Diol dehydratase, mechanism

Towards the unification of coenzyme B12-dependent diol dehydratase stereochemical and model studies the bound radical mechanism. R. G. Finke, D. A. Schiraldi and B. J. Mayer, Coord. Chem. Rev., 1984, 54,1-22 (40). 

Yamanishi, M., Yamada, S., Ishida, A., Yamauchi, J., and Toraya, T., 1998a, EPR spectroscopic evidence for the mechanism-based inactivation of adenosylcobalamin-dependent diol dehydratase by coenzyme analogs, J. Biochem. (Tokyo) 124 5989601. 

The first common step in AdoCbl-dependent readions is homolytic cleavage of the cobalt-carbon bond to generate a radical pair, cob(ii)alamin and the carbon-centered dAdo radical (Scheme 19.3). This reaction experiences a 10 -fold rate enhancement in B12 enzymes [14, 15] in the presence of substrate, and the mechanism for this rate acceleration has been the subject of extensive scrutiny. Thus, in methylmalonyl-CoA mutase and in glutamate mutase, little if any destabilization of the cobalt-carbon bond is observed in the reactant state, as revealed by resonance Raman spectroscopy [16, 17], and the intrinsic substrate binding is utilized to labilize the bond. In contrast, approximately half of the destabilization of the cobalt-carbon bond in diol dehydratase is expressed in the reactant state. This re-... 

Mori, K., Toraya, T. (1999) Mechanism of reactivation of coenzyme B12-dependent diol dehydratase by a molecular chaperone-like reactivating factor. Biochemistry 38, 13170-13178. 

Radom, L. (2001) Understanding the mechanism of Bi2-dependent diol dehydratase a synergistic retro-push-pull proposal,/. Am. Chem. Soc. 123, 1664-1675. 

Eda, M., Kamachi, T., Yoshizawa, K., Toraya, T. (2002) Theoretical study on the mechanism of catalysis of coenzyme Bi2-dependent diol dehydratase, Bull. Chem. Soc. Inn. 75, 1469-1481. 

In turning our attention to vitamin B12 and its coenzymes, it should be recognized that present diflFerences in opinions and in the interpretations oflFered on the mechanism of coenzyme Bi2-catalyzed processes are largely a consequence of the complexity of these reactions and the difficulty of interpreting the results of enzymological studies in the absence of empirical data derived from model experiments. The situation is best exemplified by first considering an early mechanism postulated for the function of coenzyme B12 in diol dehydratase, not supported by model studies. Ref. 6 contains a detailed discussion. 

Different decomposition mechanisms for the products of the reaction between Co Mea[14]-diene-N4, Co L, and carbon-centred aliphatic alcohol radicals suggest an explanation for the inhibition of diol dehydratase by NjO. 

Fig. 15. Proposed mechanism of diol dehydratases involving H-atom abstraction (step a), radical rearrangement (step b), H-atom back transfer (step c), and elimination of water (step d) (75).

From extensive analysis of recombinant proteins, and the crystal structure of A. thaliana protein, detailed reaction mechanisms have been proposed. The ANS reaction likely proceeds via stereospecific hydroxylation of the leucoanthocyanidin (flavan-3,4-cA-diol) at the C-3 to give a flavan-3,3,4-triol, which spontaneously 2,3-dehydrates and isomerizes to 2-flaven-3,4-diol, which then spontaneously isomerizes to a thermodynamically more stable anthocyanidin pseudobase, 3-flaven-2,3-diol (Figure 3.2). The formation of 3-flaven-2,3-diol via the 2-flaven-3,4-diol was previously hypothesized by Heller and Forkmann. The reaction sequence, and the subsequent formation of the anthocyanidin 3-D-glycoside, does not require activity of a separate dehydratase, which was once postulated. Recombinant ANS and uridine diphosphate (UDP)-glucose flavonoid 3-D-glucosyltransferase (F3GT, sometimes... 

Toraya, T. (2000) Radical catalysis of B12 enzymes structure, mechanism, inactivation, and reactivation of diol and glycerol dehydratases. Cell. Mol. Life Sci., 57 (1), 106-127. 

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