Hmu O, a heme degradation enzyme in the pathogen Corynebacterium diphtheriae, catalyzes the oxygen-dependent conversion of hemin to biliverdin, carbon monoxide, and free iron. A bacterial expression system using a synthetic gene 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 hemin stoichiometrically to form a hexacoordinate high spin hemin-Hmu O complex. When ascorbic acid is used as the electron donor, Hmu O converts hemin to biliverdin with alpha-hydroxyhemin and verdoheme as intermediates. The overall conversion rate to biliverdin is approximately 4-fold slower than that by rat heme oxygenase (HO) isoform 1. Reaction of the hemin-Hmu O complex 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 meta-chloroperbenzoic acid generates a ferryl oxo species. Thus, the catalytic intermediate species and the nature of the active form in the first oxygenation step of Hmu O appear to be similar to those of the mammalian HO. However, the considerably slow catalytic rate and low level of verdoheme recovery in the hydrogen peroxide reaction suggest that the active-site structure of Hmu O is different from that of its mammalian counterpart.