Reactions of Ferrous Coproheme Decarboxylase (HemQ) with O2 and H2O2 Yield Ferric Heme b

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• 作者：Bennett R. Streit ; Arianna I. Celis ; Krista Shisler ; Kenton R. Rodgers ; Gudrun S. Lukat-Rodgers ; Jennifer L. DuBois
• 刊名：Biochemistry
• 出版年：2017
• 出版时间：January 10, 2017
• 年：2017
• 卷：56
• 期：1
• 页码：189-201
• 全文大小：635K
• ISSN：1520-4995

文摘

A recently discovered pathway for the biosynthesis of heme b ends in an unusual reaction catalyzed by coproheme decarboxylase (HemQ), where the Fe(II)-containing coproheme acts as both substrate and cofactor. Because both O<sub>2sub> and H<sub>2sub>O<sub>2sub> are available as cellular oxidants, pathways for the reaction involving either can be proposed. Analysis of reaction kinetics and products showed that, under aerobic conditions, the ferrous coproheme–decarboxylase complex is rapidly and selectively oxidized by O<sub>2sub> to the ferric state. The subsequent second-order reaction between the ferric complex and H<sub>2sub>O<sub>2sub> is slow, pH-dependent, and further decelerated by D<sub>2sub>O<sub>2sub> (average kinetic isotope effect of 2.2). The observation of rapid reactivity with peracetic acid suggested the possible involvement of Compound I (ferryl porphyrin cation radical), consistent with coproheme and harderoheme reduction potentials in the range of heme proteins that heterolytically cleave H<sub>2sub>O<sub>2sub>. Resonance Raman spectroscopy nonetheless indicated a remarkably weak Fe–His interaction; how the active site structure may support heterolytic H<sub>2sub>O<sub>2sub> cleavage is therefore unclear. From a cellular perspective, the use of H<sub>2sub>O<sub>2sub> as an oxidant in a catalase-positive organism is intriguing, as is the unusual generation of heme b in the Fe(III) rather than Fe(II) state as the end product of heme synthesis.