the equilibrium towards or away from the T-state, depends on whether it's an inhibitor or a promoter.
Fig. 13.14: Concerted model. All molecules exist either in the T state or in the R state. At each level of oxygen loading, an equilibrium exists between the T and R states. The equilibrium shifts from strongly favouring the T state with no oxygen bound to strongly favouring the R state with no oxygen bound to strongly favouring the R state when the molecule is fully loaded with oxygen.
(Source of image: http://oregonstate.edu/instruct/bb450/ fall14/lecture/hemoglobinoutline.html) 13.4.3 Sequential Model
The sequential model (also known as the KNF model) was proposed in 1966 by Daniel Koshland and colleagues. It postulates that a protein's conformation changes with each binding of a ligand, thus sequentially changing its affinity for the ligand at neighbouring binding sites.
According to this model, the subunits of multimeric proteins have two conformational states. The binding of the ligand is responsible to perform the conformational change in the other subunits of the multimeric protein. Although the subunits go through conformational changes independently, the switch of one subunit makes the other subunits more likely to change, by reducing the energy needed for subsequent subunits to undergo the same conformational change. In an expansion, the binding of a ligand to one subunit of molecule performs the modifications in the protein's shape, thus making it more thermodynamically favourable for the other subunits to switch conformation to the state of high affinity. Negative cooperativity is also performed by the binding of ligands. Such situation may also result in negative or a decreased affinity for the ligand at the next binding site, which is a specific feature that makes the KNF model different from the MWC model, which proposes merely the positive cooperativity.
Binding of oxygen to haemoglobin molecule is explained by KNF model. The binding of oxygen changes the conformation of the subunit of haemoglobin to which it binds. This conformational change induces changes in the nearest subunits and facilitates the binding of another oxygen molecule. Such conformational changes are progressive and ended till the binding of fourth oxygen molecule (Fig.13.15). 216
Unit 13 Myoglobin and Haemoglobin
Fig. 13.15: Sequential model. The binding of an oxygen molecule changes the conformation of the subunit of haemoglobin to which it binds. The conformational changes induces changes in neighbouring subunits that increase their affinity for the oxygen.
Source of image: http://oregonstate.edu/instruct/bb450/fall14/ lecture/hemoglobinoutline.html 13.4.4 Hill Plot (Hill Equation and Hill Coefficient)
Cooperative binding of oxygen by haemoglobin was first analyzed by Archibald Hill in 1910. From this work came a general approach to the study of cooperative ligand binding to multi subunit proteins. He proposed an equation and also known as Hill’s equation. This equation provides a way to quantify cooperative binding by describing the fraction of saturated ligand binding sites as a function of the ligand concentration.
Hill assumed that Hb had n binding sites that had to be occupied simultaneously:
-Eq (1) and the expression for the association constant becomes
-Eq (2).
It is more common, however, to consider the dissociation constant, Kd, which is the reciprocal of Ka (Kd = 1/Ka) and is given in units of molar concentration
(M). Kd is the equilibrium constant for the release of ligand.
Here, considers the binding equilibrium from the standpoint of the fraction θ (theta), of ligand binding sites on the protein that are occupied by ligand:
-Eq (3).
By considering the Eq(2) [in terms of Kd ] and Eq (3), we have:
-Eq (4).
Rearranging and then taking the log of both sides, yields:
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-Eq (5).
-Eq (6).
n where Kd = [L] 0.5
Equation -6 is the Hill equation, and a plot of log [θ/(1-θ)] versus log [L] is called a Hill plot. Equation-6 is the equation of line. According to this equation, the Hill plot should have a slope of n. However, the experimentally determined slope actually reflects not the number of binding sites, but the degree of interaction between them. The slope of a Hill plot is therefore
denoted by nH, the Hill coefficient, which is a measure of the degree of co-
operativity. The value of nH give the inferences regarding co-operativity.
These are described below:
If nH equals to 1, ligand binding is not cooperative. Such a situation is also observed in a multi-subunit protein, which subunits are not performed the proper communication to each other.
An nH of greater than 1 indicates positive co-operativity in ligand binding. This is the situation observed in haemoglobin, in which the binding of one molecule of ligand facilitates the binding of others.
The theoretical upper limit for nH is reached, when nH = n. In this case, the binding would be completely cooperative and all the binding sites on the protein would bind ligand simultaneously, and no protein molecules partially saturated with ligand would be present under any conditions This limit is never
reached in practice, and the measured value of nH is always less than the actual number of ligand-binding sites in the protein.
