EPR properties of the Ni-Fe-C center in an enzyme complex with carbon monoxide dehydrogenase activity from acetate-grown Methanosarcina thermophila. Evidence that acetyl-CoA is a physiological substrate

Abstract

The carbon monoxide dehydrogenase complex from acetate-grown Methanosarcina thermophila was further studied by EPR spectroscopy. The as purified enzyme exhibited no paramagnetic species at 113 K; however, enzyme reduced with CO exhibited a complex EPR spectrum comprised of two paramagnetic species with g values of g1 = 2.089, g2 = 2.078, and g3 = 2.030 (signal I) and g1 = 2.057, g2 = 2.049, and g3 = 2.027 (signal II). Isotopic substitution with 61Ni, 57Fe, or 13CO resulted in broadening of the EPR spectra indicating a Ni-Fe-C spin-coupled complex. Pure signal II was obtained following treatment of the CO-reduced enzyme with acetyl-CoA but not by addition of acetyl phosphate or CoASH. Acetate-grown cells were highly enriched in acetate kinase (EC 2.7.2.1) and CoASH-dependent phosphotransacetylase (EC 2.3.1.8) activities. These results suggest acetyl-CoA is a physiological substrate for the carbon monoxide dehydrogenase complex synthesized in acetate-grown cells of M. thermophila.