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An Estimation of the First Binding Constant of O2 to Human Hemoglobin A

1978; Wiley; Volume: 87; Issue: 1 Linguagem: Inglês

10.1111/j.1432-1033.1978.tb12353.x

1432-1033

C Poyart, E Bursaux, B. Bohn,

Heme Oxygenase-1 and Carbon Monoxide

Optical difference spectrophotometry has been applied for measuring the early has been applied for measuting the early part of the O2 dissociation curve of hukman hemoglubin (HP); i.e., below 1% O2 saturation levels or log [y(1–y)] form -3.0 to -2.0, where y=fractional O2 saturation. Two matched cuvettes, sealed on glass tonometers of known volume, were filled with the same deoxygenated Hb solution (65 μM in tetramer) under atmospheric pressure of pure argon. The pO2 values were calculated after the addintion to the sample tonometer of accutate volumes of O2/argon mixtures the composition of which was varied by using a precise gas-micing pump. 4–5 points of O2 saturation below y=0.01 were thus measutred, which allowed for the calculatiojn of A1, the first association constant of O2 to the tetrametric Hb. We have compared the effects pf protons, chloride ion s, CO2 and bisphosphoglycerate (P2-glycerate) on the pH dependence (alkaline Bohr effect(of A1 and p50. At low chloride condcentration (0.005 M) the alkaline Bohr effect at A1 was identical to that measured at p50, indicatintg that exctly one-fourth of the Bohr protons were released at the first oxygenation step. A higher chloride concentration (0.15 M) A1 was decr e a sed and p50 was increased as was increased as was the Bohr effect/. CO2 addintion (pCO2 38 Torr, 5050 Pa, at I=0.15 M) lead to decreased A1 and increased p50 but to a large decrease of the alkjaline Bohr effect. The effect of CO2 was predominat at A1, indicating a large releage of carbamino adducts from the tetrameric Hb at the first step fo ligation. P2-glycerate decreased A1 adn increases p50 values. The alkaline Bohr effect was enhanced at p50 by P2-glycerate but lowered at A1. These results are compared to those already published by other workers. The vartiations of the pH dependence of A1 in the presence of effectors indicate that A1 should be consifdered as a phenomenological parameter refering to one of the posssible structures that the Hb tetramer can take in the unliganded state. They indicate also that the whole Hb molecule feels the first oxygenation step, which itsilf implies structural changes within the quanternary structure.