Heme degradation as catalyzed by a recombinant bacterial heme oxygenase (Hmu O) from Corynebacterium diphtheriae

Hmu O, a heme degradation enzyme in the pathogen Corynebacterium diphtheria e, catalyzes the oxygen-dependent conversion of hemin to biliverdin, carbon monoxide, and free iron. A bacterial expression system using a synthetic g ene coding for the 215-amino acid, full-length Hmu O has been constructed. Expressed at very high levels in Escherichia coli BL21, the enzyme binds he min stoichiometrically to form a hexacoordinate high spin hemin-Hmu O compl ex. When ascorbic acid is used as the electron donor, Hmu O converts hemin to biliverdin with alpha-hydroxyhemin and verdoheme as intermediates. The o verall conversion rate to biliverdin is approximately 4-fold slower than th at by rat heme oxygenase (HO) isoform 1. Reaction of the hemin-Hmu O comple x with hydrogen peroxide yields a verdoheme species, the recovery of which is much less compared with rat HO-1. Reaction of the hemin complex with met a-chloroperbenzoic acid generates a ferryl oxo species. Thus, the catalytic intermediate species and the nature of the active form in the first oxygen ation step of Hmu O appear to be similar to those of the mammalian HO. Howe ver, the considerably slow catalytic rate and low level of verdoheme recove ry in the hydrogen peroxide reaction suggest that the active-site structure of Hmu O is different from that of its mammalian counterpart.