Over the years, accumulating evidence has indicated that D-serine represents the endogenous ligand for the glycinemodulatory binding site on the NR1 subunit of N-methyl-D-aspartate receptors in various brain areas. Cellular concentrations of D-serine are regulated by synthesis due to the enzyme serine racemase (isomerization reaction) and by degradation due to the same enzyme (elimination reaction) as well as by the FAD-containing flavoenzyme D-amino acid oxidase (DAAO, oxidative deamination reaction). Several findings have linked low levels of D-serine to schizophrenia: D-serine concentrations in serum and cerebrospinal fluid have been reported to be decreased in schizophrenia patients while human DAAO activity and expression are increased; oral administration of Dserine improved positive, negative, and cognitive symptoms of schizophrenia as add-on therapy to typical and atypical antipsychotics. This evidence indicates that increasing NMDA receptor function, perhaps by inhibiting DAAO-induced degradation of D-serine may alleviate symptoms in schizophrenic patients. Furthermore, it has been suggested that co-administration of D-serine with a human DAAO inhibitor may be a more effective means of increasing D-serine levels in the brain. Here, we present an overview of the current knowledge of the structure-function relationships in human DAAO and of the compounds recently developed to inhibit its activity (specifically the ones recently exploited for schizophrenia treatment).

Glycine oxidase from Bacillus subtilis is a homotetrameric flavoprotein of great potential biotechnological use because it catalyzes the oxidative deamination of various amines and d-isomer of amino acids to yield the corresponding alpha-keto acids, ammonia/amine, and hydrogen peroxide. Glyphosate (N-phosphonomethylglycine), a broad spectrum herbicide, is an interesting synthetic amino acid: this compound inhibits 5-enolpyruvylshikimate-3-phosphate synthase in the shikimate pathway, which is essential for the biosynthesis of aromatic amino acids in plants and certain bacteria. In recent years, transgenic crops resistant to glyphosate were mainly generated by overproducing the plant enzyme or by introducing a 5-enolpyruvylshikimate-3-phosphate synthase insensitive to this herbicide. In this work, we propose that the enzymatic oxidation of glyphosate could be an effective alternative to this important biotechnological process. To reach this goal, we used a rational design approach (together with site saturation mutagenesis) to generate a glycine oxidase variant more active on glyphosate than on the physiological substrate glycine. The glycine oxidase containing three point mutations (G51S/A54R/H244A) reaches an up to a 210-fold increase in catalytic efficiency and a 15,000-fold increase in the specificity constant (the k(cat)/K(m) ratio between glyphosate and glycine) as compared with wild-type glycine oxidase. The inspection of its three-dimensional structure shows that the alpha2-alpha3 loop (comprising residues 50-60 and containing two of the mutated residues) assumes a novel conformation and that the newly introduced residue Arg(54) could be the key residue in stabilizing glyphosate binding and destabilizing glycine positioning in the binding site, thus increasing efficiency on the herbicide.