Characterization and gene cloning of l-xylulose reductase involved in l-arabinose catabolism from the pentose-fermenting fungus Rhizomucor pusillus.

l-Xylulose reductase (LXR) catalyzes the reduction of l-xylulose to xylitol in the fungal l-arabinose catabolic pathway. LXR (RpLXR) was purified from the pentose-fermenting zygomycetous fungus Rhizomucor pusillus NBRC 4578. The native RpLXR is a homotetramer composed of 29 kDa subunits and preferred NADPH as a coenzyme. The Km values were 8.71 mM for l-xylulose and 3.89 mM for dihydroxyacetone. The lxr3 (Rplxr3) gene encoding RpLXR consists of 792 bp and encodes a putative 263 amino acid protein (Mr = 28,341). The amino acid sequence of RpLXR showed high similarity to 3-oxoacyl-(acyl-carrier-protein) reductase. The Rplxr3 gene was expressed in Escherichia coli and the recombinant RpLXR exhibited properties similar to those of native RpLXR. Transcription of the Rplxr3 gene in R. pusillus NBRC 4578 was induced in the presence of l-arabinose and inhibited in the presence of d-glucose, d-xylose, and d-mannitol, indicating that RpLXR is involved in the l-arabinose catabolic pathway.

Purification and characterization of xylitol dehydrogenase with l-arabitol dehydrogenase activity from the newly isolated pentose-fermenting yeast Meyerozyma caribbica 5XY2.

Meyerozyma caribbica strain 5XY2, which was isolated from an alcohol fermentation starter in Thailand, was found to catabolize l-arabinose as well as d-glucose and d-xylose. The highest production amounts of ethanol from d-glucose, xylitol from d-xylose, and l-arabitol from l-arabinose were 0.45 g/g d-glucose, 0.60 g/g d-xylose, and 0.61 g/g l-arabinose with 21.7 g/L ethanol, 20.2 g/L xylitol, and 30.3 g/l l-arabitol, respectively. The enzyme with l-arabitol dehydrogenase (LAD) activity was purified from the strain and found to exhibit broad specificity to polyols, such as xylitol, d-sorbitol, ribitol, and l-arabitol. Xylitol was the preferred substrate with Km=16.1 mM and kcat/Km=67.0 min-1mM-1, while l-arabitol was also a substrate for the enzyme with Km=31.1 mM and kcat/Km=6.5 min-1 mM-1. Therefore, this enzyme from M. caribbica was named xylitol dehydrogenase (McXDH). McXDH had an optimum temperature and pH at 40°C and 9.5, respectively. The McXDH gene included a coding sequence of 1086 bp encoding a putative 362 amino acid protein of 39 kDa with an apparent homopentamer structure. Native McXDH and recombinant McXDH exhibited relative activities toward l-arabitol of approximately 20% that toward xylitol, suggesting the applicability of this enzyme with the functions of XDH and LAD to the development of pentose-fermenting Saccharomyces cerevisiae.

Structural and computational analysis of peptide recognition mechanism of class-C type penicillin binding protein, alkaline D-peptidase from Bacillus cereus DF4-B.

Alkaline D-peptidase from Bacillus cereus DF4-B, called ADP, is a D-stereospecific endopeptidase reacting with oligopeptides containing D-phenylalanine (D-Phe) at N-terminal penultimate residue. ADP has attracted increasing attention because it is useful as a catalyst for synthesis of D-Phe oligopeptides or, with the help of substrate mimetics, L-amino acid peptides and proteins. Structure and functional analysis of ADP is expected to elucidate molecular mechanism of ADP. In this study, the crystal structure of ADP (apo) form was determined at 2.1 Å resolution. The fold of ADP is similar to that of the class C penicillin-binding proteins of type-AmpH. Docking simulations and fragment molecular orbital analyses of two peptides, (D-Phe)4 and (D-Phe)2-(L-Phe)2, with the putative substrate binding sites of ADP indicated that the P1 residue of the peptide interacts with hydrophobic residues at the S1 site of ADP. Furthermore, molecular dynamics simulation of ADP for 50 nsec suggested that the ADP forms large cavity at the active site. Formation of the cavity suggested that the ADP has open state in the solution. For the ADP, having the open state is convenient to bind the peptides having bulky side chain, such as (D-Phe)4. Taken together, we predicted peptide recognition mechanism of ADP.

