The secondary α-deuterium isotope effect at the site of the enzyme-catalyzed cis-trans isomerization of maleylacetone. Mechanistic implications

An equal mixture of monodeuterated isotopomers of maleylacetone, viz., 4-hydroxy-6-oxo-2,4-heptadienoic-2-d1 acid (1) and 4-hydroxy-6-oxo-2,4-heptadienoic-3-d1 acid (2) was synthesized and subjected to catalyzed cis-trans isomerization by the maleylacetoacetate cis-trans isomerase-glutathione system. These studies were designed to probe the detailed mechanism of enzyme-catalyzed cis-trans isomerization about the C-2—C-3 bond of the substrate. In the competitive reaction, 2 accumulates with respect to 1 at high extent of reaction. 1H NMR analysis indicates that 1 reacts 15% faster than 2 in line with a nucleophilic addition mechanism at C-2 rather than a conjugate addition mechanism at C-2 and C-3, to convert the C-2—C-3 double bond to a single bond to allow facile internal rotation.Key words: enzyme-catalyzed cis-trans isomerization, secondary α-deuterium isotope effect, maleylacetoacetate cis-trans isomerase, cis-trans isomerization mechanism.