Unraveling the Molecular Determinants of Catalytic Efficiency and Substrate Specificity in <scp>l</scp>-Amino Acid Decarboxylases

l-Tryptophan decarboxylase (TDC) and l-3,4-dihydroxyphenylalanine decarboxylase (DDC) catalyze the decarboxylation of l-tryptophan, 5-hydroxytryptophan, and l-3,4-dihydroxyphenylalanine. In this study, we analyzed the amino acid compositions of the substrate-binding pockets of TDC from <i>Catharanthus roseus</i> (<i>Cr</i>TDC) and DDC from <i>Sus scrofa</i> (<i>Ss</i>DDC), explored the specificity of key amino acids within these pockets, and elucidated mechanisms influencing substrate selectivity and catalytic activity in both enzymes, using whole-cell catalysis to screen mutants and determine enzymatic kinetic parameters. The results demonstrated that residues Ala-103 and Val-122 in <i>Cr</i>TDC, along with their corresponding sites Thr-82 and Ile-101 in <i>Ss</i>DDC, significantly influence substrate selectivity and catalytic efficiency. Molecular dynamics simulations revealed that substrate selectivity and catalytic efficiency depends on the nucleophilic attack distance between the substrate's amino group and the C4' of pyridoxal 5'-phosphate. This study elucidates the catalytic mechanisms and structural bases of TDC and DDC, guiding enhancements in the related aromatic monoamine biosynthesis.