Involvement of conserved histidine, lysine and tyrosine residues in the mechanism of DNA cleavage by the caspase-3 activated DNase CAD

The caspase-activated DNase (CAD) is involved in DNA degradation during apoptosis. Chemical modification of murine CAD with the lysine-specific reagent 2,4,6-trinitrobenzenesulphonic acid and the tyrosine-specific reagent N-acetylimidazole leads to inactivation of the nuclease, indicating that lysine and tyrosine residues are important for DNA cleavage by this enzyme. The presence of DNA or the inhibitor ICAD-L protects the enzyme from modification. Amino acid substitution in murine CAD of lysines and tyrosines conserved in CADs from five different species leads to variants with little if any catalytic activity, but unaltered DNA binding (K155Q, K301Q, K310Q, Y247F), with the exception of Y170F, which retains wild-type activity. Similarly, as observed for the previously characterised H242N, H263N, H308N and H313N variants, the newly introduced His→Asp/Glu or Arg exchanges lead to variants with <1% of wild-type activity, with two exceptions: H313R shows wild-type activity, and H308D at pH 5.0 exhibits ~5% of wild-type activity at this pH. Y170F and H313R produce a specific pattern of fragments, different from wild-type CAD, which degrades DNA non-specifically. The recombinant nuclease variants produced in Escherichia coli were tested for their ability to form nucleolytically active oligomers. They did not show any significant deviation from the wild-type enzyme. Based on these and published data possible roles of the amino acid residues under investigation are discussed.     

Alternative proton donors/acceptors in the catalytic mechanism of the glutathione reductase of Escherichia coli: the role of histidine-439 and tyrosine-99

The cloned Escherichia coli gor gene encoding the flavoprotein glutathione reductase was placed under the control of the tac promoter in the plasmid pKK223-3, allowing expression of glutathione reductase at levels approximately 40,000 times those of untransformed cells. This greatly facilitated purification of the enzyme. By directed mutagenesis of the gor gene, His-439 was changed to glutamine (H439Q) and alanine (H439A). The tyrosine residue at position 99 was changed to phenylalanine (Y99F), and in another experiment, the H439Q and Y99F mutations were united to form the double mutant Y99FH439Q. His-439 is thought to act in the catalytic mechanism as a proton donor/acceptor in the glutathione-binding pocket. The H439Q and H439A mutants retain approximately 1% and approximately 0.3%, respectively, of the catalytic activity of the wild-type enzyme. This reinforces our previous finding [Berry et al. (1989) Biochemistry 28, 1264-1269] that direct protonation and deprotonation of the histidine residue are not essential for the reaction to occur. The retention of catalytic activity by the H439A mutant demonstrates further that a side chain capable of hydrogen bonding to a water molecule, which might then act as proton donor, also is not essential at this position. Tyr-99 is a further possible proton donor in the glutathione-binding pocket, but the Y99F mutant was essentially fully active, and the Y99FH439Q double mutant also retained approximately 1% of the wild-type specific activity.(ABSTRACT TRUNCATED AT 250 WORDS)     

Mutational analysis of the iron binding site of Saccharomyces cerevisiae ferroxidase Fet3. An in vivo study.

The role of residues predicted to be involved in the binding of iron by the yeast ferroxidase Fet3 has been studied by site-directed mutagenesis. The effect of Fet3 mutations E185A, E185Q, Y354F, D409V and H489D has been investigated in vivo by kinetic analyses of high affinity iron uptake. Our results indicate that Glu-185 is critical for the binding of iron, since substitution of this residue with Ala or Gln strongly affects both growth and the kinetic parameters of high affinity iron uptake, greatly increasing K(m). Mutations Y354F and D409V result in less severe alteration of high affinity iron uptake, while mutant H489D is unable to grow under conditions of iron limitation.     

Phenylmethanesulfonyl fluoride inactivates an archaeal superoxide dismutase by chemical modification of a specific tyrosine residue. Cloning, sequencing and expression of the gene coding for Sulfolobus solfataricus superoxide dismutase.

