HEME-PROTEIN INTERACTIONS - THE BINDING OF HEME COMPLEXES TO SERUM-ALBUMIN

The binding of protohaemin, deuterohaemin, deuterohaemin-monohistidine, deuterohaemin-undecapeptide, and microperoxidase-8 to human (HSA) and bovine (BSA) serum albumins has been investigated by optical and circular dichroism spectroscopy. It has been found that in all cases binding of the haemins involves coordination of a histidine residue of the proteins to one of the iron axial coordination positions, independent of the eventual presence of another axial ligand. A special binding site for protohaemin and deuterohaemin is present in both HSA and BSA that can accommodate even the bulky side-chain substituents on the porphyrin ring. The HSA binding site is characterized by a high degree of immobilization of the protein environment surrounding the haemins, while the degree of conformational mobility in the BSA binding site is much higher. The two proteins probably share a common binding site for deuterohaemin-monohistidine and microperoxidase-8.