Structure-Based Fine Mechanism of Action of a Vitamin B_<12> Enzyme

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  • TORAYA Tetsuo
    Department of Bioscience and Biotechnology, Faculty of Engineering, Okayama University.

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Other Title
  • (3)ビタミンB_<12>酵素の立体構造と精密触媒機構(シンポジウム : 「B群ビタミンはどのように働くか」)
  • ビタミンB12酵素の立体構造と精密触媒機構
  • ビタミン B12 コウソ ノ リッタイ コウゾウ ト セイミツ ショクバイ キコウ

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Abstract

Certain enzymes utilize the high reactivity of radicals to catalyze chemically difficult reactions. Coenzyme B_<12> serves as a cofactor for enzymatic radical reactions. The three-dimensional structures of coenzyme B_<12>-dependent diol dehydratase and glycerol dehydratase were determined by X-ray crystallography. The structure-based fine mechanism of action of diol dehydratase was studied to establish the general mechanism for B_<12> enzymes as well as radical enzymes. The steric strain model was proposed for the coenzyme cobalt-carbon bond homolysis. The ribosyl rotation model well explained the distance problem and the stereospecificity in hydrogen abstraction. The substrate-induced conformational change of the enzyme revealed the substrate triggering mechanism for the catalytic radical formation. Theoretical calculations as well as mutational studies suggested that the hydroxyl group migrates by the concerted pathway through a three-membered cyclic transition state which is stabilized by active-site amino acid residues. A refined catalytic mechanism for diol dehydratase is proposed here.

Journal

  • VITAMINS

    VITAMINS 77 (5-6), 297-312, 2003

    THE VITAMIN SOCIETY OF JAPAN

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