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Hemoglobin Allosteric Effects Biochemistry > Proteins > Proteins Hemoglobin Allosteric Effects Biochemistry > Proteins > Proteins HEMOGLOBIN ALLOSTERIC EFFECTS OVERVIEW DEFINITIONS Bohr effect • Low pH enhances hemoglobin oxygen dissociation 2, 3 Bisphosphoglycerate (BPG) • Molecule localized in red blood cells • Decreases hemoglobin's oxygen affinity DISSOCIATION CURVE • % oxygen saturation vs. oxygen partial pressure (torr) • Cooperative binding produces sigmoidal binding curve Bohr Effect • Decrease in blood pH shifts curve to the right • Hemoglobin requires greater pO2 in peripheral tissues to reach 50% saturation • Lowering pH decreases hemoglobin's oxygen affinity 2,3 BPG • Hemoglobin without 2,3 BPG: shifts curve to the left (hyperbolic like myoglobin) • Adding 2,3 BPG: shifts curve back to the right. 1 / 7 BOHR EFFECT Beta subunit • Aspartate (-) and histidine (+) T-form: histidine pKa = 8.0 (side chains close to each other) • T-form favored when blood pH --> has a high affinity for H+ R-form: histidine pKa drops to 7.1 (side chains move apart) • Histidine loses H+ • R-form favored when blood pH is high and H+ concentration is low Carbon dioxide • CO2 + H2O --> H2CO3 (carbonic acid) --> HCO3- (bicarbonate) + H+ (reversible) • Carbonic anhydrase catalyzes carbonic acid formation • Carbonic acid spontaneously loses proton to form bicarbonate • Bicarbonate = blood buffer • Increase in CO2 --> lowers blood pH --> favors T-form hemoglobin Carbaminohemoglobin: CO2 binds N-terminal amino acids in hemoglobin 2,3 BPG • Has strong negative charge: binds central cavity in hemoglobin • Stabilizes the T-form (only binds T-form) CARBON MONOXIDE • Binds iron center with 220 times the affinity of O2 (irreversible) • Permanently increases oxygen affinity of remaining heme groups for oxygen • Decreases oxygen release in peripheral tissues CLINICAL CORRELATION Acetazolamide 2 / 7 • Carbonic anhydrase inhibitor used to treat altitude sickness • Increases bicarbonate excretion by kidneys • Makes blood more acidic, promotes oxygen release in peripheral tissues High altitude conditions • Individuals adapted to high altitude produce more 2,3 BPG • Favors T-form hemoglobin and O2 release: more efficient O2 delivery Tobacco smoke • Smokers have elevated blood CO: hinders O2 delivery • Can produce tissue hypoxia FULL-LENGTH TEXT • Here we will learn about the Bohr effect, and discuss the effects of 2,3 bisphosphoglycerate and carbon monoxide on hemoglobin. • To begin, start a table to list out each of the key topics we'll learn. - Bohr effect, which describes the effects of pH and carbon dioxide on hemoglobin binding. - 2,3 Bisphosphoglycerate (BPG), a molecule localized in red blood cells that decreases hemoglobin's affinity for oxygen. - Carbon monoxide, which increases hemoglobin's oxygen affinity and can produce toxic effects in the body. • As a review, draw hemoglobin's sigmoidal dissociation curve. • Label the x-axis partial pressure and the y-axis percent saturation. Now, let's illustrate the first allosteric effect: the Bohr Effect. • Draw another sigmoidal curve to the right of the first one. 3 / 7 • Show that a decrease in blood pH, shifts hemoglobin's dissociation curve to the right. • What does this mean? - Extend the horizontal line that demarcates 50% saturation. - Show that it intersects the second curve at a greater partial pressure of oxygen: lowering pH decreases hemoglobin's affinity for oxygen. • To better understand this, draw the three-dimensional structure of hemoglobin: with two alpha subunits and two beta subunits. Let's take a closer look at a beta-subunit. • Within it draw two functional groups: that of aspartate (negative) and histidine (positive). • Indicate that in T-form hemoglobin, these side chains are close to each other, which raises the pKa of histidine to 8.0. - Thus, hemoglobin has a high affinity for protons in the deoxygenated state (high pKa means a high proton affinity). Now, let's draw these groups in R-form hemoglobin. • Show that they are farther apart, and that the pKa of histidine drops to 7.1. - As we have seen, the transition between T-form and R-form hemoglobin causes shifts in amino acid conformations throughout