Harpers

PROTEINS: MYOGLOBIN & HEMOGLOBIN / 45Exhaled2CO 2 + 2H 2 O2H 2 CO 3CARBONICANHYDRASE2HCO 3 – + 2H +LUNGSHb • 4O 24O 22H + –+ 2HCO 34O Hb • 2H +2(buffer)2H 2 CO 3Generated bythe Krebs cyclePERIPHERALTISSUES2CO 2 + 2H 2 OCARBONICANHYDRASEFigure 6–9. The Bohr effect. Carbon dioxide generatedin peripheral tissues combines with water to formcarbonic acid, which dissociates into protons and bicarbonateions. Deoxyhemoglobin acts as a buffer bybinding protons and delivering them to the lungs. Inthe lungs, the uptake of oxygen by hemoglobin releasesprotons that combine with bicarbonate ion,forming carbonic acid, which when dehydrated by carbonicanhydrase becomes carbon dioxide, which thenis exhaled.The hemoglobin tetramer binds one molecule ofBPG in the central cavity formed by its four subunits.However, the space between the H helices of the βchains lining the cavity is sufficiently wide to accommodateBPG only when hemoglobin is in the T state.BPG forms salt bridges with the terminal amino groupsof both β chains via Val NA1 and with Lys EF6 andHis H21 (Figure 6–10). BPG therefore stabilizes deoxygenated(T state) hemoglobin by forming additionalsalt bridges that must be broken prior to conversion tothe R state.Residue H21 of the γ subunit of fetal hemoglobin(HbF) is Ser rather than His. Since Ser cannot form asalt bridge, BPG binds more weakly to HbF than toHbA. The lower stabilization afforded to the T state byBPG accounts for HbF having a higher affinity for O 2than HbA.His H21hemoglobin. Conversion to the oxygenated R statebreaks salt bridges involving β-chain residue His 146.The subsequent dissociation of protons from His 146drives the conversion of bicarbonate to carbonic acid(Figure 6–9). Upon the release of O 2 , the T structureand its salt bridges re-form. This conformationalchange increases the pK a of the β-chain His 146residues, which bind protons. By facilitating the re-formationof salt bridges, an increase in proton concentrationenhances the release of O 2 from oxygenated (Rstate) hemoglobin. Conversely, an increase in PO 2 promotesproton release.2,3-Bisphosphoglycerate (BPG) Stabilizesthe T Structure of HemoglobinA low PO 2 in peripheral tissues promotes the synthesisin erythrocytes of 2,3-bisphosphoglycerate (BPG) fromthe glycolytic intermediate 1,3-bisphosphoglycerate.Lys EF6Val NA1His H21BPGLys EF6α-NH 3+Val NA1Figure 6–10. Mode of binding of 2,3-bisphosphoglycerateto human deoxyhemoglobin. BPG interactswith three positively charged groups on each β chain.(Based on Arnone A: X-ray diffraction study of binding of2,3-diphosphoglycerate to human deoxyhemoglobin. Nature1972;237:146. Reproduced with permission.)