The gene encoding the superoxide dismutase from the hyperthermophilic archaeon Sulfolobus solfataricus (SsSOD) was cloned and sequenced and its expression in Escherichia coli obtained. The chemicophysical properties of the recombinant SsSOD were identical with those of the native enzyme. The recombinant SsSOD possessed a covalent modification of Tyr41, already observed in native SsSOD [Ursby, T., Adinolfi, B.S., Al-Karadaghi, S., De Vendittis, E. & Bocchini, V. (1999) J. Mol. Biol. 286, 189--205]. HPLC analysis of SsSOD samples prepared from cells treated or not with phenylmethanesulfonyl fluoride (PhCH(2)SO(2)F), a protease inhibitor routinely added during the preparation of cell-free extracts, showed that the modification was caused by PhCH(2)SO(2)F. Refinement of the crystal model of SsSOD confirmed that a phenylmethanesulfonyl moiety was attached to the hydroxy group of Tyr41. PhCH(2)SO(2)F behaved as an irreversible inactivator of SsSOD; in fact, the specific activity of both native and recombinant enzyme decreased as the percentage of modification increased. The covalent modification caused by PhCH2SO2F reinforced the heat stability of SsSOD. These results show that Tyr41 plays an important role in the enzyme activity and the maintenance of the structural architecture of SsSOD.     

Mutational analysis of the thyrotropin-releasing hormone-degrading ectoenzyme. similarities and differences with other members of the M1 family of aminopeptidases and thermolysin

Thyrotropin-releasing hormone-degrading ectoenzyme (TRH-DE) is a TRH-specific peptidase which catalyzes the inactivation of the peptidergic signal substance TRH. As indicated by sequence alignment, TRH-DE and the other members of the M1 family of aminopeptidases have a distinct set of conserved amino acid residues in common. By replacing amino acid residues that are putatively involved in catalysis, we could demonstrate that the enzymatic activities of the mutants E408D, E442D, E464Q, E464D, Y528F, H507R, and H507F are dramatically decreased, essentially due to the changes of V(max). The mutant enzymes E408Q and E442Q are inactive, whereas the specific enzymatic activity of the mutants R488Q, R488A, and Y554F are similar to that of the wild-type enzyme. These data strongly suggest that E408, E442, Y528, and H507 are involved in the catalytic process of TRH-DE while E464 presumably represents the third zinc-coordinating residue and may be equivalent to E166 in thermolysin. In contrast, amino acid residues R488 and Y554 seem not to be involved in the catalytic mechanism of TRH-DE.     

Human ferrochelatase: characterization of substrate-iron binding and proton-abstracting residues

The terminal step in heme biosynthesis, the insertion of ferrous iron into protoporphyrin IX to form protoheme, is catalyzed by the enzyme ferrochelatase (EC 4.99.1.1). A number of highly conserved residues identified from the crystal structure of human ferrochelatase as being in the active site were examined by site-directed mutagenesis. The mutants Y123F, Y165F, Y191H, and R164L each had an increased K(m) for iron without an altered K(m) for porphyrin. The double mutant R164L/Y165F had a 6-fold increased K(m) for iron and a 10-fold decreased V(max). The double mutant Y123F/Y191F had low activity with an elevated K(m) for iron, and Y123F/Y165F had no measurable activity. The mutants H263A/C/N, D340N, E343Q, E343H, and E343K had no measurable enzyme activity, while E343D, E347Q, and H341C had decreased V(max)s without significant alteration of the K(m)s for either substrate. D340E had near-normal kinetic parameters, while D383A and H231A had increased K(m)s for iron. On the basis of these data and the crystal structure of human ferrochelatase, it is proposed that residues E343, H341, and D340 form a conduit from H263 in the active site to the protein exterior and function in proton extraction from the porphyrin macrocycle. The role of H263 as the porphyrin proton-accepting residue is central to catalysis since metalation only occurs in conjunction with proton abstraction. It is suggested that iron is transported from the exterior of the enzyme at D383/H231 via residues W227 and Y191 to the site of metalation at residues R164 and Y165 which are on the opposite side of the active site pocket from H263. This model should be general for mitochondrial membrane-associated eucaryotic ferrochelatases but may differ for bacterial ferrochelatases since the spatial orientation of the enzyme within prokaryotic cells may differ.     

Outer sphere mutations perturb metal reactivity in manganese superoxide dismutase

Tyrosine 34 and glutamine 146 are highly conserved outer sphere residues in the mononuclear manganese active site of Escherichia coli manganese superoxide dismutase. Biochemical and spectroscopic characterization of site-directed mutants has allowed functional characterization of these residues in the wild-type (wt) enzyme. X-ray crystallographic analysis of three mutants (Y34F, Q146L, and Q146H) reveal subtle changes in the protein structures. The Y34A mutant, as well as the previously reported Y34F mutant, retained essentially the full superoxide dismutase activity of the wild-type enzyme, and the X-ray crystal structure of Y34F manganese superoxide dismutase shows that mutation of this strictly conserved residue has only minor effects on the positions of active site residues and the organized water in the substrate access funnel. Mutation of the outer sphere solvent pocket residue Q146 has more dramatic effects. The Q146E mutant is isolated as an apoprotein lacking dismutase activity. Q146L and Q146H mutants retain only 5-10% of the dismutase activity of the wild-type enzyme. The absorption and circular dichroism spectra of the Q146H mutant resemble corresponding data for the superoxide dismutase from a hyperthermophilic archaeon, Pyrobaculum aerophilum, which is active in both Mn and Fe forms. Interestingly, the iron-substituted Q146H protein also exhibits low dismutase activity, which increases at lower pH. Mutation of glutamine 146 disrupts the hydrogen-bonding network in the active site and has a greater effect on protein structure than does the Y34F mutant, with rearrangement of the tyrosine 34 and tryptophan 128 side chains.     

