The interaction of heme-hemopexin with CO

The equilibria and kinetics of the reaction of heme-hemopexin with CO were studied. A stoichiometry of one CO/heme was determined, and the affinity of heme-hemopexin for CO was found to be pH-dependent. At pH 8.0, the affinity constant was 4.5 X 10(5) M-1 compared with 4 X 10(6) M-1 at pH 6.1. The kinetics of CO binding were also pH-dependent. A biphasic reaction at neutral pH could be resolved into a faster phase (kon = 2.2 X 10(3) M-1 s-1) solely found at pH 6.0, and a slower phase (kon = 2.0 X 10(2) M-1 s-1) solely found at pH 8.0. The dissociation reaction on the other hand was found to be independent of pH in the range examined (koff = 5 X 10(-4) s-1).