2i2x
From Proteopedia
Crystal structure of methanol:cobalamin methyltransferase complex MtaBC from Methanosarcina barkeri
Overview
Some methanogenic and acetogenic microorganisms have the catalytic capability to cleave heterolytically the C O bond of methanol. To obtain insight into the elusive enzymatic mechanism of this challenging chemical reaction we have investigated the methanol-activating MtaBC complex from Methanosarcina barkeri composed of the zinc-containing MtaB and the 5-hydroxybenzimidazolylcobamide-carrying MtaC subunits. Here we report the 2.5-A crystal structure of this complex organized as a (MtaBC)(2) heterotetramer. MtaB folds as a TIM barrel and contains a novel zinc-binding motif. Zinc(II) lies at the bottom of a funnel formed at the C-terminal beta-barrel end and ligates to two cysteinyl sulfurs (Cys-220 and Cys-269) and one carboxylate oxygen (Glu-164). MtaC is structurally related to the cobalamin-binding domain of methionine synthase. Its corrinoid cofactor at the top of the Rossmann domain reaches deeply into the funnel of MtaB, defining a region between zinc(II) and the corrinoid cobalt that must be the binding site for methanol. The active site geometry supports a S(N)2 reaction mechanism, in which the C O bond in methanol is activated by the strong electrophile zinc(II) and cleaved because of an attack of the supernucleophile cob(I)amide. The environment of zinc(II) is characterized by an acidic cluster that increases the charge density on the zinc(II), polarizes methanol, and disfavors deprotonation of the methanol hydroxyl group. Implications of the MtaBC structure for the second step of the reaction, in which the methyl group is transferred to coenzyme M, are discussed.
About this Structure
2I2X is a Protein complex structure of sequences from Methanosarcina barkeri. Full crystallographic information is available from OCA.
Reference
Insight into the mechanism of biological methanol activation based on the crystal structure of the methanol-cobalamin methyltransferase complex., Hagemeier CH, Krer M, Thauer RK, Warkentin E, Ermler U, Proc Natl Acad Sci U S A. 2006 Dec 12;103(50):18917-22. Epub 2006 Dec 1. PMID:17142327 Page seeded by OCA on Wed Apr 30 13:25:39 2008
