Proton abstraction enzyme-catalysed

The stereochemistry of /1-elimination reactions catalysed by D-galactonate dehydratase (GalD) and D-glucarate dehydratase (GlucD) enzymes is apparently not dictated by the pKas of the 7-protons of the carboxylate anion substrates.74 It had been observed previously that enzyme-catalysed dehydration initiated by abstraction of the a-proton (P a > 29) from a carboxylate anion substrate usually proceeds via anti elimination, whereas syn elimination occurs when the proton is a- to an aldehyde, ketone, or thioester and correspondingly more acidic (pKa < 25). 

The TIM-catalysed isomerization of DHAP to GAP proceeds via base-catalysed proton abstraction and formation of an enzyme-bound enediol or diolate intermediate. The organic chemistry analog of this reaction is the Lobry de Bruyn-Alberda van Ekenstein transformation, exemplified by the production of fructose when glucose is heated in base. 

During the last few years, more efficient reagents for transforming alkynes into allenes have been developed and even an enzyme catalysed reaction has been investigated The main mechanistic development has been the solution to the problem of whether rearrangements of an alkyne into an allene (and vice versa) involve an intermediate carbanion, or if the reaction can occur via a concerted mechanism in which a proton is donated from the solvent synchronously with the abstraction of a proton from the substrate by a base. 

Schlippe, Y.V.G. and Hedstrom, L. (2005) A twisted base The role of arginine in enzyme-catalysed proton abstractions. Archives of Biochemistry and Biophysics, 433, 266-278. 

Product Schiff-Base. Conversion of the substrate Schiff-base to product Schiff-base involves removal of a hydrogen from the a-carbon of the substrate. This chemically-difficult step is achieved in the enzyme by a base catalysed abstraction of a proton and shows interesting stereochemical variation amongst amine oxidases from different sources (Seaman and Palcic, 1992 Coleman et al., 1989, 1991 Shah et al., 1993). When benzylamine is the substrate, it is always the pro-S hydrogen which is removed but when tyramine or dopamine are the substrates, removal of hydrogen from C-1 can be pro-S, pro-R or random depending upon the source of the amine oxidase (see Table 3). 

Yet another example of a family of Mg -dependent enzymes is the enolase superfamily which catalyse a series of mechanistically diverse and different overall reactions. However, they all share a partial reaction in which an active site base of the enzyme abstracts the a-proton of a carboxylate substrate to generate an enolate anion intermediate... 

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