hide
Free keywords:
conformational change, methanol metabolism, x-ray structure, zinc
Abstract:
Some methanogenic and acetogenic microorganisms have the catalytic capability to cleave heterolytically the CO--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 barkericomposed of the zinc-containing MtaB and the 5-hydroxybenzimi-dazolylcobamide-carrying MtaC subunits. Here we report the 2.5-Åcrystal 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 β-barrel end and ligates to two cysteinyl sulfurs (Cys-220and Cys-269) and one carboxylate oxygen (Glu-164). MtaC is structurally related to the cobalamin-binding domain of methio-nine synthase. Its corrinoid cofactor at the top of the Rossmann domain reaches deeply into the funnel of MtaB, defining a regionbetween zinc(II) and the corrinoid cobalt that must be the binding site for methanol. The active site geometry supports a SN2 reaction mechanism, in which the CO--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 onthe 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.
