Methanol: Coenzyme M Methyltransferase from Methanosarcina Barkeri

In Methanosarcina barkeri , methanogenesis from methanol is initiated by the formation of methyl‐coenzyme M from methanol and coenzyme M. This methyl transfer reaction is catalyzed by two enzymes, designated methyltransferases 1 (MT1) and 2 (MT2). Transferase MT1, which is composed of a 50‐kDa subunit, MtaB, and a 27‐kDa corrinoid‐harbouring subunit, MtaC, has been shown recently to catalyze the methylation of free cob(I)alamin with methanol [Sauer, K., Harms, U. & Thauer, R. K. (1997) Eur. J. Biochem. 243 , 670–677]. We report here that this reaction is catalyzed by subunit MtaB overproduced in Escherichia coli. MtaB also catalyzed the formation of methanol from methylcobalamin and H 2 O, the hydrolysis being associated with a free‐energy change ΔG 0 ′ of approximately +7.0 kJ/mol. MtaB was found to contain 1 mol zinc, and its activity to be zinc dependent (p K Zn2+ = 9.3). The zinc dependence of the MT2 (MtaA)‐catalyzed reaction is also described (p K Zn2+ = 9.6).