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

Despite a large amount of work over the past 30 years, there is still no universal agreement on the differential reactivities of the individual alpha and beta 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 alpha or beta subunits to produce alpha(2)(M)beta(2)(Fe)(1) and alpha(2)(Fe)beta(2)(M) tetramers. None of the abnormal metal complexes react with dioxygen or carbon monoxide. The O-2 affinities of the resultant hemoglobins vary from 3 mu M-1 (Cr(III)/Fe(II) hybrids) to 0.003 mu M-1 (Mg(II)/Fe(II) hybrids), covering the full range expected for the various high (R) and low (T) affinity quaternary conformations, respectively, of human hemoglobin A(0). The alpha and beta subunits in hemoglobin have similar O-2 affinities in both quaternary states, despite the fact that the R to T transition causes significantly different structural changes in the alpha and beta heme pockets, This functional equivalence almost certainly evolved to maintain high n values for efficient O-2 transport.