A G-protein editor gates coenzyme B12 loading and is corrupted in methylmalonic aciduria. Proc Natl Acad Sci USA

Department of Biological Chemistry, University of Michigan Medical Center, Ann Arbor, MI 48109-5606, USA.
Proceedings of the National Academy of Sciences (Impact Factor: 9.67). 12/2009; 106(51):21567-72. DOI: 10.1073/pnas.0908106106
Source: PubMed


The mechanism by which docking fidelity is achieved for the multitude of cofactor-dependent enzymes is poorly understood. In this study, we demonstrate that delivery of coenzyme B(12) or 5'-deoxyadenosylcobalamin by adenosyltransferase to methylmalonyl-CoA mutase is gated by a small G protein, MeaB. While the GTP-binding energy is needed for the editing function; that is, to discriminate between active and inactive cofactor forms, the chemical energy of GTP hydrolysis is required for gating cofactor transfer. The G protein chaperone also exerts its editing function during turnover by using the binding energy of GTP to elicit release of inactive cofactor that is occasionally formed during the catalytic cycle of MCM. The physiological relevance of this mechanism is demonstrated by a patient mutation in methylmalonyl-CoA mutase that does not impair the activity of this enzyme per se but corrupts both the fidelity of the cofactor-loading process and the ejection of inactive cofactor that forms occasionally during catalysis. Consequently, cofactor in the incorrect oxidation state gains access to the mutase active site and is not released if generated during catalysis, leading, respectively, to assembly and accumulation of inactive enzyme and resulting in methylmalonic aciduria.

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    • "Finally, recent studies suggest that MeaB acts as a regulator of MCM cofactor binding and ejection, where the binding and hydrolysis of GTP by MeaB are important in the discrimination of MCM binding to AdoCbl versus cob(II)alamin, and promotes ejection of the latter, inactive cofactor from MCM (Ref. 130). These studies are summarised below in the proposed model for cobalamin processing. "
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    • "In addition, the same group has assigned the GTPase chaperone protein MeaB a key role in the fidelity of the AdoCbl loading process (Padovani & Banerjee 2009a). The data obtained by these authors is supported by the finding of a substitution in the human orthologue of MeaB in a patient with methylmalonic aciduria; the mutant MeaB fails to discriminate between active and non-active cofactors (Padovani & Banerjee 2009a). "
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