Transaldimination induces coenzyme reorientation in pig kidney dopa decarboxylase

Dopa decarboxylase from pig kidney is a pyridoxal-5'-phosphate (PLP) dependent enzyme that displays positive circular dichroism (CD) relative to the coenzyme absorption bands at 335 and 420 nm, which are characteristic of an asymmetrically bound coenzyme. It has been previously noted that the presence of various substrates and substrate analogs gives rise to similar absorbance changes, independently of whether or not the enzyme-ligand interaction is accompanied by the conversion of the internal aldimine to an external aldimine. The effects of various ligands on the CD spectral properties of the enzyme bound PLP are presented herein. It was observed that changes in the optical activity are seen only in the presence of ligands capable of Schiff base formation. These results imply that a reorientation of the pyridoxal phosphate ring occurs upon formation of an external aldimine. Moreover, external aldimines formed with catecholic and indolic compounds are characterized by quite dissimilar optical activities, suggesting that with respect to vicinal residues, different coenzyme microenvironments exist in these complexes.