Mechanism of Enhancement by the Fe2+-Induced Heme Degradation Activity of PM0042 From Pasteurella multocida.

PM0042 from Pasteurella multocida is a heme-degrading enzyme homologous to HutZ from Vibrio cholerae, but uniquely contains a glycine-histidine (GH) repeat motif at its C-terminal region. Unlike HutZ, PM0042 exhibits a threefold increase in heme degradation activity upon Fe2+ addition, yet the underlying mechanism has remained unclear. Here, we investigated the effect of Fe2+ on the catalytic process of PM0042. Spectroscopic analyses revealed that Fe2 + binding does not induce structural changes at either distal or proximal heme sites. Kinetic studies of individual steps in the degradation pathway demonstrated that the reduction of ferric heme to ferrous heme is the rate-determining step and is markedly accelerated by Fe2+. Other steps, including O2 binding, verdoheme formation, and biliverdin production, were unaffected. Mutational analysis introducing a His-rich motif into HutZ confirmed that Fe2+ enhances activity when bound near the proximal site. These findings suggest that Fe2+ acts as an electron mediator rather than a structural modulator, facilitating electron transfer from reductant to heme iron. This study provides mechanistic insight into metal-assisted catalysis in bacterial heme degradation and highlights the functional role of GH repeat sequences in electron transfer.