Diol dehydrase mechanism

Mechanism (1), first suggested almost three decades ago [150], continues to be the most invoked explanation of the role of the enzyme in promoting homolysis. Cleavage of the C—Co bond of sterically hindered alkylcobalamins (e.g., neopentylcobalamin) was markedly increased by diol dehydrase [72], Such cobal-amins do not function as coenzymes but convert to enzyme-bound hydroxocobal-amin in stoichiometric first-order reactions. The strong competitive inhibition by AdoB 12 indicates that labilization occurs at the active site of the enzyme and is suggested to be caused by a steric distortion of the corrin ring. 

Enzymological Properties and Mechanism of Action of Diol Dehydrase... 

Mechanism of Action. Diol dehydrase is one of the enzymes whose mechanism of action has been studied extensively (13, 14). It has been established that in general the rearrangement reactions catalyzed by adenosylcobalamin-requiring enzymes involve the migration of a hydrogen from one carbon atom of substrate to an adjacent carbon atom in exchange for a group X that moves in the opposite direction. In the case of the diol-dehydrase reaction, X represents a... 

Scheme 1. Mechanism of action of diol dehydrase RCH2— = adeno-syl [Co] = cobalamin SH = substrate PH = product...

FIGURE 19. Mechanism for the rearrangement of propane-1,2-diol radical by diol dehydrase in which the migration of the hydroxyl group if facilitated through electrostatic catalysis by a tightly-bound potassium ion at the active site (Shibata et al., 1999). 

For diol dehydrase, a radical mechanism has been implicated from the observation that glycolaldehyde deactivates the enzyme. Theory suggests that this results because the radical H(HO)C CH(=0) formed by H atom abstraction is so stable that it will not abstract an H atom from HjCAdo to allow reformation of Co—CHjAdo. 

The small value of k 10" s" for the coenzyme is noteworthy in comparison to values of 2xl0 s" estimated for the rate-determining step in the diol dehydrase and ethanolamine ammonia lyase systems. This shows one of the problems in using the radical mechanism. This difficulty usually is explained by assuming that the enzyme somehow distorts the coenzyme so that the homolysis is about 15 kcal mol" more favorable in the holoenzyme. This distortion may destabilize the reactant and/or stabilize the transition state in order to hasten homolysis in the holoenzyme. For some time, it was thought that the axial base might promote these distortions, but Hay and Finke found that... 

However, this mode of decomposition does not involve the 1,1-diol intermediate shown to be required by the elegant stereochemical and labeling studies of Arigoni and co-workers on diol dehydrase. Finke et al. proposed a bound radical mechanism for the enzyme in which formation of the 1,1-diol assists in binding the radical to the enzyme by hydrogen bonding. The bound radical mechanism involves rehydration of the radical intermediate in reaction (8.21) to give the 1,1-diol. 

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