Restoration of a defective Lactococcus lactis xylose isomerase

The genes (xylA) encoding xylose isomerase (XI) from two Lactococcus lactis subsp. lactis strains, 210 (Xyl(-)) and IO-1 (Xyl(+)), were cloned, and the activities of their expressed proteins in recombinant strains of Escherichia coli were investigated. The nucleotide and amino acid sequence homologies between the xylA genes were 98.4 and 98.6%, respectively, and only six amino acid residues differed between the two XIs. The purified IO-1 XI was soluble with K(m) and k(cat) being 2.25 mM and 184/s, respectively, while the 210 XI was insoluble and inactive. Site-directed mutagenesis on 210 xylA showed that a triple mutant possessing R202M/Y218D/V275A mutations regained XI activity and was soluble. The K(m) and k(cat) of this mutant were 4.15 mM and 141/s, respectively. One of the IO-1 XI mutants, S388T, was insoluble and showed negligible activity similar to that of 210 XI. The introduction of a K407E mutation to the IO-1 S388T XI mutant restored its activity and solubility. The dissolution of XI activity in L. lactis subsp. lactis involves a series of mutations that collectively eliminate enzyme activity by reducing the solubility of the enzyme.     

Dissolution of Xylose Metabolism in Lactococcus lactis

Xylose metabolism, a variable phenotype in strains of Lactococcus lactis, was studied and evidence was obtained for the accumulation of mutations that inactivate the xyl operon. The xylose metabolism operon (xylRAB) was sequenced from three strains of lactococci. Fragments of 4.2, 4.2, and 5.4 kb that included the xyl locus were sequenced from L. lactis subsp. lactis B-4449 (formerly Lactobacillus xylosus), L. lactis subsp. lactis IO-1, and L. lactis subsp. lactis 210, respectively. The two environmental isolates, L. lactis B-4449 and L. lactis IO-1, produce active xylose isomerases and xylulokinases and can metabolize xylose. L. lactis 210, a dairy starter culture strain, has neither xylose isomerase nor xylulokinase activity and is Xyl⁻. Xylose isomerase and xylulokinase activities are induced by xylose and repressed by glucose in the two Xyl⁺ strains. Sequence comparisons revealed a number of point mutations in the xylA, xylB, and xylR genes in L. lactis 210, IO-1, and B-4449. None of these mutations, with the exception of a premature stop codon in xylB, are obviously lethal, since they lie outside of regions recognized as critical for activity. Nevertheless, either cumulatively or because of indirect affects on the structures of catalytic sites, these mutations render some strains of L. lactis unable to metabolize xylose.     

Characterization and kinetic analysis of enzyme–substrate recognition by three recombinant lactococcal tripeptidases

Tripeptidases from Lactococcus lactis subsp. lactis (L9PepTR), L. lactis subsp. cremoris (L6PepTR), and L. lactis subsp. hordniae (hTPepTR) were cloned, overexpressed, purified, and characterized. Although these enzymes contained three to seven naturally occurring amino acid differences, both metal-binding and catalytic sites were highly conserved. The k_cat values of hTPepTR were approximately 1.5- to 2-fold higher than those of L9PepTR, while, for L6PepTR, they were approximately 0.8- to 1.4-times the L9PepTR values. The K_m of tripeptidase from subsp. lactis (L9PepTR) was considerably larger when glycine was the amino acid located at both the N- and C-terminus of the peptide substrate. In addition, the K_m values of L9PepTR increased in the following order for YGG, LGG, FGG, SGG, and α-aminoisobutyrylglycylglycine, while the k_cat/K_m decreased in the same order. These results suggest that the dipole moment and steric hindrance of the N-terminal amino acid side chain may be the most important factors controlling substrate specificity.     

