The Heme Complex of Hmu O, a Bacterial Heme Degradation Enzyme from Corynebacterium diphtheriae STRUCTURE OF THE CATALYTIC SITE

Abstract Hmu O, a heme degradation enzyme inCorynebacterium diphtheriae, forms a stoichiometric complex with iron protoporphyrin IX and catalyzes the oxygen-dependent conversion of hemin to biliverdin, carbon monoxide, and free iron. Using a multitude of spectroscopic techniques, we have determined the axial ligand coordination of the heme-Hmu O complex. The ferric complex shows a pH-dependent reversible transition between a water-bound hexacoordinate high spin neutral pH form and an alkaline form, having high spin and low spin states, with a pK a of 9. 1H NMR, EPR, and resonance Raman of the heme-Hmu O complex establish that a neutral imidazole of a histidine residue is the proximal ligand of the complex, similar to mammalian heme oxygenase. EPR of the deoxy cobalt porphyrin IX-Hmu O complex confirms this proximal histidine coordination. Oxy cobalt-Hmu O EPR reveals a hydrogen-bonding interaction between the O2and an exchangeable proton in the Hmu O distal pocket and two distinct orientations for the bound O2. Mammalian heme oxygenase has only one O2 orientation. This difference and the mixed spin states at alkaline pH indicate structural differences in the distal environment between Hmu O and its mammalian counterpart.