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 from 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, methyltransferase activation protein, and ATP. Optical and electron paramagnetic resonance spectroscopy studies were employed to determine the oxidation states and coordinating ligands of the corrinoids of MT 1 during the activation process. Purified MT 1 contained 1.7 corrinoids per 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 ligands, respectively. Reduction to the Co(II) level was accomplished by H 2 and hydrogenase. The cob(II)amide of MT 1 had the base coordinated 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-containing methyl group transferring proteins.