Identification and characterization of D-xylose reductase involved in pentose catabolism of the zygomycetous fungus Rhizomucor pusillus.

Rhizomucor pusillus NBRC 4578 efficiently produces ethanol from lignocellulosic biomass because of its ability to ferment not only d-glucose, but also d-xylose. When the strain was cultivated on d-xylose, ethanol was gradually formed in the culture medium with a decrease in d-xylose and the simultaneous accumulation of xylitol, which suggested that the strain catabolized d-xylose with d-xylose reductase (XR) and xylitol dehydrogenase (XDH). XR (RpXR) was purified to homogeneity from the crude extract prepared from the mycelia of the strain grown on d-xylose. The purified enzyme was found to be NADPH-dependent and prefer pentoses such as d-xylose, d-ribose, and l-arabinose as substrates. Isolation of the genomic DNA and cDNA of the xyl1 gene encoding RpXR revealed that the gene was interrupted by two introns and the exon of the gene encoded a protein composed of 322 amino acids with a Mr of 36,724. Phylogenetic analysis showed that RpXR is more related to 4-dihydromethyltrisporate dehydrogenases from Mucoraseae fungi rather than the previously reported fungal XRs. Quantitative real-time PCR indicated that transcription of the xyl1 gene was marked in the presence of d-xylose and l-arabinose, but was week in the presence of d-glucose. These biochemical and expression analyses suggest that RpXR is involved in the catabolism of l-arabinose as well as d-xylose. This is the first report of the purification, characterization, and gene cloning of XR from zygomycetous fungi.

Identification and characterization of D-xylulokinase from the D-xylose-fermenting fungus, Mucor circinelloides.

D-Xylulokinase catalyzes the phosphorylation of D-xylulose in the final step of the pentose catabolic pathway to form d-xylulose-5-phosphate. The D-xylulokinase activity was found to be induced by both D-xylose and L-arabinose, as well as some of the other enzymes involved in the pentose catabolism, in the D-xylose-fermenting zygomycetous fungus, Mucor circinelloides NBRC 4572. The putative gene, xyl3, which may encode D-xylulokinase, was detected in the genome sequence of this strain. The amino acid sequence deduced from the gene was more similar to D-xylulokinases from an animal origin than from other fungi. The recombinant enzyme was purified from the E. coli transformant expressing xyl3 and then characterized. The ATP-dependent phosphorylative activity of the enzyme was the highest toward D-xylulose. Its kinetic parameters were determined as Km (D-xylulose) = 0.29 mM and Km (ATP) = 0.51 mM, indicating that the xyl3 gene encoded D-xylulokinase (McXK). Western blot analysis revealed that McXK was induced by L-arabinose as well as D-xylose and the induction was repressed in the presence of D-glucose, suggesting that the enzyme may be involved in the catabolism of D-xylose and L-arabinose and is subject to carbon catabolite repression in this fungus. This is the first study on D-xylulokinase from zygomycetous fungi.

Molecular analysis of NAD⁺-dependent xylitol dehydrogenase from the zygomycetous fungus Rhizomucor pusillus and reversal of the coenzyme preference.

The zygomycetous fungus Rhizomucor pusillus NBRC 4578 is able to ferment not only d-glucose but also d-xylose into ethanol. Xylitol dehydrogenase from R. pusillus NBRC 4578 (RpXDH), which catalyzes the second step of d-xylose metabolism, was purified, and its enzymatic properties were characterized. The purified RpXDH preferred NAD(+) as its coenzyme and showed substrate specificity for xylitol, d-sorbitol, and ribitol. cDNA cloning of xyl2 gene encoding RpXDH revealed that the gene included a coding sequence of 1,092 bp with a molecular mass of 39,185 kDa. Expression of the xyl2 in R. pusillus NBRC 4578 was induced by d-xylose, and the expression levels were increased with accumulation of xylitol. The xyl2 gene was expressed in Escherichia coli, and coenzyme preference of the recombinant RpXDH was reversed from NAD(+) to NADP(+) in the double mutant D205A/I206R by site-directed mutagenesis.

