Metal Sensing by a Glycine–Histidine Repeat Sequence Regulates the Heme Degradation Activity of PM0042 from Pasteurella multocida

PM0042 protein from the Gram-negative bacterial pathogen Pasteurella multocida is homologous to the heme-degrading enzyme HutZ belonging to the pyridoxine-5-phosphate oxidase-like family. A characteristic feature of PM0042 is possession of a glycine-histidine (GH) repeat sequence at the C-terminal region. In this study, we examined the heme degradation ability of PM0042, with a particular focus on the role of the GH repeat sequence. PM0042 was expressed in Escherichia coli and successfully purified using a nickel (Ni2+)-affinity column without a histidine tag, suggesting that its GH motif facilitates binding to Ni2+. Reaction with ascorbic acid induced a significant decrease in the Soret band, suggesting the breakage of heme. While a Fe2+-ferrozine complex was not formed upon addition of ferrozine to the solution after the reaction, prior addition of metal ions to fill the metal binding site in the GH repeat sequence led to increased complex formation. In the presence of Fe2+, the heme degradation rate was accelerated ∼threefold, supporting the theory that Fe2+ binds the PM0042 protein (possibly at the GH repeat sequence) and enhances its heme degradation activity. In contrast to HutZ from Vibrio cholerae in which enzymatic activity is regulated by the protonation status of the heme proximal ligand, heme reduction is not the rate-determining step for PM0042. Rather, proton transfer to reduced oxyheme is affected, as established with the H2O/D2O isotope experiment. Based on the collective findings, the GH repeat sequence of PM0042 is proposed to function as a metal sensor that modulates iron uptake via the heme-degrading process in P. multocida.