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Globular proteins Biochemistry Team

Color index: Doctors slides Doctor’s notes Extra information Highlights Respiratory block EDITING FILE Objectives:

❖ To describe the globular proteins using common examples like hemoglobin and myoglobin. ❖ To study the structure and functions of globular proteins like:

➢ Hemoglobin (a major globular protein). ➢ γ-globulins (immunoglobulins) ➢ Myoglobin ❖ To know the different types of hemoglobin and difference between normal and abnormal hemoglobin

❖ To understand the diseases associated with globular proteins .

2 - Proteins are polymers of amino acids that are joined by Globular proteins: peptides bond, then folded in a proper way to make a Amino acid chains fold into shapes that resemble functional unit. spheres are called globular proteins - The sequence of amino acids determines the final structure This type of folding increases (shape) of a protein. solubility of proteins in - The Structure can either be: water.(because ) 1-globular (spherical) or 2-fiber (rope like structure). - The structure determines the function. - Rope like = structural 2- Hydrophobic 1- Polar”hydrophilic” functions like support and (non- polar) groups on the groups in the داﺧل) transport, they are not water protein’s surface interior ( اﻟﺑروﺗﯾن soluble e.g. collagen - Globular (spherical) = water soluble, so they can participate in systemic Fibrous proteins(collagen) are mainly reactions and interact with insoluble structural proteins metabolites. E.g. Enzymes 3 Globular proteins

There are mainly 3 types of globular proteins , ( α Alpha, β Beta , γ Gamma). These subunits have their own subtypes and combine together to make other types of globular proteins. E.g. hemoglobin is made of 2 α and 2β subunits.

γ-globulin Hemoglobin Enzymes myoglobin α1, α2, β-globulins (immunoglobulins)

Oxygen Storage Catalysis of Oxygen transport biochemical immune function Function and supply to the Various function “Humoral immunity” heart and muscle reactions

Give red color for RBCs

4 Hemoglobin Or 8 atoms of O2

A major Composed Held together by Each chain is a A Hb molecule of four subunit with a heme contain 4-heme globular non-covalent group in the center groups and carries protein in polypeptide interactions that carries oxygen 4 molecules of O2 humans chains.

Contains two Two α and dimers of αβ two β chains Extra :Haemoglobin formation does not occur randomly. but first the alpha chain binds subunits with the Beta chain forming a dimer. Two dimers form

Types of hemoglobin

Normal Abnormal Can not Can carry O2 carry O2 HbA: (97%) (Adult) Carboxy Hb

HbA2: (2%) Met Hb

HbF: (1%) (fetal) Sulf Hb Fetal HB More details are discussed HbA1c later in the lecture Structure of hemoglobin

Globin chain One Hemoglobin Consists of: - 4 globin chains “protein chains” two alpha and two beta (called subunits) - One alpha and one beta subunti bind together to form a dimer. {next slide} - each globin is bound to one heme group. (4 heme groups in total) - Heme groups consists of protoporphyrin + ferrous iron - Protoporphyrin is: organic compound forming a complex with iron (fe2 ) - each heme group makes two bonds, 1- with a ferrous iron (reduced form of iron “F+2”) 2-with the histidine on the globin - The Iron on the heme group binds to O2

Ferrous iron protoporphyrin

Heme group 6 2 dimers each HbA structure dimer consists of 2 subunits There are two types of HbA:

Hydrophobic bonds

Weaker ionic Notice that there bonds between is a lesser dimers number of ionic (interdimer) bonds because the rest were broken after O2 was bound to it

1-deoxyhemoglobin,T(taut) : 2-Oxyhemoglobin,R(relaxed): -deoxygenated state (no O2 bound to it) -oxygenated state . -Strong intradimer hydrophobic bonds - when O2 binds to it, some of the Between the subunits ionic bonds are broken, so the -weaker ionic bonds between dimers, structure becomes more relaxed these bonds make it T(taut) (tight) and allows for more movement. and causes less movement (ﻣﺷدود) Note for previous slide :The first oxygen molecule that binds with haemoglobin will be the hardest because of the conformation change that occurs the other three will bind much more easier than the first. Hemoglobin function

Normal value of hemoglobin (g/dL): ● Males 14-16 ● Females 13-15

Carries CO2 (carbon dioxide) and protons from Carries O2 from tissue back to lungs lungs to tissue

- High PO causes - Low PO causes 2 2 hemoglobin to bind hemoglobin to release to O and be 100% O 2 2 saturated (all 4 - High PCO allows O 2 2 hemoglobin binding to bind to hemoglobin places are occupied) - HB that is bound to - HB that is bound to CO2 is called oxygen is called carbaminohemoglobin oxyhemoglobin * a:alfa Types of normal Hemoglobin

