... within the flat plane by four nitrogen ligands from the porphyrin ring. The iron ion makes a fifth bond to a histidine side chain from one of the helices that form the heme pocket. This fifth coordination bond is to histidine 87 in the human ? chain and histidine 92 in the human ? chain. Both histidine residues are part of the F helix in each globin chain. The Bohr Effect The ability of hemoglobin to release oxygen, is affected by pH, CO2 and by the differences in the oxygen-rich environment of the lungs and the oxygen-poor environment of the tissues. The pH in the tissues is considerably lower (more acidic) than in the lungs. Protons are generated from the reaction between carbon dioxide and water to form bicarbonate: CO2 + H20 -----------------> HCO3- + H+ This increased acidity serves a twofold purpose. First, protons lower the affinity of hemoglobin ...

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