the entire protein. • Show that because its pKa decreases, histidine loses a proton. • Thus, write that R-form is favored when blood pH is high, and the proton concentration is low. • Now, write that T-form is favored when the blood pH is low, and the proton concentration is high. - Thus, a low pH enhances oxygen dissociation and shifts hemoglobin's dissociation curve to the right. Now, we know that a low pH enhances oxygen dissociation. But what produces low blood pH in the first place? - Carbon dioxide! • Write out the following equation: 4 / 7 - Carbon dioxide plus water reversibly converts to carbonic acid. • Show that the enzyme carbonic anhydrase catalyzes this reaction. • Next, indicate that carbonic acid spontaneously loses its proton to form bicarbonate. - Thus bicarbonate releases protons into the blood stream. • Indicate that bicarbonate functions as a buffer in the blood. • As a clinical correlation, write that acetazolamide is a carbonic anhydrase inhibitor that is often used to treat altitude sickness. - How does it work? It produces an increase in bicarbonate excretion by the kidneys, via a mechanism we will not cover here. - As a result, it makes the blood more acidic, and promotes the release of oxygen in the peripheral tissues. • Finally, write that an increase in carbon dioxide in the body lowers blood pH and favors T-form hemoglobin. - This facilitates hemoglobin's physiologic function in the body. • Note that carbon dioxide can also form carbaminohemoglobin by binding to N-terminal amino acids in hemoglobin. We will not discuss this, here. Now that we have learned the Bohr effect, let's move on to 2,3-bisphosphoglycerate (BPG), a molecule synthesized by red blood cells. Our current sigmoidal dissociation curve already accounts for the allosteric effects of normal 2,3-BPG levels in red blood cells. • To visualize 2,3-BPG's effects, draw a hyperbolic curve to the left of the hemoglobin curve. - It should resemble myoglobin's dissociation curve. • Indicate that this curve represents hemoglobin without 2,3-BPG. - Thus, without 2,3-BPG, hemoglobin's oxygen affinity increases dramatically. • Show that adding 2,3-BPG shifts the curve back to the right. 5 / 7 How does 2,3-BPG lower hemoglobin's oxygen affinity? Let's illustrate this, now. • To do this, take a closer look at the center of our hemoglobin molecule: the cavity created at the intersection of all four subunits. • Draw histidine residues at the periphery of each of the beta subunits. • Show that these histidines are positively charged; they repel each other. • Now, importantly, label this diagram the T-form (deoxygenated hemolgobin). Now, let's add 2,3-BPG. • Draw a 2,3-BPG molecule within this central cavity. • Indicate that it has a strong negative charge, which stabilizes the T-form. - Other positively charged amino acid side chains also bind 2,3 BPG here, but we won't draw all of them. • Now, write that 2,3-BPG only binds the T-form. - It encourages oxygen dissociation, and facilitates oxygen delivery! • As a clinical correlation, indicate that individuals that have adapted to high altitude conditions produce more BPG. - Why? More BPG favors T-form hemoglobin and oxygen release; it allows hemoglobin to deliver more oxygen to the peripheral tissues. Finally, let's illustrate carbon monoxide. • Draw a simplified hemoglobin iron center. • Show that carbon monoxide irreversibly binds it. - We draw it at an angle because a distal histidine causes it to bend. • Write that carbon monoxide binds iron with an affinity 220 times greater than oxygen! 6 / 7 • Indicate that by irreversibly binding iron, it permanently increases the oxygen affinity of the remaining heme groups for oxygen. - Thus, it pushes the hemoglobin dissociation curve to the left. What are the physiological consequences? • Indicate that carbon monoxide leads to decreased oxygen release in the peripheral tissues. • As a clinical correlation, write that people who regularly smoke tobacco have elevated levels of carbon monoxide in their blood, which hinders hemoglobin's ability to deliver oxygen. - Thus, a consequence of elevated carbon monoxide is tissue hypoxia. Powered by TCPDF (www.tcpdf.org) 7 / 7.
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