Gly or Ala substitutions for ProThrAsn at the β8-β9 turn of subtilisin Carlsberg increase the catalytic rate and decrease thermostability

A comparison of the primary structures among psychrophilic, mesophilic, and thermophilic subtilases revealed that the turn between the β8 and β9 strands (β8-β9 turn, BPN′ numbering) of psychrophilic subtilases are more flexible than those of their mesophilic and thermophilic counterparts. To investigate the relationship between structure of this turn and enzyme activity as well as thermostability of mesophilic subtilisin Carlsberg (sC), we analyzed 6 mutants of sC with a single, double, or triple Gly or Ala substitutions for Pro²¹⁰Thr²¹¹Asn²¹² at the β8-β9 turn. Among the single Gly substitutions, the P210G substitution most significantly (1.5-fold) increased the specific activity on N-succinyl-Ala-Ala-Pro-Phe-p-nitroanilide (AAPF) substrate and 12-fold decreased the thermostability. All mutants tested showed the increased k_cat for the AAPF substrate and reduced thermostability compared with the wild-type sC. The k_cat values of the P210G, P210G/T211G, and P210G/T211G/N212G mutants were 1.5-, 1.7-, and 1.8-fold higher than that of the wild-type sC. There were significant positive correlations between k_cat and thermal inactivation rates as well as k_cat and K_m of the wild-type and mutants. These results demonstrate that the structure of β8-β9 turn, despite its distance from the active site, has significant effects on the catalytic rate and thermostability of sC through a global network of intramolecular interactions and suggest that the lack of flexibility of this turn stabilizes the wild-type sC against thermal inactivation in compensation for some loss of catalytic activity.     

GH52 xylosidase from Geobacillus stearothermophilus: characterization and introduction of xylanase activity by site-directed mutagenesis of Tyr509

A xylosidase gene, gsxyn, was cloned from the deep-sea thermophilic Geobacillus stearothermophilus, which consisted of 2,118 bp and encoded a protein of 705 amino acids with a calculated molecular mass of 79.8 kDa. The GSxyn of glycoside hydrolase family 52 (GH52) displayed its maximum activity at 70 °C and pH 5.5. The K _m and k _cat values of GSxyn for ρNPX were 0.48 mM and 36.64 s^?1, respectively. Interestingly, a new exo-xylanase activity was introduced into GSxyn by mutating the tyrosine509 into glutamic acid, whereas the resultant enzyme variant, Y509E, retained the xylosidase activity. The optimum xylanase activity of theY509E mutant displayed at pH 6.5 and 50 °C, and retained approximately 45 % of its maximal activity at 55 °C, pH 6.5 for 60 min. The K _m and k _cat values of the xylanase activity of Y509E mutant for beechwood xylan were 5.10 mg/ml and 22.53 s^?1, respectively. The optimum xylosidase activity of theY509E mutant displayed at pH 5.5 and 60 °C. The K _m and k _cat values of the xylosidase activity of Y509E mutant for ρNPX were 0.51 mM and 22.53 s^?1, respectively. This report demonstrated that GH52 xylosidase has provided a platform for generating bifunctional enzymes for industrially significant and complex substrates, such as plant cell wall.     

Recombinant Deamidated Mutants of Erwinia chrysanthemi l-Asparaginase Have Similar or Increased Activity Compared to Wild-Type Enzyme

