On the mechanism of Rhodotorula gracilis d-amino acid oxidase: role of the active site serine 335

Enhancement of the substrate specificity of D-amino acid oxidase based on tunnel-pocket engineering

D-Amino acid oxidase (DAAO) selectively catalyzes the oxidative deamination of  D-amino acids, making it one of the most promising routes for synthesizing optically pure  L-amino acids, including  L-phosphinothricin ( L-PPT), a chiral herbicide with significant market potential. However, the native DAAOs that have been reported have low activity against unnatural acid substrate  D-PPT. Herein, we designed and screened a DAAO from Rhodotorula taiwanensis (RtwDAAO), and improved its catalytic potential toward  D-PPT through protein engineering. A semirational design approach was employed to create a mutation library based on the tunnel-pocket engineering. After three rounds of iterative saturation mutagenesis, the optimal variant M3rd -SHVG was obtained, exhibiting a >2000-fold increase in relative activity. The kinetic parameters showed that M3rd -SHVG improved the substrate binding affinity and turnover number. This is the optimal parameter reported so far. Further, molecular dynamics simulation revealed that the M3rd -SHVG reshapes the tunnel-pocket and corrects the direction of enzyme-substrate binding, allowing efficiently catalyze unnatural substrates. Our strategy demonstrates that the redesign of tunnel-pockets is effective in improving the activity and kinetic efficiency of DAAO, which provides a valuable reference for enzymatic catalysis. With the M3rd -SHVG as biocatalyst, 500 mM D, L-PPT was completely converted and the yield reached 98%. The results laid the foundation for further industrial production.

Enhancement in the catalytic efficiency of D-amino acid oxidase from Glutamicibacter protophormiae by multiple amino acid substitutions

D-Amino acid oxidase (DAAO) is an imperative oxidoreductase that oxidizes D-amino acids to corresponding keto acids, producing ammonia and hydrogen peroxide. Previously, based on the sequence alignment of DAAO from Glutamicibacter protophormiae (GpDAAO-1) and (GpDAAO-2), 4 residues (E115, N119, T256, T286) at the surface regions of GpDAAO-2, were subjected to site-directed mutagenesis and achieved 4 single-point mutants with enhanced catalytic efficiency (k_cat/K_m) compared to parental GpDAAO-2. In the present study, to further enhance the catalytic efficiency of GpDAAO-2, a total of 11 (6 double, 4 triple, and 1 quadruple-point) mutants were prepared by the different combinations of 4 single-point mutants. All mutants and wild types were overexpressed, purified and enzymatically characterized. A triple-point mutant E115A/N119D/T286A exhibited the most significant improvement in catalytic efficiency as compared to wild-type GpDAAO-1 and GpDAAO-2. Structural modeling analysis elucidated that residue Y213 in loop region C209-Y219 might act as the active-site lid for controlling substrate access, the residue K256 substituted by threonine (K256T) might change the hydrogen bonding interaction between residue Y213 and the surrounding residues, and switch the conformation of the active-site lid from the closed state to the open state, resulting in the enhancement in substrate accessibility and catalytic efficiency.

Modulating D-amino acid oxidase (DAAO) substrate specificity through facilitated solvent access

D-amino acid oxidase (DAAO) degrades D-amino acids to produce α-ketoacids, hydrogen peroxide and ammonia. DAAO has often been investigated and engineered for industrial and clinical applications. We combined information from literature with a detailed analysis of the structure to engineer mammalian DAAOs. The structural analysis was complemented with molecular dynamics simulations to characterize solvent accessibility and product release mechanisms. We identified non-obvious residues located on the loops on the border between the active site and the secondary binding pocket essential for pig and human DAAO substrate specificity and activity. We engineered DAAOs by mutating such critical residues and characterised the biochemical activity of the resulting variants. The results highlight the importance of the selected residues in modulating substrate specificity, product egress and enzyme activity, suggesting further steps of DAAO re-engineering towards desired clinical and industrial applications.