Active site residues in Mycobacterium tuberculosis pantothenate synthetase required in the formation and stabilization of the adenylate intermediate

Pantothenate synthetase (EC 6.3.2.1) catalyzes the formation of pantothenate from ATP, D-pantoate, and beta-alanine in bacteria, yeast, and plants. The three-dimensional structural determination of pantothenate synthetase from Mycobacterium tuberculosis has indicated specific roles for His44, His47, Asn69, Gln72, Lys160, and Gln164 residues in the binding of substrates and the pantoyl adenylate intermediate. To evaluate the functional roles of these strictly conserved residues, we constructed six Ala mutants and determined their catalytic properties. The substitution of alanine for H44, H47, N69, Q72, and K160 residues in M. tuberculosis pantothenate synthetase caused a greater than 1000-fold reduction in enzyme activity, while the Q164A mutant exhibited 50-fold less activity. The rate of the isolated adenylation reaction in single turnover studies was also reduced 40-1000-fold by the replacement of one of these six amino acids with alanine, suggesting that these residues are essential for the formation of the pantoyl adenylate intermediate. The rate of pantothenate formation from the adenylate and beta-alanine in the second half reaction could not be measured for the H44A, H47A, N69A, Q72A, and K160A mutants and was reduced 40-fold in the Q164A mutants. The activity of the K160C mutant enzyme was markedly enhanced by the alkylation of cysteine with bromoethylamine, further supporting the critical role of the K160 residue in pantoyl adenylate formation. Isothermal titration microcalorimetry analysis demonstrated that the substitution of either H47 or K160 for Ala resulted in a decreased affinity of the enzyme for ATP. These results indicate that the highly conserved His44, His47, Asn69, Gln72, Lys160 and residues are essential for the formation and stabilization of pantoyl adenylate intermediate in the pantothenate synthetase reaction.     

Increased alkali stability in Trichoderma reesei endo-1, 4-beta-xylanase II by site directed mutagenesis

A number of engineered Trichoderma reesei endo-beta-1,4-xylanase (Xyn II) mutants were created and activity tests were performed for increased stability. The stability of the earlier characterized mutant Y5 (T2C, T28C, K58R, +191D) was further increased by the mutations creating the constructs P9 (N97R+F93W+H144K), P12 (H144C+N92C), P15 (F180Q+H144C+N92C) and P21 (H22K+F180Q+H144C+N92C). The resistance towards thermal inactivation at alkaline pH was increased in all of the mutants. Residual activity T(50%) was increased 4-5 degrees C for P9 at pH 9. The performance of the P9 mutant in sulphate pulp bleaching was also tested and was shown to increase brightness markedly compared to the reference. The bleaching results showed the industrial potential of the obtained mutant.     

Pronounced conversion of the metal-specific activity of superoxide dismutase from Porphyromonas gingivalis by the mutation of a single amino acid (Gly155Thr) located apart from the active site

Glycine 155, which is located approximately 10 A from the active metal sites, is mostly conserved in aligned amino acid sequences of manganese-specific superoxide dismutases (Mn-SODs) and cambialistic SOD (showing the same activity with Fe and Mn) from Porphyromonas gingivalis, but is substituted for threonine in most Fe-SODs. Since Thr155 is located between Trp123 and Trp125, and Trp123 is one member of the metal-surrounding aromatic amino acids, there is a possibility that the conversion of this amino acid may cause a conversion of the metal-specific activity of cambialistic P. gingivalis SOD. To clarify this possibility, we have prepared a mutant of the P. gingivalis SOD with conversion of Gly155 to Thr. The ratios of the specific activities of Fe- to Mn-reconstituted enzyme, which are measured by the xanthine oxidase/cytochrome c method, increased from 0.6 in the wild-type to 11.2 in the mutant SODs, indicating the conversion of the metal-specific activity of the enzyme from a cambialistic type to an Fe-specific type. The visible absorption spectra of the Fe- and Mn-reconstituted mutant SODs closely resembled those of Fe-specific SOD. Furthermore, the EPR spectra of the Fe- and Mn-reconstituted mutant SODs also closely resembled those of Fe-specific SOD. Three-dimensional structures of the Fe-reconstituted wild-type SOD and Mn-reconstituted mutant SOD have been determined at 1.6 A resolution. Both structures have identical conformations, orientations of residues involved in metal binding, and hydrogen bond networks, while the side chain of Trp123 is moved further toward the metal-binding site than in wild-type SOD. A possible contribution of the structural differences to the conversion of the metal-specific activity through rearrangement of the hydrogen bond network among Trp123, Gln70, Tyr35, and the metal-coordinated solvent is discussed.     