An nH is less than 1 indicates negative co-operativity, in which the binding of one molecule of ligand obstructs the binding of others. The suitable cases of negative co-operativity are rare.
To adapt the Hill equation to the binding of oxygen to haemoglobin and here again substituted the pO2 for [L] and P50 n for Kd, the equation represented below:
-Eq (7).
Hill plots for myoglobin and haemoglobin are given in Fig. 13.16.
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Unit 13 Myoglobin and Haemoglobin
Fig. 13.16: Hill plots for the binding of oxygen to myoglobin and haemoglobin
(When nH = 1, there is no evident co-operativity. The maximum degree of co- operativity observed for hemoglobin corresponds approximately to nH =3).
(Source of image: https://www.studocu.com/en-au/document/university-of- melbourne/biochemistry-and-molecular-biology/lecture-notes/mst1-lecture- notes-mst1/4184630/view) SAQ 3
1. Indicate whether the following statements are true or false: i) Co-operativity is a phenomenon observed by a receptor that has multiple sites. ( ) ii) Measurement of the degree of co-operativity is explained by Hill plot. ( ) iii) The concerted model hypothesized that all the subunits of a co- operatively binding protein are functionally identical. ( ) iv) The sequential model postulates that a protein's conformation changes with each binding of a ligand, thus sequentially changing its affinity for the ligand at neighbouring binding sites. ( ) v) Co-operative binding of oxygen by haemoglobin was first analyzed by Archibald Hill in 1910. ( )
2. Match the following in column A with those given in column B:
Column A Column B
(1) Hill plot (a) MWC Model
(2) Concerted Model (b) Degree of cooperativity
(3) Sequential model (c) KNF model
(4) Co-operative binding of oxygen (d) Haemoglobin
13.5 Haemoglobin Disorders Introduction
Haemoglobin disorders are also known as the hemoglobinopathies. Haemoglobinopathies are a group of genetic disorders. In such disorders, an 219
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abnormal production or structure of the hemoglobin molecule is take place. These abnormal haemoglobins are differentiated by their characteristic electrophoretic mobilities. 13.5.1 Types and Causes
Haemoglobinopathies are categorized in to following:
(1) Change in structure and quality of haemoglobin
Haemoglobinopathies, where the mutation results in a change to the structure and quality of haemoglobin, are known as haemoglobin variants. The most important example of sickle cell variant is Hb S. Other haemoglobin variants, which have a genetic significance, and also occur most frequently in the populations in England are Hb C and Hb D.
(2) Reduction in quantity of haemoglobin
The thalassaemias is the name for a group of related conditions where the amount of haemoglobin that the body produces is reduced, and these impacts on its oxygen carrying capacity. These usually affect either the alpha or beta globin chain.
Classical examples of haemoglobinopathies
Haemoglobinopathies are not gender (x) linked and more prevalent in certain parts of the world. Two classical examples of haemoglobinopathies are sickle cell diseases and thalassaemias.
Sickle cell disease is a recessively inherited genetic condition of the haemoglobin. It appears due to transfer of abnormal haemoglobin genes to the baby by both parents and the baby has no normal haemoglobin (Hb A). The quality of the haemoglobin is affected by abnormal beta globin chains
The most common types of sickle cell disease are:
(1) Sickle cell anemia (HbSS); In such disease, patients inherited both S genes from their parents. It is also the most severe form of the disease with potential for serious complications in later life.
(2) Sickle cell disease SC ( HbSC): In such disease, patients has the genotype SC, which means a sufferer inherits one sickle S gene mutation and one Hbc gene mutation from each parent. The symptoms are similar to Hb SS and it is the second most common and severe form of the disease.
(3) Sickle cell beta thalassaemia (SβO Thalassaemia): In such disease, haemoglobin in red blood cells is affected. Sufferers inherit S gene from one parent and a thalassaemia from another parent.
(4) Sickle cell traits (HbAS): In such disease, patients inherit one sickle gene S from one parent and a normal A from another parent. They are sometimes referred to as carriers and can pass it to their children if they marry 220
Unit 13 Myoglobin and Haemoglobin someone with similar genes; while, this group is usually not sick, some have been known to manifest mild symptoms of sickle cell warriors.
