EPR OBSERVATION OF CARBON-MONOXIDE DEHYDROGENASE, METHYLREDUCTASE AND CORRINOID IN INTACT METHANOSARCINA-BARKERI DURING METHANOGENESIS FROM ACETATE

Carbon monoxide dehydrogenase, corrinoid, and methylreductase can be observed with EPR spectroscopy during active methanogenesis from acetate in whole cells of Methanosarcina barkeri. During methanogenesis, center 2 of carbon monoxide dehydrogenase (g = 2.01, 1.91, 1.76) shows similar changes to those seen in the isolated enzyme upon addition of CO, with the appearance of a new feature at g = 1.73. This indicates that the cleavage of acetate yields a moiety that carbon monoxide dehydrogenase recognizes as CO. Cyanide rendered center 2 of carbon monoxide dehydrogenase EPR-silent, suggesting that this potent inhibitor of methanogenesis from acetate acts at the level of this enzyme. Cyanide also induced formation of two very fast relaxing centers with overlapping rhombic signals in the iron-sulfur region of the spectrum. Transient electron flow through iron-sulfur clusters could be correlated with methanogenesis, and could be prevented by iodopropane or cyanide. Cells before and after methanogenesis possessed a putative nickel signal at g = 2.28, 2.27 and 2.18. During methane production this signal was replaced by an axial signal (g = 2.24, 2.05). These signals are similar to those reported for methylreductase from Methanobacterium. EPR-detectable corrinoid underwent no significant change during methanogenesis and was unaffected by the presence of either cyanide or iodopropane. © 1990.