Hb Hb A A2 Hb F 1C

● Appears shortly ● Major hemoglobin found in ● HbA undergoes before birth. the fetus and newborn non-enzymatic ● Constitutes ~2% ● Tetramer (has 4 monomers) glycosylation of total Hb. with two alpha and two ● Glycosylation depends gamma (γ) chains ● Composed of two on plasma glucose ● Higher affinity for O2 than levels (a) and two ( Hb A Why? Because there is ● HbA1c levels are high delta) globin a molecule that binds to beta in patients with chain*. subunit which release O2 diabetes mellitus. ● It produced by easily, and found less with ● When glucose is high, it will the 8th month of gamma subunit. bind to the hemoglobin. ● It allows the fetus to take the pregnancy in the ● It’s a diagnostic tool of O2 from the mother's blood. diabetes since it gives a fetus and it ● Transfers O2 from maternal reflect image about the last replaces Hb F to fetal circulation across 2-3 months .placenta ﺑﺎﻟﺣﺎﻻت) .gradually ● 5.8% is the normal .It’s produced from the 5th range ● اﻟﻣرﺿﯾﺔ ﯾﺻﯾر ﻓﻲ طﻔرة week until 6 months after ● 6.4% is high risk ﺑﺎﻟﺑﯾﺗﺎ ﺗؤدي اﻟﻰ ﺗﻛﺎﺛر born. range. indicates %6.5 =< ● ( ھﺎﻟﻧوﻋﯾن diabetes. Abnormal hemoglobin

Unable to transport O2 due to abnormal structure

Carboxy-Hb: Met- Hb: Sulf-HB

3+ Contains oxidized Fe (ferric) ● Forms due to high sulfur levels in ● CO “carbon monoxide” replaces O ● 2 blood and binds 200X tighter than O2 (~2%) that cannot carry O (normal 2 ● (irreversible reaction) ● Found in in smokers, and causes HB binds to ferrous “reduced form of iron) ● Seen in people treated with sulfate or shortness of breath ● reversible reaction cases of chronic constipation ● Also found in carbon monoxide ● This kind of hemoglobin is normally produced in the body. But we convert it to poisoning normal hemoglobin by the enzyme Nadh ● It is treatable by high amounts of ● Nadh reduces ferric iron to ferrous. A 100% saturated oxygen deficiency (genetic) in this enzyme causes this kind of abnormal hemoglobin ● Drugs like nitrate, acute inflammation, and production of oxygen reactive species are other causes of this abnormality

10 Hemoglobinopathies Disorders of hemoglobin caused by: -Beta subunit is represented by one -Synthesis of structurally abnormal Hb Or -Synthesis of insufficient quantities of normal Hb gene -Alpha is represented by two genes (qualitative) Or (quantitative) “genetic defects are less common because there are two gene -A combination of both responsible for the same functions

Sickle cell (HbS) disease Hemoglobin C disease Methemoglobinemia Thalassemia

-Caused by oxidation of Hb to -Caused by a single mutation -Defective synthesis of either α or -Caused by a single mutation in ferric (Fe3+) state in b-globin gene b-globin chain due to gene b-globin gene (point mutation) -Methemoglobin cannot bind -Glutamic acid at position 6 in mutation. Has two types: -Glutamic acid at position 6 in HbA oxygen HbA is replaced by lysine (1) α-thalassemia: is replaced by valine -Caused by certain drugs, (polar +) (notice it is the same -Synthesis of α-globin chain is -The mutant HbS (abnormal, reactive oxygen species and position as sickle cell anemia decreased or absent modified HB) contains βs chain NADH-cytochrome b5 reductase but instead of valine it -Causes mild to moderate 2copys deficiency becomes lysine) hemolytic anemia -The shape of RBCs become sickled, -can be treated except in case ( اﺣﺗﻣﺎﻟﯾﺔ اﻹﺻﺎﺑﺔ اﺳﮭل):Causes a mild form of (2) b-thalassemia- with a lifespan of 20 days of enzyme deficiency hemolytic anemia -Synthesis of b-globin chain is -Causes sickle cell anemia -Chocolate cyanosis: decreased or absent brownish-blue color of the skin -Causes severe anemia Glutamic acid is has a polar group while and blood valine is non polar, this leads to -Patients need regular blood structural changes where the modified transfusions hemoglobin starts attaching to itself -can be minor which doesn’t require making polymers of hemoglobin giving it blood transfusions , or major which its sickled shape requires blood transfusions Summary of Hemoglobinopathies Summary Myoglobin Looks like one subunit of HB but with some differences

