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 K. oxytoca 4-HCD occurs via a para-quinone methide intermediate. Stereoselective protonation and the liberation of CO2 by an intermediary molecule are most likely the key reaction steps in the stereochemical specificity of the newly incorporated hydrogen.