The enzyme Erwinia chrysanthemi l-asparaginase (ErA) is an important biopharmaceutical product used in the treatment of acute lymphoblastic leukaemia. Like all proteins, certain asparagine (Asn) residues of ErA are susceptible to deamidation to aspartic acid (Asp), which may be a concern with respect to enzyme activity and potentially to pharmaceutical efficacy. Recombinant ErA mutants containing Asn to Asp changes were expressed, purified and characterised. Two mutants with single deamidation sites (N41D and N281D) were found to have approximately the same specific activity (1,062 and 924 U/mg, respectively) as the wild-type (908 U/mg). However, a double mutant (N41D N281D) had an increased specific activity (1261 U/mg). The N41D mutation conferred a slight increase in the catalytic constant (k _cat 657 s^?1) when compared to the WT (k _cat 565 s^?1), which was further increased in the double mutant, with a k _cat of 798 s^?1. Structural analyses showed that the slight changes caused by point mutation of Asn₄₁ to Asp may have reduced the number of hydrogen bonds in this α-helical part of the protein structure, resulting in subtle changes in enzyme turnover, both structurally and catalytically. The increased α-helical content observed with the N41D mutation by circular dichroism spectroscopy correlates with the difference in k _cat, but not K _m. The N281D mutation resulted in a lower glutaminase activity compared with WT and the N41D mutant, however the N281D mutation also imparted less stability to the enzyme at elevated temperatures. Taken as a whole, these data suggest that ErA deamidation at the Asn₄₁ and Asn₂₈₁ sites does not affect enzyme activity and should not be a concern during processing, storage or clinical use. The production of recombinant deamidated variants has proven an effective and powerful means of studying the effect of these changes and may be a useful strategy for other biopharmaceutical products.     

OptZyme: Computational Enzyme Redesign Using Transition State Analogues

OptZyme is a new computational procedure for designing improved enzymatic activity (i.e., k_cat or k_cat/K_M) with a novel substrate. The key concept is to use transition state analogue compounds, which are known for many reactions, as proxies for the typically unknown transition state structures. Mutations that minimize the interaction energy of the enzyme with its transition state analogue, rather than with its substrate, are identified that lower the transition state formation energy barrier. Using Escherichia coli β-glucuronidase as a benchmark system, we confirm that K_M correlates (R² = 0.960) with the computed interaction energy between the enzyme and the para-nitrophenyl- β, D-glucuronide substrate, k_cat/K_M correlates (R² = 0.864) with the interaction energy of the transition state analogue, 1,5-glucarolactone, and k_cat correlates (R² = 0.854) with a weighted combination of interaction energies with the substrate and transition state analogue. OptZyme is subsequently used to identify mutants with improved K_M, k_cat, and k_cat/K_M for a new substrate, para-nitrophenyl- β, D-galactoside. Differences between the three libraries reveal structural differences that underpin improving K_M, k_cat, or k_cat/K_M. Mutants predicted to enhance the activity for para-nitrophenyl- β, D-galactoside directly or indirectly create hydrogen bonds with the altered sugar ring conformation or its substituents, namely H162S, L361G, W549R, and N550S.     

pH-rate profiles support a general base mechanism for galactokinase (Lactococcus lactis)

Galactokinase (GALK), a member the Leloir pathway for normal galactose metabolism, catalyzes the conversion of α-d-galactose to galactose-1-phosphate. For this investigation, we studied the kinetic mechanism and pH profiles of the enzyme from Lactococcus lactis. Our results show that the mechanism for its reaction is sequential in both directions. Mutant proteins D183A and D183N are inactive (<10 000 fold), supporting the role of Asp183 as a catalytic base that deprotonates the C-1 hydroxyl group of galactose. The pH-k_cat profile of the forward reaction has a pK_a of 6.9 ± 0.2 that likely is due to Asp183. The pH-k_cat/K_Gal profile of the reverse reaction further substantiates this role as it is lacking a key pK_a required for a direct proton transfer mechanism. The R36A and R36N mutant proteins show over 100-fold lower activity than that for the wild-type enzyme, thus suggesting that Arg36 lowers the pK_a of the C-1 hydroxyl to facilitate deprotonation.     

Solvent isotope and viscosity effects on the steady-state kinetics of the flavoprotein nitroalkane oxidase

The flavoprotein nitroalkane oxidase catalyzes the oxidative denitrification of a broad range of primary and secondary nitroalkanes to yield the respective aldehydes or ketones, hydrogen peroxide and nitrite. With nitroethane as substrate the ^D2O(k_cat/K_M) value is 0.6 and the ^D2Ok_cat value is 2.4. The k_cat proton inventory is consistent with a single exchangeable proton in flight, while the k_cat/K_M is consistent with either a single proton in flight in the transition state or a medium effect. Increasing the solvent viscosity did not affect the k_cat or k_cat/K_M value significantly, establishing that nitroethane binding is at equilibrium and that product release does not limit k_cat.     