Mechanism of FAD reduction and role of active site residues His-225 and Tyr-259 in Arthrobacter globiformis dimethylglycine oxidase: analysis of mutant structure and catalytic function

Residues His-225 and Tyr-259 are located close to the FAD in the dehydrogenase active site of the bifunctional dimethylglycine oxidase (DMGO) of Arthrobacter globiformis. We have suggested [Leys, D., Basran, J., and Scrutton, N. S. (2003) EMBO J. 22, 4038-4048] that these residues are involved in abstraction of a proton from the substrate amine group of dimethylglycine prior to C-H bond breakage and FAD reduction. To investigate this proposal, we have isolated two mutant forms of DMGO in which (i) His-225 is replaced with Gln-225 (H225Q mutant) and (ii) Tyr-259 is replaced with Phe-259 (Y259F mutant). Both mutant enzymes retain the ability to oxidize substrate, but the steady-state turnover of the Y259F mutant is attenuated more than 200-fold. Only modest changes in kinetic parameters are observed for the H225Q mutant during steady-state turnover. Stopped-flow studies indicate that the rate of FAD reduction in the Y259F enzyme is substantially impaired by a factor of approximately 1500 compared with that of the wild-type enzyme, suggesting a key role for this residue in the reductive half-reaction of the enzyme. The kinetics of FAD reduction in the H225Q enzyme are complex and involve three discrete kinetic phases that are attributed to different conformational states of this mutant, evidence for which is provided by crystallographic analysis. Neither the H225Q enzyme nor the Y259F enzyme stabilizes the FADH(2)-iminium charge-transfer complex observed previously in stopped-flow studies with the wild-type enzyme. Our studies are consistent with a key role for Tyr-259, but not His-225, in deprotonation of the substrate amine group prior to FAD reduction. We infer that residue His-225 is likely to modulate the acid-base properties of Tyr-259 by perturbing the pK(a) of Tyr-259 and thus fine-tunes the reaction chemistry to facilitate proton abstraction under physiological conditions. Our data are discussed in the context of the crystallographic data for DMGO and also in relation to contemporary mechanisms for flavoprotein-catalyzed oxidation of amine substrates.     

Alteration of the Substrate Specificity of the Angular Dioxygenase Carbazole 1,9a-Dioxygenase

Carbazole 1,9a-dioxygenase (CARDO) consists of terminal oxygenase (CARDO-O) and electron transport components. CARDO can catalyze specific oxygenation for various substrates: angular dioxygenation for carbazole and dibenzo-p-dioxin, lateral dioxygenation for anthracene, and monooxygenation for methylene carbon of fluorene and sulfide sulfur of dibenzothiophene. To elucidate the molecular mechanism determining its unique substrate specificity, 17 CARDO-O site-directed mutants at amino acid residues I262, F275, Q282, and F329, which form the substrate-interacting wall around the iron active site by CARDO-O crystal structure, were generated and characterized. F329 replacement dramatically reduced oxygenation activity. However, several mutants produced different products from the wild-type enzyme to a large extent: I262V and Q282Y (1-hydroxycarbazole), F275W (4-hydroxyfluorene), F275A (unidentified cis-dihydrodiol of fluoranthene), and I262A and I262W (monohydroxydibenzothiophenes). These results suggest the possibility that the respective substrates bind to the active sites of CARDO-O mutants in a different orientation from that of the wild-type enzyme.     

