The vitamin B6-derived pyridoxal 5-phosphate (PLP) is the cofactor of enzymes catalyzing a large variety of chemical reactions mainly involved in amino acid metabolism. These enzymes have been divided in five families and fold types on the basis of evolutionary relationships and protein structural organization. Almost 1.5% of all genes in prokaryotes code for PLP-dependent enzymes, whereas the percentage is substantially lower in eukaryotes. Although about 4% of enzyme-catalyzed reactions catalogued by the Enzyme Commission are PLP-dependent, only a few enzymes are targets of approved drugs and about twenty are recognised as potential targets for drugs or herbicides. PLP-dependent enzymes for which there are already commercially available drugs are DOPA decarboxylase (involved in the Parkinson disease), GABA aminotransferase (epilepsy), serine hydroxymethyltransferase (tumors and malaria), ornithine decarboxylase (African sleeping sickness and, potentially, tumors), alanine racemase (antibacterial agents), and human cytosolic branched-chain aminotransferase (pathological states associated to the GABA/glutamate equilibrium concentrations). Within each family or metabolic pathway, the enzymes for which drugs have been already approved for clinical use are discussed first, reporting the enzyme structure, the catalytic mechanism, the mechanism of enzyme inactivation or modulation by substrate-like or transition state-like drugs, and on-going research for increasing specificity and decreasing side-effects. Then, PLP-dependent enzymes that have been recently characterized and proposed as drug targets are reported. Finally, the relevance of recent genomic analysis of PLP-dependent enzymes for the selection of drug targets is discussed.

Pyridoxine 5′‐phosphate oxidase catalyzes the terminal step in the synthesis of pyridoxal 5′‐phosphate. The cDNA for the human enzyme has been cloned and expressed in Escherichia coli. The purified human enzyme is a homodimer that exhibits a low catalytic rate constant of ∼0.2 sec⁻¹ and K_m values in the low micromolar range for both pyridoxine 5′phosphate and pyridoxamine 5′‐phosphate. Pyridoxal 5′‐phosphate is an effective product inhibitor. The three‐dimensional fold of the human enzyme is very similar to those of the E. coli and yeast enzymes. The human and E. coli enzymes share 39% sequence identity, but the binding sites for the tightly bound FMN and substrate are highly conserved. As observed with the E. coli enzyme, the human enzyme binds one molecule of pyridoxal 5′‐phosphate tightly on each subunit.

L‐Threonine aldolases (l‐TAs) represent a family of homologous pyridoxal 5′‐phosphate‐dependent enzymes found in bacteria and fungi, and catalyse the reversible cleavage of several l‐3‐hydroxy‐α‐amino acids. l‐TAs have great biotechnological potential, as they catalyse the formation of carbon–carbon bonds, and therefore may be exploited for the bioorganic synthesis of l‐3‐hydroxyamino acids that are biologically active or constitute building blocks for pharmaceutical molecules. Many l‐TAs, showing different stereospecificity towards the Cβ configuration, have been isolated. Because of their potential to carry out diastereoselective syntheses, l‐TAs have been subjected to structural, functional and mechanistic studies. Nevertheless, their catalytic mechanism and the structural bases of their stereospecificity have not been elucidated. In this study, we have determined the crystal structure of low‐specificity l‐TA from Escherichia coli at 2.2‐Å resolution, in the unliganded form and cocrystallized with l‐serine and l‐threonine. Furthermore, several active site mutants have been functionally characterized in order to elucidate the reaction mechanism and the molecular bases of stereospecificity. No active site catalytic residue was revealed, and a structural water molecule was assumed to act as the catalytic base in the retro‐aldol cleavage reaction. Interestingly, the very large active site opening of E. coli l‐TA suggests that much larger molecules than l‐threonine isomers may be easily accommodated, and l‐TAs may actually have diverse physiological functions in different organisms. Substrate recognition and reaction specificity seem to be guided by the overall microenvironment that surrounds the substrate at the enzyme active site, rather than by one ore more specific residues.Structured digital eTA and eTA bind by x-ray crystallography (1, 2). Database: RCSB PDB (www.pdb.org): structural data are available in the Protein Data Bank/BioMagResBank databases under the accession numbers 4LNJ, 4LNM and 4LNL for the unliganded, eTA‐Thr and eTA‐Ser structures.