Nickel(II) ion-catalyzed hydrolysis of acetyl esters of 2-pyridineoxime derivatives: Effects of geometry around central metal atom on the efficiency of metal ion catalysis

The kinetics of the Ni(II)-catalyzed ester hydrolysis of O-acetyl-2-pyridine-carboxaldoxime, O-acetyl-2-acetylpyridineketoxime, and O-acetyl-6-carboxy-2-pyridine-carboxaldoxime are measured and the values of various k_cat parameters are calculated for reaction paths involving one metal ion (k_cat^W and k_cat^OH) and two metal ions k_cat^A and k_cat^B). Examination of the kinetic data reveals that the k_cat^W and k_cat^OH paths for the Ni(II)-catalyzed reactions involve the same mechanism as those for the previously reported Cu(II)-catalyzed reactions. For the k_cat^A and k_cat^B paths, the mechanism involving binuclear Ni(II) ions is preferred by analogy with the previously reported Zn(II)-catalyzed reactions. Comparison of k_cat^OH values for the Cu(II)- and Ni(II)-catalyzed hydrolysis of 1–3 indicates that markedly different steric effects are exerted by the substituents of 2 and 3 on the catalytic behavior of the two metal ions. This is explained in terms of differences in the fit of the metal ions in the metal complexes of 1–3. Present results demonstrate that slight changes in the geometry around the central metal atom can affect the catalytic outcome significantly. The implications of the present results on metal substitution in metalloenzymes are also discussed.     

Genetic Evidence for a Defective Xylan Degradation Pathway in Lactococcus lactis

Genetic and biochemical evidence for a defective xylan degradation pathway was found linked to the xylose operon in three lactococcal strains, Lactococcus lactis 210, L. lactis IO-1, and L. lactis NRRL B-4449. Immediately downstream of the xylulose kinase gene (xylB) (K. A. Erlandson, J.-H. Park, W. El Khal, H.-H. Kao, P. Basaran, S. Brydges, and C. A. Batt, Appl. Environ. Microbiol. 66:3974–3980, 1999) are two open reading frames encoding a mutarotase (xylM) and a xyloside transporter (xynT) and a partial open reading frame encoding a β-xylosidase (xynB). These are functions previously unreported for lactococci or lactobacilli. The mutarotase activity of the putative xylM gene product was confirmed by overexpression of the L. lactis enzyme in Escherichia coli and purification of recombinant XylM. We hypothesize that the mutarotase links xylan degradation to xylose metabolism due to the anomeric preference of xylose isomerase. In addition, Northern hybridization experiments suggested that the xylM and xynTB genes are cotranscribed with the xylRAB genes, responsible for xylose metabolism. Although none of the three strains appeared to metabolize xylan or xylobiose, they exhibited xylosidase activity, and L. lactis IO-1 and L. lactis NRRL B-4449 had functional mutarotases.     

Evaluation of kinetic data: What the numbers tell us about PRMTs

Protein arginine N-methyltransferase (PRMT) kinetic parameters have been catalogued over the past fifteen years for eight of the nine mammalian enzyme family members. Like the majority of methyltransferases, these enzymes employ the highly ubiquitous cofactor S-adenosyl-l-methionine as a co-substrate to methylate arginine residues in peptidic substrates with an approximately 4-μM median K_M. The median values for PRMT turnover number (k_cat) and catalytic efficiency (k_cat/K_M) are 0.0051 s⁻¹ and 708 M⁻¹ s⁻¹, respectively. When comparing PRMT metrics to entries found in the BRENDA database, we find that while PRMTs exhibit high substrate affinity relative to other enzyme-substrate pairs, PRMTs display largely lower k_cat and k_cat/K_M values. We observe that kinetic parameters for PRMTs and arginine demethylase activity from dual-functioning lysine demethylases are statistically similar, paralleling what the broader enzyme families in which they belong reveal, and adding to the evidence in support of arginine methylation reversibility.     

