METHANOL-COENZYME-M METHYLTRANSFERASE FROM METHANOSARCINA-BARKERI - ZINC DEPENDENCE AND THERMODYNAMICS OF THE METHANOL-COB(I)ALAMIN METHYLTRANSFERASE REACTION

In Methanosarcina barkeri, methanogenesis from methanol is initiated b y the formation of methyl-coenzyme M from methanol and coenzyme M. Thi s methyl transfer reaction is catalyzed by two enzymes, designated met hyltransferases 1 (MT1) and 2 (MT2). Transferase MT1, which is compose d of a 50-kDa subunit, MtaB, and a 27-kDa corrinoid-harbouring subunit , MtaC, has been shown recently to catalyze the methylation of free co b(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 c atalyzed by subunit MtaB overproduced in Escherichia coli. MtaB also c atalyzed the formation of methanol from methylcobalamin and H2O, the h ydrolysis being associated with a free-energy change Delta G degrees' of approximately +7.0 kJ/mol. MtaB was found to contain 1 mol zinc, an d its activity to be zinc dependent (pK(Zn2+) = 9.3). The zinc depende nce of the MT2 (MtaA)-catalyzed reaction is also described (pK(Zn2+) = 9.6).