The -hydrogenase SoHydAB of Shewanella oneidensis - An Atypical M2(D)-type Enzyme that Drives Formate Oxidation
The gamma-proteobacterium Shewanella oneidensis harbours one [NiFe]- and one [FeFe]-hydrogenase. This paper presents the first biochemical characterisation of SoHydAB, a M2(D)-type [FeFe]-hydrogenase consisting of a small subunit featuring a twin-arginine-targeting motif (TAT) and a large subunit harbouring the active site cofactor and two accessory iron-sulfur clusters. There are three main reasons to investigate this enzyme. Firstly, S. oneidensis is a highly studied, metabolically complex, and widely utilised organism, especially in the context of wastewater treatment, CO2 conversion, and microbial fuel cells, including H2-producing systems. Advanced understanding of its hydrogen metabolism can improve biotechnological applications. Secondly, S. oneidensis has been a mainstay of hydrogenase research for decades as its enzymes HydE, F and G are the most widely studied and utilised maturases - enzymes assembling the active site cofactor of [FeFe]-hydrogenases. Lastly, while enzymes like NdHydAB from Nitratidesulfovibrio vulgaris (formerly DdHydAB, Desulfovibrio vulgaris) with high similarity to SoHydAB have been characterised extensively in the past, the metabolic context of the enzyme in Shewanella is different from Nitratidesulfovibrio. While NdHydAB receives electrons from ferredoxin, SoHydAB accepts electrons from the menaquinone pool of the outer membrane while being a soluble enzyme in the periplasm. This is facilitated by interaction with a membrane-integrated cytochrome b transcribed from the same operon. Oxidation of the menaquinone pool drives oxidation of formate toward CO2 via formate dehydrogenase. This study therefore lays the groundwork for understanding H2-dependent CO2 turnover, a new type of metabolic niche for [FeFe]-hydrogenases, while simultaneously improving our understanding of a widely used platform organism.