Characterisation of the DNA-dependent ATPase activity of human DNA topoisomerase IIbeta: mutation of Ser165 in the ATPase domain reduces the ATPase activity and abolishes the in vivo complementation ability

We report for the first time an analysis of the ATPase activity of human DNA topoisomerase (topo) IIβ. We show that topo IIβ is a DNA-dependent ATPase that appears to fit Michaelis–Menten kinetics. The ATPase activity is stimulated 44-fold by DNA. The k_cat for ATP hydrolysis by human DNA topo IIβ in the presence of DNA is 2.25 s^–1. We have characterised a topo IIβ derivative which carries a mutation in the ATPase domain (S165R). S165R reduced the k_cat for ATP hydrolysis by 7-fold, to 0.32 s^–1, while not significantly altering the apparent K_m. The specificity constant for the interaction between ATP and topo IIβ (k_cat/K_mapp) showed a 90% reduction for βS165R. The DNA binding affinity and ATP-independent DNA cleavage activity of the enzyme are unaffected by this mutation. However, the strand passage activity is reduced by 80%, presumably due to reduced ATP hydrolysis. The mutant enzyme is unable to complement ts yeast topo II in vivo. We have used computer modelling to predict the arrangement of key residues at the ATPase active site of topo IIβ. Ser165 is predicted to lie very close to the bound nucleotide, and the S165R mutation could thus influence both ATP binding and ADP dissociation.     

Engineering GST M2-2 for high activity with indene 1,2-oxide and indication of an H-site residue sustaining catalytic promiscuity

The substrate-binding H-site of human glutathione transferase (GST) M2-2 was subjected to iterative saturation mutagenesis in order to obtain an efficient enzyme with the novel epoxide substrate indene 1,2-oxide. Residues 10, 116, and 210 were targeted, and the activities with the alternative substrates, benzyl isothiocyanate and the prodrug azathioprine, undergoing divergent chemical reactions were monitored for comparison. In general, increased activities were found when the smaller residues Gly, Ser, and Ala replaced the original Thr210. The most active mutant T210G was further mutated at position 116, but no mutant showed enhanced catalytic activity. However, saturation mutagenesis of position 10 identified one double mutant T210G/I10C with 100-fold higher specific activity with indene 1,2-oxide than wild-type GST M2-2. This enhanced epoxide activity of 50 μmol min^− 1 mg^− 1 resulted primarily from an increased k_cat value (70 s^− 1). The specific activity is 24-fold higher than that of wild-type GST M1-1, which is otherwise the most proficient GST enzyme with epoxide substrates. A second double mutant T210G/I10W displayed 30-fold increased activity with azathioprine, 0.56 μmol min^− 1 mg^− 1. In both double mutants, the replacement of Ile10 led to narrowed acceptance of alternative substrates. Ile10 is evolutionarily conserved in related class Mu GSTs. Conservation usually indicates preservation of a particular function, and in the Mu class, it would appear that the conserved Ile10 is not necessary to maintain catalytic functions but to prevent loss of broad substrate acceptance. In summary, our data underscore the facile transition between alternative substrate selectivity profiles in GSTs by a few mutations.     