A globular heme protein in heart and “skeletal” muscle

structure : Function: ★ Contains a single polypeptide Myoglobin in diseases: ★ Stores and supplies chain forming a single subunit ★ Myoglobinuria: oxygen to the heart and with eight a-helix structures Myoglobin is excreted skeletal muscle only. ★ The interior of the subunit is in urine due to muscle composed of nonpolar amino damage ★ It gives red color to acids (rhabdomyolysis) it skeletal muscles. ★ The charged (polar) amino means the destruction of acids are located on the muscle cells ★ Supplies oxygen during surface (forming hydrogen ★ May cause acute renal aerobic exercise. bond) while The heme group failure (block of urine) is present at the center of the ★ Specific marker for molecule muscle injury ★ Has one heme group which means it ★ Less specific marker binds to one O2 for heart attack Differences between a subunit of HB Immunoglobulins and myoglobin

●

Subunit of HB Myoglobin ● hydrophilic groups on hydrophilic groups on the surface the surface ●

Some hydrophobic groups are on the All hydrophobic ● surface for intra-dimer groups are inside binding ● Two histidine groups. - one involved in One histidine group binding ● involved in binding - The other facilitates the binding Summary Take home messages

16 Key concept map for hemoglobin structure and function (from lippincott) Extra

17 Key concept map for hemoglobinopathies (from lippincott) Extra

18 Extra Lippincott summary

Hemoglobin A, the major hemoglobin in adults, is composed of four polypeptide chains (two α chains and two β chains, α2β2) held together by noncovalent interactions (Figure 3.24). The subunits occupy different relative positions in deoxyhemoglobin compared with oxyhemoglobin. The deoxy form of hemoglobin is called the “T,” or taut (tense) form. It has a constrained structure that limits the movement of the polypeptide chains. The T form is the low-oxygen-affinity form of hemoglobin. The binding of oxygen to hemoglobin causes rupture of some of the ionic and hydrogen bonds. This leads to a structure called the “R,” or relaxed form, in which the polypeptide chains have more freedom of movement. The R form is the high-oxygen-affinity form of hemoglobin. The oxygen dissociation curve for hemo globin is sigmoidal in shape (in contrast to that of myoglobin, which is hyperbolic), indicating that the subunits cooperate in binding oxygen. Cooperative binding of oxygen by the four subunits of hemoglobin means that the binding of an oxygen molecule at one heme group increases the oxygen affinity of the remaining heme groups in the same hemoglobin molecule. Hemoglobin’s ability to bindoxygen reversibly is affected by the pO2 (through heme-heme interactions), the pH of the environment, the pCO2, and the availability of 2,3-bisphosphoglycerate (2,3-BPG). For example, the release of O2 from Hb is enhanced when the pH is lowered or the pCO2 is increased (the Bohr effect), such as in exercising muscle, and the oxygen dissociation curve of Hb is shifted to the right. To cope long-term with the effects of chronic hypoxia or anemia, the concentration of 2,3-BPG in RBCs increases. 2,3-BPG binds to the Hb and decreases its oxygen affinity, and it, therefore, also shifts the oxygen-dissociation curve to the right. Carbon monoxide (CO) binds tightly (but reversibly) to the hemoglobin iron, forming carbon monoxy hemoglobin (Hb CO). Hemoglobinopathies are disorders caused either by production of a structurally abnormal hemoglobin molecule, synthesis of insufficient quantities of normal hemoglobin subunits, or, rarely, both (Figure 3.25). The sickling diseases sickle cell anemia (Hb S disease) and hemoglobin SC disease, as well as hemoglobin C disease and the thalassemia syndromes are representative hemoglobinopathies that can have severe clinical consequences. 19 MCQs:

1- What is the function of Myoglobin ? A- oxygen transport B- oxygen storage C- exchange gas D-none

2- A Hb molecule contains …….. Carries ……………………. A- 4 heme groups , 2 molecules of O2 B- 2 heme groups , 2 molecules of O2 C- 4 molecules of O2 , 4 heme groups D-4 heme groups , 4 molecules of O2 Answers 1-B 3- Which of the following has highest affinity to O2 ? 2-D A- HbA B-HbA2 C-HbF D-HbA1c 3-C 4-D 4- Caused by Glutamic acid at position 6 in HbA is replaced by lysine A-Thalassemia B- sickle cell anemia C-Methemoglobinemia D-Hemoglobin C disease 20 SAQ

A 67-year-old man presented to the emergency department with a 1 week history of angina and shortness of breath. He complained that his face and extremities had a “blue color.” His medical history included chronic stable angina treated with isosorbide dinitrate and nitroglycerin. Blood obtained for analysis was chocolate-colored.

What is the most likely diagnosis?

What is the cause of the disease ?

Correct answer = . Methemoglobinemia; Oxidation of the heme component of hemoglobin to the ferric (Fe3+) state forms methemoglobin. This may be caused by the action of certain drugs, such as nitrates. The methemoglobinemias are characterized by chocolate cyanosis (a brownish-blue coloration of the skin and mucous membranes), and chocolate-colored blood as a result of the dark-colored methemoglobin. Symptoms are related to tissue hypoxia, and include anxiety, headache, dyspnea. In rare cases, coma and death can occur.

21 Girls team Boys team Team leaders

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