Hemoglobin proton release

The ATP can then power another contraction. Eventually, the amount of ATP available approaches a level too low to be bound by myosin in the muscle, even though it is by no means exhausted. The protons (acid) from metabolism cause hemoglobin to release its oxygen more readily, promoting a switch to aerobic metabolism. Lactate and protons from glycolysis may also lead to fatigue and an inability to sustain the level of speed that was possible earlier. In most humans, this seems to occur after a run of about 400 meters, which is why running quarters is one of the most unpleasant exercises for any athlete, no matter how well conditioned. 

Protons released by hemoglobin as it is oxygenated (Step 2) react with bicarbonate ions to form carbonic acid. 

The Bohr Effect is the Release of H on Binding of O2. Hemoglobin releases about 0.6 protons for each O2 bound. These protons derive from partial proton release from the Ct2-chain VaP(a-NH3 ) and the P2-chain H146(imi-dazolium). These charged species are part of the ion-paired clusters shown in Figure 7.8. On... 

As with hemoglobin, discussed in Chapter 7, a Bohr effect occurs with cytochrome c oxidase. Again from the viewpoint of the hydrophobic consilient mechanism, these phenomena are analogous. Formation of the less polar states on reduction of Complex IV and on forming deoxyhemoglobin result in proton uptake, whereas formation of the more polar oxidized state of Complex IV and the more polar oxygenated state of hemoglobin result in proton release. This is as expected from the AG, of the comprehensive hydrophobic effect, as discussed above. 

After Releasing O2 at the Tissues, Hemoglobin Transports CO2 Protons to the Lungs... 

Figure 6-9. The Bohr effect. Carbon dioxide generated in peripheral tissues combines with water to form carbonic acid, which dissociates into protons and bicarbonate ions. Deoxyhemoglobin acts as a buffer by binding protons and delivering them to the lungs. In the lungs, the uptake of oxygen by hemoglobin releases protons that combine with bicarbonate ion, forming carbonic acid, which when dehydrated by carbonic anhydrase becomes carbon dioxide, which then is exhaled.

Hemoglobin also functions in CO2 and proton transport from tissues to lungs. Release of O2 from oxyHb at the tissues is accompanied by uptake of protons due to lowering of the of histidine residues. 

Figure 7.9 Delivery of oxygen to peripheral tissue by oxyhemoglobin. Production of protons from COz and HzO, catalyzed by carbonic anhydrase (CA) and their uptake by hemoglobin are also shown. The pC02 in peripheral tissues is high and p02 is low, causing the conversion of hemoglobin R forms into the T forms with the concomitant release of 02. (Reproduced by permission from Bunn HF, Forget BG. Hemoglobin Molecular, Genetic, and Clinical Aspects. Philadelphia WB Saunders, 1986, p. 41.)...

ROOH = LOOH, a lipid alcohol is released and no initiation or branching can occur. In aprotic solvents or acid environments, however, H abstraction is delayed and the radicals remain active. When the heme is myoglobin, hemoglobin, or a heme protein where an internal proton source is not available, the reaction mechanism is more likely to be homolytic, yielding alkoxyl radicals with no radical on the porphyrin. 

Fig. 44.14. Binding of CO2 to hemoglobin. CO2 forms carbamates with the N-terminal amino gronps of Hb chains. Approximately 15% of the CO2 in blood is carried to the Inngs bound to Hb. The reaction releases protons, which contribnte to the Bohr effect. The overall effect is the stabilization of the deoxy form of hemoglobin.

Fig. 44.13. Effect of on oxygen binding by hemoglobin (Hb). A. In the tissues, CO2 is released. In the red blood cell, this CO2 forms carbonic acid, which releases protons. The protons bind to Hb, causing it to release oxygen to the tissues. B. In the lungs, the reactions are reversed. O2 binds to protonated Hb, causing the release of protons. They bind to bicarbonate (HCO3 ), forming carbonic acid, which is cleaved to water and CO2, which is exhaled.

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