Cloning,Heterologous Expression,Purification and Characterization of M12 Mutant of Aspergillus niger Glucose Oxidase in Yeast Pichia pastoris KM71H

Aspergillus niger glucose oxidase (GOx) genes for wild-type (GenBank accession no. X16061, swiss-Prot; P13006) and M12 mutant (N2Y, K13E, T30 V, I94 V, K152R) were cloned into pPICZαA vector for expression in Pichia pastoris KM71H strain. The highest expression level of 17.5 U/mL of fermentation media was obtained in 0.5 % (v/v) methanol after 9 days of fermentation. The recombinant GOx was purified by cross-flow ultrafiltration using membranes of 30 kDa molecular cutoff and DEAE ion-exchange chromatography at pH 6.0. Purified wt GOx had k _cat of 189.4 s^?1 and K _m of 28.26 mM while M12 GOx had k _cat of 352.0 s^?1 and K _m of 13.33 mM for glucose at pH 5.5. Specificity constants k _cat/K _m of wt (6.70 mM^?1 s^?1) and M12 GOx (26.7 mM^?1 s^?1) expressed in P. pastoris KM71H were around three times higher than for the same enzymes previously expressed in Saccharomyces cerevisiae InvSc1 strain. The pH optimum and sugar specificity of M12 mutant of GOx remained similar to the wild-type form of the enzyme, while thermostability was slightly decreased. M12 GOx expressed in P. pastoris showed three times higher activity compared to the wt GOx toward redox mediators like N,N-dimethyl-nitroso-aniline used for glucose strips manufacturing. M12 mutant of GOx produced in P. pastoris KM71H could be useful for manufacturing of glucose biosensors and biofuel cells.     

Engineering Streptomyces clavuligerus Deacetoxycephalosporin C Synthase for Optimal Ring Expansion Activity toward Penicillin G

The deacetoxycephalosporin C synthase (DAOCS) from Streptomyces clavuligerus was engineered with the aim of enhancing the conversion of penicillin G into phenylacetyl-7-aminodeacetoxycephalosporanic acid, a precursor of 7-aminodeacetoxycephalosporanic acid, for industrial application. A single round of random mutagenesis followed by the screening of 5,500 clones identified three mutants, G79E, V275I, and C281Y, that showed a two- to sixfold increase in the k_cat/K_m ratio compared to the wild-type enzyme. Site-directed mutagenesis to modify residues surrounding the substrate resulted in three mutants, N304K, I305L, and I305M, with 6- to 14-fold-increased k_cat/K_m values. When mutants containing all possible combinations of these six sites were generated to optimize the ring expansion activity for penicillin G, the double mutant, YS67 (V275I, I305M), showed a significant 32-fold increase in the k_cat/K_m ratio and a 5-fold increase in relative activity for penicillin G, while the triple mutant, YS81 (V275I, C281Y, I305M), showed an even greater 13-fold increase in relative activity toward penicillin G. Our results demonstrate that this is a robust approach to the modification of DAOCS for an optimized DAOCS-penicillin G reaction.     

Engineering the substrate-binding domain of an esterase enhances its hydrolytic activity toward fatty acid esters

The poor solubility and dispersibility of fatty acids in aqueous reaction media may limit the catalytic activity of fatty acid transformation enzymes. Therefore, we studied a novel method to increase the catalytic activity of an esterase by introducing a presumed substrate-binding domain. The primary structure of an esterase from Pseudomonas fluorescens WI SIK (PFEI) is similar to that of an esterase in P. fluorescens DSM 50106 (PFEII) but not Bacillus subtilis DSM 402 (BS2). However, the reaction kinetics for the formation of octylacetate and a ricinoleic acid-derived ester (3) were more similar to the kinetics in BS2. For instance, the k_cat value of PFEI with 3 was similar to that of BS2, which was approximately 12-fold lower than the k_cat value of PFEII. Furthermore, fusion of PFEI to the N-terminal hydrophobic domain of PFEII led to a substantial increase (an approximate 6-fold increase in the k_cat value) in its hydrolytic activity of 3. These results indicate that the N-terminal domain of PFEII, which is assumed to be involved in anchoring the enzyme in the membrane, interacts with fatty acid-like substrates, resulting in an improved enzymatic activity. Therefore, we conclude that the membrane-anchoring domains can be used to increase the catalytic activity of fatty acid transformation enzymes.     

The purification and kinetic properties of biophosphoglycerate synthase from horse red blood cells.

