PLP-dependent Enzymes: a Powerful Tool for Metabolic Synthesis of Non-canonical Amino Acids

Pyridoxal 5’-phosphate (PLP), the biologically active of vitamin B6, was first identified in the mid-forties as the cofactor for the transamination reaction. Since then, PLP-dependent enzymes have been the focus of extensive biochemical research. The interest aroused by these enzymes is due to their unrivalled catalytic versatility and their widespread involvement in cellular metabolism. As a matter of fact, PLP acts as cofactor in more than 160 different enzymes classified by the Enzyme Commission, representing 4% of all known cellular catalytic activities. PLP-dependent enzymes serve vital roles in all living organisms and catalyze a number of diverse chemical reactions, such as transamination, decarboxylation, racemization, carbon-carbon bond cleavage and formation. PLP-dependent activities are involved in essential biosynthetic pathways including glucose and lipid metabolism, amino acid metabolism, heme and nucleotide synthesis, and neurotransmitter production. As a consequence of their crucial metabolic relevance, a number of these enzymes are widely recognized drug targets. The mechanism of many B6 enzymes has been studied extensively over the last 50 years, with respect to structure, function, substrate and reaction specificity. Because of their catalytic versatility, in the recent years PLP-dependent enzymes have acquired extraordinary importance in biotechnology, to be exploited in the semi-synthetic production of compounds for medical and industrial use.