Characterization and application of aminoamide-oxidizing enzyme from Aspergillus carbonarius AIU 205.

We isolated Aspergillus carbonarius AIU 205 as a new producer of an enzyme catalyzing oxidative deamination of 4-aminobutanamide (4-ABAD) to 4-oxobutanamide with the subsequent release of ammonia and hydrogen peroxide. Since the strain produced three enzymes with different Km values for 4-ABAD, the enzyme with lowest Km value (0.31 mM) was purified and revealed certain remarkable properties. The enzyme also oxidized aliphatic monoamines, aromatic amines and aliphatic aminoalcohols, but did not oxidize l-amino acids and aliphatic diamines. The Vmax/Km values for aliphatic monoamines were higher than that for 4-ABAD, and the enzyme activity was strongly inhibited by inhibitors of copper-containing amine oxidases. Thus, it was concluded that the enzyme might belong to a group of copper-containing amine oxidase. The 4-ABAD oxidase activity of this enzyme was optimum at pH 7.0, and the enzyme activity at pH 6.0 was 65% of that at pH 7.0. The enzyme was useful for increasing the sensitivity of l-lysine assay using l-amino acid oxidase/monooxygenase from Pseudomonas sp. AIU 813.

Functional expression of a plant hydroxynitrile lyase in Escherichia coli by directed evolution: creation and characterization of highly in vivo soluble mutants.

Low protein solubility of recombinantly expressed proteins in Escherichia coli is a major factor hindering their application and analysis. We generated highly in vivo soluble mutants of a hydroxynitrile lyase in E.coli using protein engineering. Structure-guided saturation mutagenesis caused high solubility of single Lys-Pro mutations at positions 176, 199 and 224 of this low soluble wild-type enzyme. The triple Lys-Pro mutant generated at these surface conserved residues showed up to 8-fold increase in specific activity in the cell-free extract. Random mutagenesis also created a mutant of His103Met with 18.5-fold increase. The main expression form was reversed from insoluble to the soluble fraction following both types of above-mentioned mutations in E.coli at 37°C. The findings challenge the rationale of producing recombinant proteins in this host at 37°C. Formerly wild type low soluble protein was then present as soluble protein by these mutations, which also elevated the total soluble protein fraction in E.coli. Saturation mutagenesis of His103 provided other highly soluble mutants with hydrophobic substitutions. These mutations caused only minor secondary structural changes as determined by circular dichroism and Fourier-transform infrared spectroscopy and affected catalytic efficiency slightly for the purified mutants (0.82-1.6-fold for benzaldehyde and 0.9-1.9-fold for mandelonitrile). The stability of the mutants was differed from that of the wild type at high temperatures and at pH >8. Exchanging the buried basic-polar residue His103 with hydrophobic amino acids is in line with the overall structure of the enzyme, i.e. having hydrophilic residues in solvent-exposed areas and hydrophobic residues in the core.

Characterization of a new (R)-hydroxynitrile lyase from the Japanese apricot Prunus mume and cDNA cloning and secretory expression of one of the isozymes in Pichia pastoris.

PmHNL, a hydroxynitrile lyase from Japanese apricot ume (Prunus mume) seed was purified to homogeneity by ammonium sulfate fractionation and chromatographic steps. The purified enzyme was a monomer with molecular mass of 58 kDa. It was a flavoprotein similar to other hydroxynitrile lyases of the Rosaceae family. It was active over a broad temperature, and pH range. The N-terminal amino acid sequence (20 amino acids) was identical with that of the enzyme from almond (Prunus dulcis). Based on the N-terminal sequence of the purified enzyme and the conserved amino acid sequences of the enzymes from Pr. dulcis, inverse PCR method was used for cloning of a putative PmHNL (PmHNL2) gene from a Pr. mume seedling. Then the cDNA for the enzyme was cloned. The deduced amino acid sequence was found to be highly similar (95%) to that of an enzyme from Pr. serotina, isozyme 2. The recombinant Pichia pastoris transformed with the PmHNL2 gene secreted an active enzyme in glycosylated form.

