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Volume 271, Number 37, Issue of September 13, 1996 pp. 22346-22351
©1996 by The American Society for Biochemistry and Molecular Biology, Inc.

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Activation Mechanism of Methanol:5-Hydroxybenzimidazolylcobamide Methyltransferase from Methanosarcina barkeri

(Received for publication, December 29, 1995, and in revised form, June 7, 1996)

Piet J. H. Daas , Wilfred R. Hagen ^§ , Jan T. Keltjens , Chris van der Drift and Godfried D. Vogels

From the  Department of Microbiology, Faculty of Science, University of Nijmegen, Toernooiveld, NL-6525 ED Nijmegen, The Netherlands and the ^§ Department of Biochemistry, Agricultural University, NL-6703 HA Wageningen, The Netherlands

Methanol:5-hydroxybenzimidazolylcobamide methyltransferase (MT₁) is the first of two enzymes involved in the transmethylation reaction from methanol to 2-mercaptoethanesulfonic acid in Methanosarcina barkeri. MT₁ 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 H₂, 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 MT₁ during the activation process. Purified MT₁ 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 H₂ and hydrogenase. The cob(II)amide of MT₁ had the base coordinated at this stage. Subsequent addition of methyltransferase activation protein and ATP resulted in the formation of base-uncoordinated Co(II) MT₁. 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.

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