Reaction and substrate specificity of recombinant pig kidney Dopa decarboxylase under aerobic and anaerobic conditions

Dopa decarboxylase (DDC) catalyzes as main reaction the stereospecific CO2 abstraction from L-Dopa and L-5-hydroxytryptophan (5-HTP), generating the corresponding aromatic amines, dopamine and serotonin, respectively. Side reactions with turnover time of minutes are also catalyzed by the enzyme. In particular, DDC exhibits half-transaminase activity toward D-aromatic amino acids and oxidative deaminase activity toward aromatic amines. The latter reaction could represent a new activity for this class of enzymes. Studies on the effect exerted by O-2 on reaction specificity of DDC revealed that under anaerobic conditions decarboxylation Of L-aromatic amino acids takes place with a kat approximately half of that measured in the presence Of O-2, and is accompanied by a decarboxylation-dependent transamination, whereas oxidative deamination of aromatic amines is replaced by half-transamination. Half-transamination Of D-aromatic amino acids is unaffected by the presence or absence Of O-2. Some structural elements relevant for the control of reaction and substrate specificity of DDC have been identified by means of limited tryptic digestion and site-directed mutagenesis studies. All together, the data indicate that the chemical nature of the substrate, the presence Of O-2, the integrity of a mobile loop, the absence of perturbation in the coenzyme-binding cleft and pH are important requirements for the achievement of a closed conformational state where the highest level of reaction specificity is reached. (C) 2003 Elsevier Science B.V. All rights reserved.