High yield synthesis of 12-aminolauric acid by "enzymatic transcrystallization" of omega-laurolactam using omega-laurolactam hydrolase from Acidovorax sp. T31.

The genes encoding omega-laurolactam hydrolases from Cupriavidus sp. T7, Acidovorax sp. T31, Cupriavidus sp. U124, and Sphingomonas sp. U238 were cloned and sequenced. Nucleotide and amino acid sequence analysis of the four genes indicated that the primary structures of these omega-laurolactam hydrolases are significantly similar to the 6-aminohexanoate-cyclic-dimer hydrolase (EC 3.5.2.12). These genes were expressed in Escherichia coli, and the omega-laurolactam hydrolysing activity of the recombinant enzymes was compared with that of 6-aminohexanoate-cyclic-dimer hydrolase from Arthrobacter sp. KI72. The enzyme from Acidovorax sp. T31 was most successfully expressed in E. coli. Cell-free extract of the recombinant strain was used for the synthesis of 12-aminolauric acid from omega-laurolactam by "enzymatic transcrystallization," because crystalline omega-laurolactam added into the enzyme solution was converted to crystalline 12-aminolauric acid (> or =97.3% yield). Under the optimum conditions, 208 g/l of 12-aminolauric acid was produced in 17 h. The resulting pure product was identical to authentic 12-aminolauric acid.

The novel structure of a pyridoxal 5'-phosphate-dependent fold-type I racemase, alpha-amino-epsilon-caprolactam racemase from Achromobacter obae.

Alpha-amino-epsilon-caprolactam (ACL) racemase (ACLR) from Achromobacter obae catalyzes the interconversion of l- and d-ACL. ACLR belongs to the fold-type I group of pyridoxal 5'-phosphate (PLP) dependent enzymes. In this study, the first crystal structures of a fold-type I racemase are solved for the native form and epsilon-caprolactam-complexed form of ACLR at 2.21 and 2.40 A resolution, respectively. Based on the location of epsilon-caprolactam in the complex structure, the substrate-binding site is assigned between Trp49 and Tyr137. The carboxyl group of Asp210 is a reasonable candidate that recognizes the nitrogen atom of a lactam or amide in the substrate. Based on a structural comparison with fold-type III alanine racemase, Tyr137 is potentially the acid/base catalytic residue that is essential for the two-base racemization mechanism. The overall structure of ACLR is similar to that of fold-type I enzymes. A structural comparison with these enzymes explains the different reaction specificities.

The screening, characterization, and use of omega-laurolactam hydrolase: a new enzymatic synthesis of 12-aminolauric acid.

Several omega-laurolactam degrading microorganisms were isolated from soil samples. These strains were capable of growing in a medium containing omega-laurolactam as sole source of carbon and nitrogen. Among them, five strains (T7, T31, U124, U224, and U238) were identified as Cupriavidus sp. T7, Acidovorax sp. T31, Cupriavidus sp. U124, Rhodococcus sp. U224, and Sphingomonas sp. U238, respectively. The omega-laurolactam hydrolyzing enzyme from Rhodococcus sp. U224 was purified to homogeneity, and its enzymatic properties were characterized. The enzyme acts on omega-octalactam and omega-laurolactam, but other lactam compounds, amides and amino acid amides, cannot be substrates. The enzyme gene was cloned, and the deduced amino acid sequence showed high homology with 6-aminohexanoate-cyclic-dimer hydrolase (EC 3.5.2.12) from Arthrobacter sp. KI72 and Pseudomonas sp. NK87. Enzymatic synthesis of 12-aminolauric acid was performed using partially purified omega-laurolactam hydrolase from Rhodococcus sp. U224.

Purification and characterization of a novel (R)-hydroxynitrile lyase from Eriobotrya japonica (Loquat).

