Residues D271, H192, H302 and N300 of l‐3,4‐dihydroxyphenylalanine decarboxylase (DDC), a homodimeric pyridoxal 5′‐phosphate (PLP) enzyme, were mutated in order to acquire information on the catalytic mechanism. These residues are potential participants in catalysis because they belong to the common PLP‐binding structural motif of group I, II and III decarboxylases and other PLP enzymes, and because they are among the putative active‐site residues of structural modelled rat liver DDC. The spectroscopic features of the D271E, H192Q, H302Q and N300A mutants as well as their dissociation constants for PLP suggest that substitution of each of these residues causes alteration of the state of the bound coenzyme molecule and of the conformation of aromatic amino acids, possibly in the vicinity of the active site. This supports, but does not prove, the possibility that these residues are located in the coenzyme‐binding cleft. Interestingly, mutation of each residue generates an oxidative decarboxylase activity towards l‐3,4‐dihydroxyphenylalanine (l‐Dopa), not inherent in the wild‐type in aerobiosis, and reduces the nonoxidative decarboxylase activity of l‐Dopa from 3‐ to 390‐fold. The partition ratio between oxidative and nonoxidative decarboxylation ranges from 5.7 × 10−4 for N300A mutant to 946 × 10−4 for H302Q mutant. Unlike wild‐type enzyme, the mutants catalyse these two reactions to the same extent either in the presence or absence of O2. In addition, all four mutants exhibit an extremely low level of the oxidative deaminase activity towards serotonin with respect to wild‐type. All these findings demonstrate that although D271, H192, H302 and N300 are not essential for catalysis, mutation of these residues alters the nature of catalysis. A possible relationship among the integrity of the PLP cleft, the productive binding of O2 and the transition to a closed conformational state of DDC is discussed.