Rate constants for O2 and CO binding to the α and β subunits within the R and T states of human hemoglobin

Abstract

Despite a large amount of work over the past 30 years, there is still no universal agreement on the differential reactivities of the individual α and β subunits in human hemoglobin. To address this question systematically, we prepared a series of hybrid hemoglobins in which heme was replaced by chromium(III), manganese(III), nickel(II), and magnesium(II) protoporphyrin IXs in either the α or β subunits to produce α2(M)β2(Fe)1 and α2(Fe)β2(M) tetramers. None of the abnormal metal complexes react with dioxygen or carbon monoxide. The O2 affinities of the resultant hemoglobins vary from 3 μM-1 (Cr(III)/Fe(II) hybrids) to 0.003 μM-1 (Mg(H)/Fe(II) hybrids), covering the full range expected for the various high (R) and low (T) affinity quaternary conformations, respectively, of human hemoglobin A0. The α and β subunits in hemoglobin have similar O2 affinities in both quaternary states, despite the fact that the R to T transition causes significantly different structural changes in the α and β heme pockets. This functional equivalence almost certainly evolved to maintain high n values for efficient O2 transport.