Diol Dehydratases and Ethanolamine Ammonia Lyase

Diol dehydratase (DD) and glycerol dehydrates (GD) are isofunctional enzymes that catalyze the dehydration of glycerol, ethane-1,2-diol and propane-1,2-diol to 3-hydroxypropanal, acetaldehyde and propanal, respectively (other glycols can be dehydrated in an analogous fashion) [210] (see Fig. 19). DD has about a twofold preference for propane-1,2-diol to glycerol, whereas GD prefers glycerol to propane-1,2-diol as a substrate [178]. Conversion of glyc- 

1- diol-2-yl radicals then comes close to the CH3 group of 5 -deoxyadenosine and back-abstracts a hydrogen atom, producing 1,1-gem-diols and the adenosyl radical. The stereospecific dehydration of the 1,1-gem-diols to the final product propionaldehyde is catalyzed by Asp335 and Glul70 [ 178). 

Three crystal structures of DD [20,217,219] and two of GD [215,220] have been studied, confirming the base-on nature of the boimd corrinoid. Analysis of the crystal structure of DD reconstituted with the coenzyme B12 analogue 5-adeninyl-pentyl-cobalamin (53) [217] helped identify an adenine-binding pocket in the substrate-binding a-subunit. The observed mode of adenine binding allowed the inference of a build-up of strain in the bound adenosyl-cobamide cofactor and activation towards homolysis of the Co - C bond. 

Ethanolamine ammonia lyase (EAL) converts ethanolamine (2-hydroxy-ethylamine) to acetaldehyde, with the loss of ammonia. The cobamide bovmd to the enzyme of Clostridium sp. was identified as a pseudocoenzyme B12 (37) [221] but a range of other adenosyl-cobamides are accepted as cofactors. The active enzyme is multimeric, has an apparent molecular mass of about 560-600 kDa, but further studies are much less advanced with EAL than those of the AdoCbl-dependent enzymes described above. From ESR studies [222], and the fact that the cobalamin binding motif (Gly-X-X-His-X-Asp) is absent [223], EAL was concluded to be a base-on B -dependent 

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Diol dehydratase and ethanolamine ammonia lyase exhibit the largest overall tritium isotope effects that have been measured in Bi2-dependent enzymes [44, 45], the overall deuterium kinetic isotope effect is also substantial [10, 34, 45]. The observation of a deuterium isotope effect on the pre-steady-state formation of cob(ii)alamin in diol dehyratase [10] and in ethanolamine ammonia lyase [25] is consistent with kinetic coupling between the homolysis and H-transfer steps. 

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