Spectroscopic and enzymatic evidence for membrane-bound electron transport carriers and hydrogenase and their relation to cytochrome b function in Methanosarcina barkeri

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Abstract
Membranes prepared from Methanosarcina barkeri cultured on acetate were examined for electron carriers using electron paramagnetic resonance (EPR) and optical spectroscopy. EPR analysis of membrane suspensions demonstrated multiple iron-sulfur centers of the 4Fe-4S type, a hihg-spin heme-like species and possibly rebredoxin. Optical spectroscopy demonstrated that a b-type cytochrome was reduced by molecular hydrogen and oxidized by methyl coenzyme M. A membrane-bound hydrogenase activity (14 μM · min−1 (mg protein)−1) was detected. This suggests a putative role for cytochrome b and hydrogenase in electron transfer and methyl-group reduction during aceticlastic methanogenesis.