A hydroxynitrile lyase was isolated and purified to homogeneity from seeds of Eriobotrya japonica (loquat). The final yield, of 36% with 49-fold purification, was obtained by 30-80% (NH(4))(2)SO(4) fractionation and column chromatography on DEAE-Toyopearl and Concanavalin A Sepharose 4B, which suggested the presence of a carbohydrate side chain. The purified enzyme was a monomer with a molecular mass of 72 kDa as determined by gel filtration, and 62.3 kDa as determined by SDS-gel electrophoresis. The N-terminal sequence is reported. The enzyme was a flavoprotein containing FAD as a prosthetic group, and it exhibited a K(m) of 161 microM and a k(cat)/K(m) of 348 s(-1) mM(-1) for mandelonitrile. The optimum pH and temperature were pH 5.5 and 40 degrees C respectively. The enzyme showed excellent stability with regard to pH and temperature. Metal ions were not required for its activity, while activity was significantly inhibited by CuSO(4), HgCl(2), AgNO(3), FeCl(3), beta-mercaptoethanol, iodoacetic acid, phenylmethylsulfonylfluoride, and diethylpyrocarbonate. The specificity constant (k(cat)/K(m)) of the enzyme was investigated for the first time using various aldehydes as substrates. The enzyme was active toward aromatic and aliphatic aldehydes, and showed a preference for smaller substrates over bulky one.

Deduced catalytic mechanism of D-amino acid amidase from Ochrobactrum anthropi SV3.

D-Amino acid amidase (DAA) from Ochrobactrum anthropi SV3 catalyzes D-stereospecific hydrolysis of amino acid amides. DAA has attracted attention as a catalyst for the stereospecific production of D-amino acids, although the mechanism that drives the reaction has not been clear. Previously, the structure of DAA was classified into two types, a substrate-bound state with an ordered Omega loop, and a ground state with a disordered Omega loop. Because the binding of the substrate facilitates ordering, this transition was regarded to be induced fit motion. The angles and distances of hydrogen bonds at Tyr149 Oeta, Ser60 Ogamma and Lys63 Nzeta revealed that Tyr149 Oeta donates an H atom to a water molecule in the substrate-bound state, and that Tyr149 Oeta donates an H atom to Ser60 Ogamma or Lys63 Nzeta in the ground state. Taking into consideration the locations of the H atoms of Tyr149 Oeta, Ser60 Ogamma and Lys63 Nzeta, a catalytic mechanism of DAA activity is presented, wherein a shift of an H atom at Tyr149 Oeta in the substrate-bound versus the ground state plays a significant role in the reaction. This mechanism explains well why acylation proceeds and deacylation does not proceed in the substrate-bound state.

Structures of D-amino-acid amidase complexed with L-phenylalanine and with L-phenylalanine amide: insight into the D-stereospecificity of D-amino-acid amidase from Ochrobactrum anthropi SV3.

The crystal structures of D-amino-acid amidase (DAA) from Ochrobactrum anthropi SV3 in complex with L-phenylalanine and with L-phenylalanine amide were determined at 2.3 and 2.2 A resolution, respectively. Comparison of the L-phenylalanine amide complex with the D-phenylalanine complex reveals that the D-stereospecificity of DAA might be achieved as a consequence of three structural factors: (i) the hydrophobic cavity in the region in which the hydrophobic side chain of the substrate is held, (ii) the spatial arrangement of Gln310 O and Glu114 O epsilon2 that fixes the amino N atom of the substrate and (iii) the existence of two cavities that keep the carboxyl/amide group of the substrate near or apart from Ser60 O gamma.

New enzymatic method of chiral amino acid synthesis by dynamic kinetic resolution of amino acid amides: use of stereoselective amino acid amidases in the presence of alpha-amino-epsilon-caprolactam racemase.

D- and L-amino acids were produced from L- and D-amino acid amides by D-aminopeptidase from Ochrobactrum anthropi C1-38 and L-amino acid amidase from Pseudomonas azotoformans IAM 1603, respectively, in the presence of alpha-amino-epsilon-caprolactam racemase from Achromobacter obae as the catalyst by dynamic kinetic resolution of amino acid amides.

Crystal structure and functional characterization of a D-stereospecific amino acid amidase from Ochrobactrum anthropi SV3, a new member of the penicillin-recognizing proteins.