B.Thalassaemia
Thalassemias are a group of genetic hemolytic disorders. These disorders are characterized by impairment in the synthesis of globin chains of Hb. Thalassemias are characterized by a defect in the production of α-or β-globin chain. There is however, no abnormality in the amino acids of the individual chains. Thalassemias also occur due to a variety of molecular defects:
1. Gene deletion or substitution,
2. Underproduction or instability of mRNA,
3. Defect in the initiation of chain synthesis,
4. Premature chain termination. i) α-Thalassemias
α-Thalassemias are caused by the reduction in the synthesis or complete deficiency of α-globin chain of haemoglobin (Hb). There are four copies of α- globin gene and situated two on each one of the chromosome 16. Four types of α-thalassemias are observed due to the number of missing α-globin genes.
1. Silent carrier state is due to loss of one of the four α-globin genes with no physical manifestations.
2. α-Thalassemia trait is observed by loss of two genes (both from the same gene pair or one from each gene pair). Slight anaemia is observed.
3. Hemoglobin H disease is observed due to missing of three genes. This disease is associated with moderate anaemia.
4. Hydrops fetails is the most severe form of α-Thalassemias due to lack of all the four genes. The foetus usually survives until birth and then dies. ii) β-Thalassemias
In such disease, the decrease in synthesis rate or complete absence of β- globin chain is responsible for β-thalassemias. The production of α-globin chain continues to be normal, leading to the formation of a globin tetramer (α4) that precipitate. This causes premature death of erythrocytes. There are mainly two types of β-thalassemias.
1. β-Thalassemia minor: This is an heterozygous state with a defect in only one of the two β-globin gene pairs on chromosome 11. This disorder, also known as β-thalassemia trait, usually asymptomatic, since the individuals can make some amount of β-globin from the affected gene.
2. β-Thalassemia major: This is a homozygous state with a defect in both the genes responsible for β-globin synthesis. The infants born with β- 221
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thalassemia major are healthy at birth since β-globin is not synthesized during the foetal development. They become severely anaemic and die within 1-2 years. Frequent blood transfusion is required for these children. This is associated with iron overload which in turn may lead to death within 15-20 years of age. SAQ 4
1. Indicate whether the following statements are true or false:
i) Haemoglobin disorders are also known as the hemoglobinopathies.
ii) Haemoglobinopathies are a group of genetic disorders.
iii) Abnormal haemoglobins are differentiated by their characteristic electrophoretic mobilities.
iv) Two classical examples of haemoglobinopathies are sickle cell diseases and thalassaemias.
v) Sickle cell disease is a recessively inherited genetic condition of the haemoglobin.
2. Match the following in column A with those given in column B:
Column A Column B
(1) Sickle cell anemia (a) HbSC
(2) Sickle cell disease SC (b) HbSS
(3) Sickle cell traits (c) HbAS
(4) Thalassemias (d) Hemolytic disorders
13.6 SUMMARY
• A comparative analysis of myoglobin and haemoglobin showed the key characteristics of these two proteins in context of structure and function. These two proteins are evolutionary related and have similarity in structure and physiological functions. Haemoglobin and myoglobin are very important proteins regarding the oxygen carrying capacity.
• Haemoglobin has a 90% of its potential oxygen-carrying capacity, whereas, myoglobin has only the 7% of its potential oxygen-carrying capacity. Myoglobin is an enormously compressed molecule with the dimensions of 45 × 35 × 25 Å. This protein is a single polypeptide chain of 153 amino acids (molecular weight =17,000 Da). There are 70% part of the main chain is folded into eight α- helices and much of the rest of the chain produces turns and loops between helices.
• Haemoglobin is actually a conjugated protein containing heme as prosthetic group and globin as the protein part apoprotein. Heme is 222 present as a prosthetic group in haemoglobin as well as in myoglobin.
Unit 13 Myoglobin and Haemoglobin
Each haemoglobin molecule consists of a tetramer of globin polypeptide chains. Each polypeptide chain contains heme in the heme-pocket. Then one haemoglobin molecule contains four heme units. The subunits of haemoglobin are arranged in a tetrahedral array with a tight spherical overall appearance and each individual polypeptide is folded in such a manner to maximize polar residues being on the exposed surface and non-polar interactions being internal, making this large protein water soluble. The interior surface of the molecule lined with nonpolar groups, form a hydrophobic pocket into which heme is inserted. Myoglobin is a molecule of oxygen transporter and it facilitates respiration in rapidly respiring muscle tissue.
