Cellular and Molecular Life Sciences CMLS

, Volume 57, Issue 1, pp 106–127 | Cite as

Radical catalysis of B12 enzymes: structure, mechanism, inactivation, and reactivation of diol and glycerol dehydratases

  • T. Toraya


Enzymatic radical catalysis is defined as a mechanism of catalysis by which enzymes catalyze chemically difficult reactions by utilizing the high reactivity of free radicals. Adenosylcobalamin (coenzyme B12) serves as a cofactor for enzymatic radical reactions. The recent structural analysis of adenosylcobalamin-dependent diol dehydratase revealed that the substrate 1,2-propanediol and an essential potassium ion are located inside a (β/α)8 barrel. Two hydroxyl groups of the substrate coordinate directly to the potassium ion which binds to the negatively charged inner part of the cavity. Cobalamin bound in the base-on mode covers the cavity to isolate the active site from solvent. Based on the three-dimensional structure and theoretical calculations, a new mechanism for diol dehydratase is proposed in which the potassium ion plays a direct role in the catalysis. The mechanisms for generation of a catalytic radical by homolysis of the coenzyme Co-C bond and for protection of radical intermediates from undesired side reactions during catalysis are discussed based on the structure. The reactivating factors for diol and glycerol dehydratases have been identified. These factors are a new type of molecular chaperone which participate in reactivation of the inactivated holoenzymes by mediating ATP-dependent exchange of the modified coenzyme for free intact coenzyme.

Key words. Coenzyme B12; adenosycobalamin; diol dehydratase; glycerol dehydratase; enzymatic radical catalysis; enzyme structure and mechanism; mechanism-based inactivation; reactivating factor 


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Copyright information

© Birkhäuser Verlag Basel, 2000

Authors and Affiliations

  • T. Toraya
    • 1
  1. 1.Department of Bioscience and Biotechnology, Faculty of Engineering, Okayama University, Tsushima-Naka, Okayama 700-8530 (Japan), Fax +81 86 251 8264, e-mail:

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