ELECTRON-PARAMAGNETIC-RESONANCE SPECTROSCOPIC AND ELECTROCHEMICAL CHARACTERIZATION OF THE PARTIALLY PURIFIED N-5-METHYLTETRAHYDROMETHANOPTERIN-COENZYME-M METHYLTRANSFERASE FROM METHANOSARCINA-MAZEI GO1

The N-5-methyltetrahydromethanopterin:coenzyme M methyltransferase is a membrane-bound cobalamin containing protein of Methanosarcina mazei Go1 that couples the methylation of coenzyme M by methyltetrahydrosarc inopterin to the translocation of Na+ across the cell membrane (B. Bec her, V. Muller, and G. Gottschalk, J, Bacteriol. 174:7656-7660, 1992). We have partially purified this enzyme and shown that, in addition to the cobamide, at least one iron-sulfur cluster is essential for the t ransmethylation reaction. The membrane fraction or the partly purified protein contains a ''base-on'' cobamide with a standard reduction pot ential (E(0)') for the Co-2+/1+ couple of -426 mV. The iron-sulfur clu ster appears to be a [4Fe-4S](2+/1+) type with an E(0)' value of -215 mV. We have determined the methyltransferase activity at various contr olled redox potentials and demonstrated that the enzyme activity is ac tivated by a one-electron reduction with half-maximum activity occurri ng at -235 mV in the presence of ATP and -450 mV in its absence. No ac tivation was observed when ATP was replaced by other nucleoside tripho sphates or nonhydrolyzable ATP analogs.