Coenzyme B-12-dependent enzymatic dehydration of 1,2-diols: simple reaction, complex mechanisms!

The conversion of glycerol to acrolein is an undesirable event in whisky production, caused by infection of the broth with Klebsiella pneumoniae. This organism uses glycerol dehydratase to transform glycerol into 3-hydroxypropanal, which affords acrolein on distillation. The enzyme requires adenosylcobalamin (coenzyme B-12) as cofactor and a monovalent cation (e.g. K+). Diol dehydratase is a similar enzyme that converts 1,2-diols (C-2-C-4) including glycerol into an aldehyde and water. The subtle stereochemical features of these enzymes are exemplified by propane-1,2-diol: both enantiomers are substrates but different hydrogen and oxygen atoms are abstracted. The mechanism of action of the dehydratases has been elucidated by protein crystallography and ab initio molecular orbital calculations, aided by stereochemical and model studies. The 5'-deoxyadenosyl (adenosyl) radical from homolysis of the coenzyme's Co-C sigma- bond abstracts a specific hydrogen atom from C-1 of diol substrate giving a substrate radical that rearranges to a product radical by 1,2 shift of hydroxyl from C-2 to C-1. The rearrangement mechanism involves an acid-base 'push-pull' in which migration of OH is facilitated by partial protonation by His alpha 143, synergistically assisted by partial deprotonation of the non-migrating (C-1) OH by the carboxylate of Glu alpha 170. The active site K+ ion holds the two hydroxyl groups in the correct conformation, whilst not significantly contributing to catalysis. Recently, diol dehydratases not dependent on coenzyme B,, have been discovered. These enzymes utilize the same kind of diol radical chemistry as the coenzyme B-12-dependent enzymes and they also use the adenosyl radical as initiator, but this is generated from S-adenosylmethionine.