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 4.5x10⁵ M^-1 compared with 4x10⁶ 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 (k(on)=2.2x10³ M^-1s^-1) solely found at pH 6.0, and a slower phase (k(on)=2.0x10² M^-1s^-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 (k(off)=5x10^-4s^-1).