• Haemoglobin is an important molecule of oxygen transporter (red blood cells) in vivo and this is it transports oxygen and partially carbon dioxide. Oxygen binding is defined as a capacity of binding of an oxygen molecule to a specific functional protein for either transport or storage in vivo. An oxygen-binding curve is a plot that shows fractional saturation versus the concentration of oxygen. By definition, fractional saturation indicates the presence of binding sites that have oxygen. Fractional saturation can range from zero (all sites are empty) to one (all sites are filled). Oxygen binding is defined as a capacity of binding of an oxygen molecule to a specific functional protein for either transport or storage in vivo (process that take place in living organisms). An oxygen-binding curve is a plot that shows fractional saturation versus the concentration of oxygen. Partial pressure determined the concentration of oxygen. The oxygen dissociation curve is known by several other names such as oxygen haemoglobin dissociation curve, the oxy-hemoglobin dissociation curve. The oxygen dissociation curve is a curve that draws the proportion of hemoglobin in its saturated form on the vertical axis adjacent to the existing oxygen pressure on the horizontal axis. This curve explains the mechanism of binding of oxygen to haemoglobin and its releases. Binding capacity of oxygen to haemoglobin is influenced by several factors. These factors are: (1) Temperature, (2) pH or hydrogen
ion concentration, (3) CO2 (Carbon dioxide) and (4) 2, 3- bisphosphoglycerate (BPG). Bohr effect explains the uptake of oxygen in lungs and its releases in the tissue as well as its regulation by the + concentration of CO2 (carbon dioxide), H ion (hydrogen ions) and the
partial pressure of oxygen (pO2). Chloride shift is a course of action, which take place in a cardiovascular system. Under this course of action, − − substitute of bicarbonate (HCO3 ) and chloride (Cl ) across the membrane of red blood cells (RBCs) is take place.
• Co-operativity is a phenomenon observed by a receptor that has multiple sites. The affinity of the binding sites for a ligand may be increased (positive co-operativity) or decreased (negative cooperativity). Binding of oxygen to the haemoglobin molecule is based on the phenomenon of co-operativity. Co-operativity maximizes the
ability of haemoglobin both to load O2 at the PO2 of the lungs and to
deliver O2 at the PO2 of the tissues. Two models are proposed to explain
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this phenomenon. These are concerted model and sequential model. Measurement of the degree of cooperativity is explained by Hill plot.
• Haemoglobinopathies are a group of genetic disorders. In such disorders, an abnormal production or structure of the hemoglobin molecule is take place. These abnormal haemoglobins are differentiated by their characteristic electrophoretic mobilities. Two classical examples of haemoglobinopathies are sickle cell diseases and thalassaemias. 13.7 TERMINAL QUESTIONS
1. Describe the structure and functions of myoglobin and haemoglobin?
2. Outline the important differences between the myoglobin and haemoglobin?
3. What do you understand by oxygen binding curve and oxygen dissociation curve? Describe both the curves in the context of myoglobin and haemoglobin.
4. What is the meaning of co-operativity? What is the importance of Hill plot in the context of the co-operativity?
5. Describe all the factors which influence the oxygen dissociation curve?
6. Describe the concerted and sequential models in the context of haemoglobin?
7. Write a short note on haemoglobin disorders? 13.8 ANSEWERS
SAQ 1
1. i) True ii) True iii) False iv) True v) True.
2. (1)-(b), (2)-(a), (3)-(d), (4)-(c).
SAQ 2
1. i) True ii) True iii) True iv) True v) False.
2. (1)-(b), (2)-(a), (3)-(c), (4)-(d).
SAQ 3
1. i) True ii) True iii) True iv) True v) True.
2. (1)-(b), (2)-(a), (3)-(c), (4)-(d).
SAQ 4
1. i) True ii) True iii) True iv) True v) True.
2. (1)-(b), (2)-(a), (3)-(c), (4)-(d).
Terminal Questions
1. Refer to section 13.2.1 224 2. Refer to section 13.2.3.
Unit 13 Myoglobin and Haemoglobin
3. Refer to section 13.3.1 & 13.3.2. 4. Refer to section 13.4.1 & 13.4.4.
5. Refer to section 13.3.3.
6. Refer to section 13.4.2 & 13.4.3.
7. Refer to section 13.5. 13.9 FURTHER READINGS
1. Voet, Donald, and Judith G. Voet. 1995. Biochemistry. New York: J. Wiley & Sons.
2. Lehninger, Albert L., David L. Nelson, and Michael M. Cox. 2000. Lehninger principles of biochemistry. New York: Worth Publishers.
3. Berg, Jeremy M., John L. Tymoczko, Lubert Stryer, and Lubert Stryer. 2002. Biochemistry. New York: W.H. Freeman.
4. Devlin, Thomas M. 2011. Textbook of biochemistry: with clinical correlations. Hoboken, NJ: John Wiley & Sons.
5. U Satyanarayana.2013. Biochemistry, 4th edition, Elsevier India.
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