No abstract

The vitamin B(6)-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.

Thiamin (vitamin B1) is a pharmacological agent boosting central metabolism through the action of the coenzyme thiamin diphosphate (ThDP). However, positive effects, including improved cognition, of high thiamin doses in neurodegeneration may be observed without increased ThDP or ThDP-dependent enzymes in brain. Here, we determine protein partners and metabolic pathways where thiamin acts beyond its coenzyme role. Malate dehydrogenase, glutamate dehydrogenase and pyridoxal kinase were identified as abundant proteins binding to thiamin- or thiazolium-modified sorbents. Kinetic studies, supported by structural analysis, revealed allosteric regulation of these proteins by thiamin and/or its derivatives. Thiamin triphosphate and adenylated thiamin triphosphate activate glutamate dehydrogenase. Thiamin and ThDP regulate malate dehydrogenase isoforms and pyridoxal kinase. Thiamin regulation of enzymes related to malate-aspartate shuttle may impact on malate/citrate exchange, responsible for exporting acetyl residues from mitochondria. Indeed, bioinformatic analyses found an association between thiamin- and thiazolium-binding proteins and the term acetylation. Our interdisciplinary study shows that thiamin is not only a coenzyme for acetyl-CoA production, but also an allosteric regulator of acetyl-CoA metabolism including regulatory acetylation of proteins and acetylcholine biosynthesis. Moreover, thiamin action in neurodegeneration may also involve neurodegeneration-related 14-3-3, DJ-1 and β-amyloid precursor proteins identified among the thiamin- and/or thiazolium-binding proteins.

L-Threonine aldolases (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. TAs have great biotechnological potential, since they catalyse the formation of carbon-carbon bonds, and therefore may be exploited for bioorganic synthesis of l-3-hydroxyamino acids that are biologically active or constitute building blocks for pharmaceutical molecules. Many TAs, showing different stereospecificity towards the Cβ configuration, have been isolated. Because of their potential to carry out diastereoselective syntheses, TAs have been the subject of 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-threonine aldolase from Escherichia coli at 2.2 Å resolution, in the unliganded form and co-crystallized with l-serine and l-threonine. Furthermore, several active-site mutants have been functionally characterised 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 catalytic base in the retro-aldol cleavage reaction. Interestingly, the very large active site opening of E. coli TA suggests that a much larger molecule than l-threonine isomers may be easily accommodated, and threonine aldolases may actually play 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 to one ore more specific residues.

Adaptive metabolic reprogramming gives cancer cells a proliferative advantage. Tumour cells extensively use glycolysis to sustain anabolism and produce serine, which not only refuels the one-carbon units necessary for the synthesis of nucleotide precursors and for DNA methylation, but also affects the cellular redox homeostasis. Given its central role in serine metabolism, serine hydroxymethyltransferase (SHMT), a pyridoxal 5 0 -phosphate (PLP)-dependent enzyme, is an attractive target for tumour chemotherapy. In humans, the cytosolic isoform (SHMT1) and the mitochondrial isoform (SHMT2) have distinct cellular roles, but high sequence identity and comparable catalytic properties, which may complicate development of successful therapeutic strategies. Here, we investigated how binding of the cofactor PLP controls the oligomeric state of the human isoforms. The fact that eukaryotic SHMTs are tetrameric proteins while bacterial SHMTs function as dimers may suggest that the quaternary assembly in eukaryotes provides an advantage to fine-tune SHMT function and differentially regulate intertwined metabolic fluxes, and may provide a tool to address the specificity problem. We determined the crystal structure of SHMT2, and compared it to the apo-enzyme structure, showing that PLP binding triggers a disorder-to-order transition accompanied by a large rigid-body movement of the two cofactor-binding domains. Moreover, we demonstrated that SHMT1 exists in solution as a tetramer, both in the absence and presence of PLP, while SHMT2 undergoes a dimer-to-tetramer transition upon PLP binding. These findings indicate an unexpected structural difference between the two human SHMT isoforms, which opens new perspectives for understanding their differing behaviours, roles or regulation mechanisms in response to PLP availability in vivo.

Reprogramming of cellular metabolism towards de novo serine production fuels the growth of cancer cells, providing essential precursors such as amino acids and nucleotides and controlling the antioxidant and methylation capacities of the cell. The enzyme serine hydroxymethyltransferase (SHMT) has a key role in this metabolic shift, and directs serine carbons to one-carbon units metabolism and thymidilate synthesis. While the mitochondrial isoform of SHMT (SHMT2) has recently been identified as an important player in the control of cell proliferation in several cancer types and as a hot target for anticancer therapies, the role of the cytoplasmic isoform (SHMT1) in cancerogenesis is currently less defined. In this paper we show that SHMT1 is overexpressed in tissue samples from lung cancer patients and lung cancer cell lines, suggesting that, in this widespread type of tumor, SHMT1 plays a relevant role. We show that SHMT1 knockdown in lung cancer cells leads to cell cycle arrest and, more importantly, to p53-dependent apoptosis. Our data demonstrate that the induction of apoptosis does not depend on serine or glycine starvation, but is because of the increased uracil accumulation during DNA replication.

