The fluorescence of porphyringlobin is quenched on adding haemoglobin to its solutions. It is suggested that this result indicates the formation of hybrids (comprising a dimer of porphyringlobin and a dimer of haemoglobin) in which quenching occurs by energy transfer from the porphyrin to the haem groups of the protein. From an analysis of fluorescence quenching, dissociation constants were calculated for the hybrids of oxy- and deoxyhaemoglobin with the fast- and slow-moving porphyringlobin species isolated by chromatography on CM-Sephadex (Treffry & Ainsworth, 1974). The values obtained are: deoxyhaemoglobin–fast-moving porphyringlobin, 0.8X10−9m; deoxyhaemoglobin–slow-moving porphyringlobin, 5X10−10m; oxyhaemoglobin–fast-moving porphyringlobin, 0.8X10−6m; oxyhaemoglobin–slow-moving porphyringlobin, 1.2X10−7m. The rates of reactions of solutions of haemoglobin and porphyringlobin, containing hybrids, with the thiol reagent 4,4′-dithiodipyridine showed that the thiol groups of the hybrids deoxyhaemoglobin–fast-moving porphyringlobin and oxyhaemoglobin–slow-moving porphyringlobin react more slowly than expected on the basis of composition alone: this result indicates that the deoxy and slow-moving conformations are the more stable, imposing themselves partially on to the fast-moving or oxy dimer of the hybrid. Also the rate of the reaction of CO with deoxyhaemoglobin is decreased when slow-moving porphyringlobin is added to its solutions: this is reflected in a movement of the oxygen equilibrium curve of such a mixture to higher oxygen partial pressures. Similar experiments with deoxyhaemoglobin solutions containing fast-moving porphyringlobin, showed an initial increase in the rate of CO uptake. Correspondingly, the oxygen equilibrium curve of the mixture showed an increased affinity for oxygen. Approximate calculations to determine the oxygen equilibria of the hybrids indicate that a functional dimer retains co-operative characteristics even when the dimer accompanying it within the tetramer has the reacted conformation.
The Optimal Oxygen Equilibrium Curve: A Comparison Between Environmental Hypoxia and Anemia
