12: Hemoglobin and allosteric effects

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Reading & Problems: LNC p. 163-172, 157- 163, p. 189 - 200; p. 184 prob. 3, 6, 7, 8, 9; Segel p. 93 prob. 55

I. A variety of molecules affect the O₂ binding by Hb (and their binding is in turn affected by O₂ binding). These are allosteric effectors of Hb binding of O₂. Some of these molecules are also transported by Hb.

A. O₂ - positively affects binding of O₂ (positive cooperativity). This results from the breakage of salt bridges between Hb subunits that help stabilize the "tense" deoxy forms of the subunits. The illustration below shows salt bridges that are present in the deoxy (tense) form of Hb, but that are broken due to conformational changes when oxygen binds (the relaxed form).

B. H⁺ negatively affects binding of O₂. For example, H⁺ is bound by a histidine residue in the beta chain that forms a salt bridge with an aspartate residue only in the deoxy form. These salt bridges stabilize the deoxy form. This histidine has a higher pK_a in the deoxy form than it does in the oxy form of Hb. As a result,more H⁺ is bound to the deoxy form than to the oxy form. H⁺ is preferentially carried from the tissues to the lungs.

C. CO₂ negatively affects binding of O₂. CO₂ is bound by Hb in a carbamate linkage to the N-terminus of the beta chain and is carried from the tissues to the lungs.

D. 2,3-Bisphosphoglycerate (BPG) negatively affects binding of O₂. Changes in BPG concentration are associated with adaptation to altitude. Smokers also have elevated BPG because the oxygen carrying capacity of their blood is reduced due to the inhalation of CO in tobacco smoke.

external link: Hb structure in JSmol including Oxy and Deoxy states.

II. Mammalian fetal hemoglobins

A. Fetal mammals have alternative Hb where the beta-globin subunit is substituted initially with an epsilon subunit (alpha₂ epsilon₂, embryonic Hb) and then by a gamma subunit (alpha₂ gamma₂, fetal Hb).

B. In these alternative subunits, the position occupied by His146 (one of the His residues that binds BPG) in beta-globin is replaced by a Ser. As a result, Hb containing these subunits doesn't bind BPG as tightly, this preferentially favors the unbound state and increases affinity for O₂.

C. The higher affinity of embryonic and fetal Hb for O₂ allows the developing fetus to take O₂ from the adult hb in the mother's blood through the placenta.

D. BPG is a small molecule and so its concentration equalizes between maternal and fetal blood.

III. Enzymes - protein catalysts (catalyst - a compound that accelerates a reaction without itself being changed)

A. Enzymes have active sites where "substrates" are bound to facilitate reactions.

B. Enzymes cannot affect the delta G of a reaction so they do not change the direction of a reaction. The only thing they can do is make the reaction move more rapidly toward equilibrium than in the uncatalyzed reaction (but this acceleration can be tens of orders of magnitude).

C. Enzymes catalyze reactions by providing an alternative mechanism that lowers the activation energy for the reaction.

D. By directing some reactions to accelerate and not others, enzymes actually determine which reactions occur in cells.

Some take home points:

Hemoglobin:

O₂ is a positive regulator of O₂ binding.
H⁺, CO₂, and BPG are negative effectors of O₂ binding.
O₂ is a negative effector of H⁺, CO₂, and BPG binding.
H⁺, CO₂, and BPG each positively affect the binding of the others.
Hb "always does the right thing" and responds to the different environments in the lungs and tissues (capillaries) to load and unload the appropriate molecules:

in the lungs there is high O₂ and low H⁺ and CO₂ and Hb responds to all of these by picking up O₂ and droping off H⁺ and CO₂
in the capillaries there is high H⁺ and CO₂ and low O₂ and and Hb responds to all of these by picking up H⁺ and CO₂ and droping off O₂

6. Fetal Hb has a higher affinity for O₂ than does adult Hb due to a lesser affinity of fetal Hb for BPG.

Enzymes:
• see III A-D AND IV A-D above!

Contributors

Charles S. Gasser (Department of Molecular & Cellular Biology; UC Davis)

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