ACTIVATION MECHANISM OF METHANOL-5-HYDROXYBENZIMIDAZOLYLCOBAMIDE METHYLTRANSFERASE FROM METHANOSARCINA-BARKERI

Methanol:5-hydroxybenzimidazolylcobamide methyltransferase (MT(1)) is the first of two enzymes involved in the transmethylation reaction fro m methanol to 2-mercaptoethanesulfonic acid in Methanosarcina barkeri. MT(1) only binds the methyl group of methanol when the cobalt atom of its corrinoid prosthetic groups is present in the highly reduced Co(I ) state. Formation of this redox state requires H-2, hydrogenase, meth yltransferase activation protein, and ATP. Optical and electron parama gnetic resonance spectroscopy studies were employed to determine the o xidation states and coordinating ligands of the corrinoids of MT(1) du ring the activation process. Purified MT(1) contained 1.7 corrinoids p er enzyme with cobalt in the fully oxidized Co(III) state. Water and N -3 of the 5-hydroxybenzimidazolyl base served as the upper and lower l igands, respectively. Reduction to the Co(II) level was accomplished b y H-2 and hydrogenase. The Co(II)amide of MT(1) had the base coordinat ed at this stage. Subsequent addition of methyltransferase activation protein and ATP resulted in the formation of base-uncoordinated Co(II) MT(1). The activation mechanism is discussed within the context of a proposed model and compared to those described for other corrinoid-con taining methyl group transferring proteins.