A G-protein editor gates coenzyme B12 loading and is corrupted in methylmalonic aciduria.

Department of Biological Chemistry, University of Michigan Medical Center, Ann Arbor, MI 48109-5606, USA.
Proceedings of the National Academy of Sciences (Impact Factor: 9.81). 12/2009; 106(51):21567-72. DOI: 10.1073/pnas.0908106106
Source: PubMed

ABSTRACT 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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    ABSTRACT: Fidelity during cofactor assembly is essential for the proper functioning of metalloenzymes and is ensured by specific chaperones. MeaB, a G-protein chaperone for the coenzyme B12-dependent radical enzyme methylmalonyl-CoA mutase (MCM), uses the energy of GTP binding, hydrolysis or both to regulate cofactor loading into MCM, protect MCM from inactivation and rescue MCM that is inactivated during turnover. Typically, G proteins signal to client proteins using the conformationally mobile switch I and II loops. Crystallographic snapshots of MeaB reported herein reveal a new switch III element that has substantial conformational plasticity. Using alanine-scanning mutagenesis, we demonstrate that the switch III motif is critical for bidirectional signal transmission of the GTPase-activating protein activity of MCM and the chaperone functions of MeaB in the MeaB-MCM complex. Mutations in the switch III loop identified in patients corrupt this interprotein communication and lead to methylmalonic aciduria, an inborn error of metabolism.
    Nature Chemical Biology 07/2013; · 12.95 Impact Factor
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    ABSTRACT: The reactivity of the cobalt-carbon bond in cobalamins is the key to their chemical versatility, supporting both methyltransfer and isomerization reactions. During evolution of higher eukaryotes that utilize B12, the high-reactivity of the cofactor coupled with its low abundance, pressured development of an efficient system for uptake, assimilation and delivery of the cofactor to client B12-dependent enzymes. While most proteins suspected to be involved in B12 trafficking were discovered by 2009, the recent identification of a new protein reveals that the quest for elucidating the intracellular B12 highway is still far from complete. Herein, we review the biochemistry of cobalamin trafficking.
    Journal of Biological Chemistry 03/2013; · 4.65 Impact Factor
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May 21, 2014