Competitive Inhibitors Unveil Structure/Function Relationships in Human D-Amino Acid Oxidase

D-amino acid oxidase (DAAO) catalyzes the oxidative deamination of several neutral D-amino acids and is the enzyme mainly responsible (together with serine racemase) for degrading D-serine (D-Ser) in the central nervous system of mammals. This D-amino acid, which binds the coagonist site of the N-methyl-D-aspartate receptor, is thus a key neuromodulator of glutamatergic neurotransmission. Altered D-Ser metabolism results in several pathological conditions (e.g., amylotrophic lateral sclerosis or schizophrenia, SZ) for which effective "broad spectrum" pharmaceutical drugs are not yet available. In particular, the correlation between reduced D-Ser concentration and SZ led to a renaissance of biochemical interest in human DAAO (hDAAO). In the last 10 years, public and corporate research laboratories undertook huge efforts to study the structural, enzymatic, and physiological properties of the human flavoenzyme and to identify novel effective inhibitors which, acting as pharmaceutical drugs, could decrease hDAAO activity, thus restoring the physiological concentration of D-Ser. Although, none of the identified hDAAO inhibitors has reached the market yet, from a biochemical point of view, these compounds turned out to be invaluable for gaining a detailed understanding of the structure/function relationships at the molecular level in the mammalian DAAO, in particular of the interaction between ligand and the enzyme. This detailed knowledge, together with several recent studies concerning the interaction of the human enzyme with other protein regulative partners, its subcellular localization, and in vivo degradation, contributed to gaining comprehensive knowledge of the structure, function, and physiopathological role of this important human enzyme.

Hyperactive mutants of mouse D-aspartate oxidase: mutagenesis of the active site residue serine 308

The role of Ser-308 of murine D-aspartate oxidase (mDASPO), particularly its side chain hydroxyl group, was investigated through the use of site-specific mutational analysis of Ser-308. Recombinant mDASPO carrying a substitution of Gly, Ala, or Tyr for Ser-308 was generated, and fused to either His (His-mDASPO), or glutathione S-transferase, His, and S (GHS-mDASPO) at its N-terminus. Wild-type His-mDASPO or GHS-mDASPO or their mutant derivatives were expressed in Escherichia coli and purified by affinity chromatography. All purified recombinant proteins had functional DASPO activity. The Gly-308 and Ala-308 mutants had significantly higher catalytic efficiency towards D-Asp and N-methyl-D-Asp, and a higher affinity for flavin adenine dinucleotide (FAD) compared to the wild-type enzyme. The Tyr-308 mutant had lower catalytic efficiency and binding capacity. These results suggest that the side chain hydroxyl group of a critical residue of mDASPO, Ser-308, down-regulates enzymatic activity, substrate binding, and FAD binding. This study provides information on the active site of DASPO that will considerably enhance our understanding of the biological significance of this enzyme.

Investigating the role of active site residues of Rhodotorula gracilis d-amino acid oxidase on its substrate specificity

D-amino acid oxidase (DAAO) is a flavoprotein that catalyzes stereospecifically the oxidative deamination of D-amino acids. The wild-type DAAO is mainly active on neutral D-amino acids, while basic D-amino acids are poor substrates and the acidic ones are virtually not oxidized. To present a comprehensive picture of how the active site residues can modulate the substrate specificity a number of mutants at position M213, Y223, Y238, R285, S335, and Q339 were prepared in the enzyme from the yeast Rhodotorula gracilis. All DAAO mutants have spectral properties similar to those of the wild-type enzyme and are catalytically active, thus excluding an essential role in catalysis; a lower activity on neutral and basic amino acids was observed. Interestingly, an increase in activity and (k(cat)/K(m))(app) ratio on D-aspartate was observed for all the mutants containing an additional charged residue in the active site. The active site of yeast DAAO appears to be a highly evolved scaffold built up through evolution to optimize the oxidative deamination of neutral D-amino acids without limiting its substrate specificity. It is noteworthy, that introduction of a sole, additional, positively charged residue in the active site is sufficient to optimize the reactivity on acidic D-amino acids, giving rise to kinetic properties similar to those of D-aspartate oxidase.

Overproduction and characterization of a recombinant D-amino acid oxidase from Arthrobacter protophormiae

A screening of soil samples for D-amino acid oxidase (D-AAO) activity led to the isolation and identification of the gram-positive bacterium Arthrobacter protophormiae. After purification of the wild-type D-AAO, the gene sequence was determined and designated dao. An alignment of the deduced primary structure with eukaryotic D-AAOs and D-aspartate oxidases showed that the D-AAO from A. protophormiae contains five of six conserved regions; the C-terminal type 1 peroxisomal targeting signal that is typical for D-AAOs from eukaryotic origin is missing. The dao gene was cloned and expressed in Escherichia coli. The purified recombinant D-AAO had a specific activity of 180 U mg protein(-1) for D-methionine and was slightly inhibited in the presence of L-methionine. Mainly, basic and hydrophobic D-amino acids were oxidized by the strictly enantioselective enzyme. After a high cell density fermentation, 2.29 x 10(6) U of D-AAO were obtained from 15 l of fermentation broth.