D-amino acid amidase (DAA) from Ochrobactrum anthropi SV3, which catalyzes the stereospecific hydrolysis of D-amino acid amides to yield the D-amino acid and ammonia, has attracted increasing attention as a catalyst for the stereospecific production of D-amino acids. In order to clarify the structure-function relationships of DAA, the crystal structures of native DAA, and of the D-phenylalanine/DAA complex, were determined at 2.1 and at 2.4 A resolution, respectively. Both crystals contain six subunits (A-F) in the asymmetric unit. The fold of DAA is similar to that of the penicillin-recognizing proteins, especially D-alanyl-D-alanine-carboxypeptidase from Streptomyces R61, and class C beta-lactamase from Enterobacter cloacae strain GC1. The catalytic residues of DAA and the nucleophilic water molecule for deacylation were assigned based on these structures. DAA has a flexible Omega-loop, similar to class C beta-lactamase. DAA forms a pseudo acyl-enzyme intermediate between Ser60 O(gamma) and the carbonyl moiety of d-phenylalanine in subunits A, B, C, D, and E, but not in subunit F. The difference between subunit F and the other subunits (A, B, C, D and E) might be attributed to the order/disorder structure of the Omega-loop: the structure of this loop cannot assigned in subunit F. Deacylation of subunit F may be facilitated by the relative movement of deprotonated His307 toward Tyr149. His307 N(epsilon2) extracts the proton from Tyr149 O(eta), then Tyr149 O(eta) attacks a nucleophilic water molecule as a general base. Gln214 on the Omega-loop is essential for forming a network of water molecules that contains the nucleophilic water needed for deacylation. Although peptidase activity is found in almost all penicillin-recognizing proteins, DAA lacks peptidase activity. The lack of transpeptidase and carboxypeptidase activities may be attributed to steric hindrance of the substrate-binding pocket by a loop comprised of residues 278-290 and the Omega-loop.

L: -Stereoselective amino acid amidase with broad substrate specificity from Brevundimonas diminuta: characterization of a new member of the leucine aminopeptidase family.

Brevundimonas diminuta TPU 5720 produces an amidase acting L-stereoselectively on phenylalaninamide. The enzyme (LaaA(Bd)) was purified to electrophoretic homogeneity by ammonium sulfate fractionation and four steps of column chromatography. The final preparation gave a single band on SDS-PAGE with a molecular weight of approximately 53,000. The native molecular weight of the enzyme was about 288,000 based on gel filtration chromatography, suggesting that the enzyme is active as a homohexamer. It had maximal activity at 50 degrees C and pH 7.5. LaaA(Bd) lost its activity almost completely on dialysis against potassium phosphate buffer (pH 7.0), and the amidase activity was largely restored by the addition of Co(2+) ions. The enzyme was, however, inactivated in the presence of ethylenediaminetetraacetic acid even in the presence of Co(2+), suggesting that LaaA(Bd) is a Co(2+)-dependent enzyme. LaaA(Bd) had hydrolyzing activity toward a broad range of L-amino acid amides including L-phenylalaninamide, L-glutaminamide, L-leucinamide, L-methioninamide, L-argininamide, and L-2-aminobutyric acid amide. Using information on the N-terminal amino acid sequence of the enzyme, the gene encoding LaaA(Bd) was cloned from the chromosomal DNA of the strain and sequenced. Analysis of 4,446 bp of the cloned DNA revealed the presence of seven open-reading frames (ORFs), one of which (laaA ( Bd )) encodes the amidase. LaaA(Bd) is composed of 491 amino acid residues (calculated molecular weight 51,127), and the deduced amino acid sequence exhibits significant similarity to that of ORFs encoding hypothetical cytosol aminopeptidases found in the genomes of Caulobacter crescentus, Bradyrhizobium japonicum, Rhodopseudomonas palustris, Mesorhizobium loti, and Agrobacterium tumefaciens, and leucine aminopeptidases, PepA, from Rickettsia prowazekii, Pseudomonas putida ATCC 12633, and Escherichia coli K-12. The laaA ( Bd ) gene modified in the nucleotide sequence upstream from its start codon was overexpressed in an E. coli transformant. The activity of the recombinant LaaA(Bd) in cell-free extracts of the E. coli transformant was 25.9 units mg(-1) with L-phenylalaninamide as substrate, which was 50 times higher than that of B. diminuta TPU 5720.