STEREOCHEMICALLY SPECIFIC PROTON-TRANSFER IN DECARBOXYLATION OF 4-HYDROXYCINNAMIC ACIDS BY 4-HYDROXYCINNAMATE DECARBOXYLASE FROM KLEBSIELLA-OXYTOCA

The stereochemical specificity in the decarboxylation of E-4-hydroxyci nnamic acid catalyzed by E-4-hydroxycinnamate decarboxylase (4-HCD) of Klebsiella oxytoca was investigated. Unlike the pyrolytic decarboxyla tion of 8-deuterated E-4-hydroxycinnamic acid to yield an equimolecula r mixture of 8-Z- and 8-E-deuterated 4-hydroxystyrenes, treating 8-deu terated E-4-hydroxycinnamic acid with the enzyme in H2O-based buffer y ielded 8-Z-deuterated 4-hydroxystyrene selectively. The specific E-ori entation in catalysis and the substrate specificity requiring 4-OH in the substrates suggest that decarboxylation by IL oxytoca 4-HCD occurs via a para-quinone methide intermediate. Stereoselective protonation and the liberation of CO, by an intermediary molecule are most likely the key reaction steps in the stereochemical specificity of the newly incorporated hydrogen. (C) 1998 Academic Press.