Stereochemically Specific Proton Transfer in Decarboxylation of 4-Hydroxycinnamic Acids by 4-Hydroxycinnamate Decarboxylase fromKlebsiella oxytoca

The stereochemical specificity in the decarboxylation ofE-4-hydroxycinnamic acid catalyzed byE-4-hydroxycinnamate decarboxylase (4-HCD) ofKlebsiella oxytocawas investigated. Unlike the pyrolytic decarboxylation of 8-deuteratedE-4-hydroxycinnamic acid to yield an equimolecular mixture of 8-Z- and 8-E-deuterated 4-hydroxystyrenes, treating 8-deuteratedE-4-hydroxycinnamic acid with the enzyme in H2O-based buffer yielded 8-Z-deuterated 4-hydroxystyrene selectively. The specificE-orientation in catalysis and the substrate specificity requiring 4-OH in the substrates suggest that decarboxylation byK. oxytoca4-HCD occurs via apara-quinone methide intermediate. Stereoselective protonation and the liberation of CO2by an intermediary molecule are most likely the key reaction steps in the stereochemical specificity of the newly incorporated hydrogen.