The three-dimensional structure of glutamate-1-semialdehyde aminomutase (EC 5.4.3.8), an ␣ 2 -dimeric enzyme from Synechococcus, has been determined by x-ray crystallography using heavy atom derivative phasing. The structure, refined at 2.4-Å resolution to an R-factor of 18.7% and good stereochemistry, explains many of the enzyme's unusual specificity and functional properties. The overall fold is that of aspartate aminotransferase and related B 6 enzymes, but it also has specific features. The structure of the complex with gabaculine, a substrate analogue, shows unexpectedly that the substrate binding site involves residues from the N-terminal domain of the molecule, notably Arg-32. Glu-406 is suitably positioned to repel ␣-carboxylic acids, thereby suggesting a basis for the enzyme's reaction specificity. The subunits show asymmetry in cofactor binding and in the mobilities of the residues 153-181. In the unliganded enzyme, one subunit has the cofactor bound as an aldimine of pyridoxal phosphate with Lys-273 and, in this subunit, residues 153-181 are disordered. In the other subunit in which the cofactor is not covalently bound, residues 153-181 are well defined. Consistent with the crystallographically demonstrated asymmetry, a form of the enzyme in which both subunits have pyridoxal phosphate bound to Lys-273 through a Schiff base showed biphasic reduction by borohydride in solution. Analysis of absorption spectra during reduction provided evidence of communication between the subunits. The crystal structure of the reduced form of the enzyme shows that, despite identical cofactor binding in each monomer, the structural asymmetry at residues 153-181 remains.Enzymes using pyridoxal-5Ј-phosphate (PLP) as cofactor provide an excellent example of the evolutionary development of catalytic versatility in homologous enzymes. Thus the ␣-family of PLP-dependent enzymes (1) contains aminotransferases, racemases, decarboxylases, mutases, and synthetases. Understanding how the related proteins confer separate reaction specificities on the coenzyme requires knowledge of the threedimensional structures of representative enzymes that catalyze the different reactions.

A flexible loop (residues 328 -339), presumably covering the active site upon substrate binding, has been revealed in 3,4-dihydroxyphenylalanine decarboxylase by means of kinetic and structural studies. The function of tyrosine 332 has been investigated by substituting it with phenylalanine. Y332F displays coenzyme content and spectroscopic features identical to those of the wild type. Unlike wild type, during reactions with L-aromatic amino acids under both aerobic and anaerobic conditions, Y332F does not catalyze the formation of aromatic amines. However, analysis of the products shows that in aerobiosis, L-aromatic amino acids are converted into the corresponding aromatic aldehydes, ammonia, and Studies on the effect exerted by O 2 on reaction specificity of the enzyme have shown that under anaerobic conditions, Reaction 1 takes place with a k cat value approximately half that occurring in the presence of O 2 and is accompanied by a decarboxylation-dependent transamination (4), and Reaction 2 occurs at the same extent either in the presence or absence of O 2 (4). Reaction 3 does not occur in anaerobiosis and is replaced by half-transamination (1, 5). Tancini et al. (6) reported the presence of an exposed and flexible region in the native pig kidney DDC molecule susceptible to tryptic digestion by which two fragments cleaved at the Lys-334 -His-335 bond were formed. Although the nicked enzymatic species does not exhibit either decarboxylase or oxidative deamination activities, it retains a large percentage of the native transaminase activity toward D-aromatic amino acids and displays a slow transaminase activity toward aromatic amines (1). Steady-state kinetic studies of native and nicked enzymatic species together with protection experiments against limited proteolysis of DDC by various substrates have suggested that ligand conformational changes occur at or near the tryptic cleavage region (1). The finding that recombinant rat liver DDC lacking this loop is incompetent for decarboxylation supports this view (7).The spatial structure of ligand-free DDC and its complex with the anti-Parkinson drug carbiDopa has been recently solved, but the flexible loop between residues 328 and 339 is invisible in the electron density map (8). A model based on the coordinates of the enzyme with the flexible loop in its hypothetical open form was built. Although in this modeled structure the loop is located at the dimer interface, far away from the active site, it is expected to extend toward the active site of the other monomer in a closed conformation upon substrate binding. Such loop closure could be an essential step in the catalytic mechanism of the enzyme, and it is also reasonable to suppose that some loop residues could even take part in the catalytic * This work was supported by funding from the Italian Ministero dell'Istruzione, dell'Università e Ricerca and from Consiglio Nazionale delle Ricerche (CNR) (to C. B. V.). The costs of publication of this article were defrayed in part by the payment of page charges. This...