Bisphosphoglycerate synthase from horse red cells has been purified to apparent homogeneity by a simple and efficient new procedure incorporating chromatography on a column of Sepharose 4B derivatized with blue dextran. The enzyme is similar to the human red cell synthase in subunit size. It is phosphorylated by either glycerate-1,3-P₂ or glycerate-2,3-P₂ to form a phosphoenzyme with the acid-lability of a histidyl phosphate. In addition to the synthase activity (glycerate-1,3-P₂ → glycerate-2,3-P₂), k_cat 12.5 s^?1, the enzyme has bisphosphoglycerate phosphatase activity in the presence of glycolate-2-P (glycerate-2,3-P₂ → glycerate-P + P_i), k_cat 2.6 s^?1 and phosphoglycerate mutase activity (3-PGA ? 2-PGA), k_cat 1.7 s^?1. The energy of activation for the synthase reaction is 9.38 kcal/mol. Lineweaver-Burk plots of the kinetic data are parallel lines. In contrast intersecting patterns were obtained from similar experiments done with the human red cell enzyme. Further investigation is required to explain these differences. This enzyme may function as both synthase and phosphatase for bisphosphoglycerate in the red blood cell.     

Engineering steroid hormone specificity into aldo-keto reductases

Steroid hormone transforming aldo-keto reductases (AKRs) include virtually all mammalian 3α-hydroxysteroid dehydrogenases (3α-HSDs), 20α-HSDs, as well as the 5β-reductases. To elucidate the molecular determinants of steroid hormone recognition we used rat liver 3α-HSD (AKR1C9) as a starting structure to engineer either 5β-reductase or 20α-HSD activity. 5β-Reductase activity was introduced by a single point mutation in which the conserved catalytic His (H117) was mutated to Glu117. The H117E mutant had a k_cat comparable to that for homogeneous rat and human liver 5β-reductases. pH versus k_cat profiles show that this mutation increases the acidity of the catalytic general acid Tyr55. It is proposed that the increased TyrOH₂⁺ character facilitates enolization of the Δ⁴-3-ketosteroid and subsequent hydride transfer to C5. Since 5β-reductase precedes 3α-HSD in steroid hormone metabolism it is likely that this metabolic pathway arose by gene duplication and point mutation. 3α-HSD is positional and stereospecific for 3-ketosteroids and inactivates androgens. The enzyme was converted to a robust 20α-HSD, which is positional and stereospecific for 20-ketosteroids and inactivates progesterone, by the generation of loop-chimeras. The shift in log₁₀(k_cat/K_m) from androgens to progestins was of the order of 10¹¹. This represents a rare example of how steroid hormone specificity can be changed at the enzyme level. Protein engineering with predicted outcomes demonstrates that the molecular determinants of steroid hormone recognition in AKRs will be ultimately rationalized.     

Cloning, Expression, and Enzymatic Characterization of Isocitrate Dehydrogenase from Helicobacter pylori

Isocitrate dehydrogenase (IDH) catalyzes the oxidative decarboxylation of isocitrate to α-ketoglutarate with NAD(P) as a cofactor in the tricarboxylic acid cycle. As a housekeeping protein in Helicobacter pylori, IDH was considered as a possible candidate for serological diagnostics and detection. Here, we identified a new icd gene encoding IDH from H. pylori strain SS1. The recombinant H. pylori isocitrate dehydrogenase (HpIDH) was cloned, expressed, and purified in E. coli system. The enzymatic characterization of HpIDH demonstrates its activity with k _cat of 87 s^?1, K _m of 124 μM and k _cat/K _m of 7 × 10⁵ M^?1s^?1 toward isocitrate, k _cat of 80 s^?1, K _m of 176 μM and k _cat/K _m of 4.5 × 10⁵ M^?1s^?1 toward NADP. The optimum pH of the enzyme activity is around 9.0, and the optimum temperature is around 50 °C. This current work is expected to help better understand the features of HpIDH and provide useful information for H. pylori serological diagnostics and detection.