An essential arginine residue at the binding site of pig kidney 3,4-dihydroxyphenylalanine decarboxylase.

Pig kidney 3,4-dihydroxyphenylalanine (Dopa) decarboxylase is inactivated by the arginine-specific reagent phenylglyoxal. Under these experimental conditions, the reaction follows pseudo-first-order kinetics with a second-order rate constant of 25 m-1 min-1. Holo- and apo-enzyme were inactivated at the same rate. However, inactivation seems to be related to modification of 1 and 2 arginyl residues per mol of holo- and apo-enzyme, respectively. Only one of these two residues was essential to decarboxylase activity of the enzyme. Phenylglyoxal-modified apo-Dopa decarboxylase retained the capacity to bind pyridoxal-P. Neither this reconstituted species nor the phenylglyoxal-modified holoenzyme were able to form Schiff base intermediates with aromatic amino acids in L and D forms. These data together with protection experiments suggest that the susceptible arginine residue in holoenzyme may somehow perturb the substrate binding site. However, unlike in other pyridoxal-P enzymes, this critical arginine in Dopa decarboxylase does not seem to behave as an anionic recognition site for the phosphate group of the coenzyme or the carboxy group of the substrate. It is speculated that this guanidyl group could function in hydrogen bonding of substrate side chain.