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-hydroxycinnamic acid catalyzed by E-4-hydroxycinnamate decarboxylase (4-HCD) of Klebsiella oxytoca was investigated. Unlike the pyrolytic decarboxylation of 8-deuterated E-4-hydroxycinnamic acid to yield an equimolecular mixture of 8-Z- and 8-E-deuterated 4-hydroxystyrenes, treating 8-deuterated E-4-hydroxycinnamic acid with the enzyme in H2O-based buffer yielded 8-Z-deuterated 4-hydroxystyrene selectively. The specific E-orientation 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.