Reaction and substrate specificity of 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 O2 on reaction specificity of DDC revealed that under anaerobic conditions decarboxylation of L-aromatic amino acids takes place with a kcat approximately half of that measured in the presence of O2, 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 O2. 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 O2, 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. © 2003 Elsevier Science B.V. All rights reserved.