Enzymatic properties of human hemalbumin.

The binding of hemin to the primary site of human serum albumin (HSA) has been reinvestigated using UV-Vis, CD and
NMR techniques. The major fraction of bound hemin contains a five-coordinated high-spin iron(III) center, but a minor
fraction of the metal appears to be in a six-coordinated, low-spin state, where a `distal' residue, possibly a second histidine
residue, completes the coordination sphere. The reduced, iron(II) form of the adduct contains six-coordinated low-spin heme.
The distal residue hinders the access to the iron(III) center of hemin^HSA to small anionic ligands like azide and cyanide and
destabilizes the binding of neutral diatomics like dioxygen and carbon monoxide to the iron(II) form. In spite of these
limitations, the hemin^HSA complex promotes hydrogen peroxide activation processes that bear the characteristics of
enzymatic reactions and may have biological relevance. The complex is in fact capable of catalyzing peroxidative reactions on
phenolic compounds related to tyrosine and hydrogen peroxide dismutation. Kinetic and mechanistic studies confirm that
the low efficiency with which peroxidative processes occur depends on the limited rate of the reaction between hydrogen
peroxide and the iron(III) center, to form the active species, and by the competitive peroxide degradation reaction.