Catalytic and DNA-binding properties of the human Ogg1 DNA N-glycosylase/AP lyase: biochemical exploration of H270, Q315 and F319, three amino acids of the 8-oxoguanine-binding pocket

The human Ogg1 protein (hOgg1) is an antimutator DNA glycosylase/AP lyase that catalyzes the excision of 8-oxo-7,8-dihydroguanine (8-oxoG) and the incision of apurinic and apyrimidinic (AP) sites in DNA. In this study, we have investigated the functional role of H270, Q315 and F319, three amino acids that are located in the 8-oxoG-binding pocket of hOgg1. Wild-type and mutant hOgg1 proteins (H270A, H270R, H270L, Q315A and F319A) were purified to apparent homogeneity. The catalytic activities and the DNA-binding properties of the various hOgg1 mutants were compared to those of the wild-type. The results show that hOgg1 mutated at H270 (H270A and H270L) or F319 (F319A) exhibits greatly reduced (50- to 1000-fold) DNA glycosylase activity, whereas the AP lyase activity is only moderately affected (<4-fold). The affinity of the hOgg1 mutants (H270A, H270L and F319A) for 8-oxoG.C-containing DNA is also greatly reduced (>30-fold), whereas their affinity for THF.C-containing DNA is only moderately reduced (<7-fold). The results also show that hOgg1 mutated at Q315 (Q315A) exhibits catalytic and DNA-binding properties similar to those of the wild-type. Therefore, H270 and F319 are essential to form the functional 8-oxoG-binding pocket, whereas Q315 is less crucial. In contrast, H270, Q315 and F319 are not required for efficient binding of THF.C and cleavage of AP sites. Finally, hOgg1 mutant proteins with a substitution of H270A or F319A are members of a new type of hOgg1 that is deficient in DNA glycosylase but proficient in AP lyase.     

Directed evolution of Streptomyces clavuligerus deacetoxycephalosporin C synthase for enhancement of penicillin G expansion

The deacetoxycephalosporin C synthase from Streptomyces clavuligerus was directly modified for enhancement of penicillin G expansion into phenylacetyl-7-aminodeacetoxycephalosporanic acid, an important intermediate in the industrial manufacture of cephalosporin antibiotics. Nine new mutants, mutants M73T, T91A, A106T, C155Y, Y184H, M188V, M188I, H244Q, and L277Q with 1.4- to 5.7-fold increases in the kcat/Km ratio, were obtained by screening 6,364 clones after error-prone PCR-based random mutagenesis. Subsequently, DNA shuffling was carried out to screen possible combinations of substitutions, including previous point mutations. One quaternary mutant, the C155Y/Y184H/V275I/C281Y mutant, which had a kcat/Km ratio that was 41-fold higher was found after 10,572 clones were assayed. The distinct mutants obtained using different mutagenesis methods demonstrated the complementarity of the techniques. Interestingly, most of the mutated residues that result in enhanced activities are located within or near the unique small barrel subdomain, suggesting that manipulation of this subdomain may be a constructive strategy for improvement of penicillin expansion. Several mutations had very distinct effects on expansion of penicillins N and G, perhaps due to different penicillin-interacting modes within the enzyme. Thus, the present study provided not only promising enzymes for cephalosporin biosynthesis but also a large number of mutants, which provided new insights into the structure-function relationship of the protein that should lead to further rational engineering.     

Role of permissive neuraminidase mutations in influenza A/Brisbane/59/2007-like (H1N1) viruses

Neuraminidase (NA) mutations conferring resistance to NA inhibitors were believed to compromise influenza virus fitness. Unexpectedly, an oseltamivir-resistant A/Brisbane/59/2007 (Bris07)-like H1N1 H275Y NA variant emerged in 2007 and completely replaced the wild-type (WT) strain in 2008-2009. The NA of such variant contained additional NA changes (R222Q, V234M and D344N) that potentially counteracted the detrimental effect of the H275Y mutation on viral fitness. Here, we rescued a recombinant Bris07-like WT virus and 4 NA mutants/revertants (H275Y, H275Y/Q222R, H275Y/M234V and H275Y/N344D) and characterized them in vitro and in ferrets. A fluorometric-based NA assay was used to determine Vmax and Km values. Replicative capacities were evaluated by yield assays in ST6Gal1-MDCK cells. Recombinant NA proteins were expressed in 293T cells and surface NA activity was determined. Infectivity and contact transmission experiments were evaluated for the WT, H275Y and H275Y/Q222R recombinants in ferrets. The H275Y mutation did not significantly alter Km and Vmax values compared to WT. The H275Y/N344D mutant had a reduced affinity (Km of 50 vs 12 μM) whereas the H275Y/M234V mutant had a reduced activity (22 vs 28 U/sec). In contrast, the H275Y/Q222R mutant showed a significant decrease of both affinity (40 μM) and activity (7 U/sec). The WT, H275Y, H275Y/M234V and H275Y/N344D recombinants had comparable replicative capacities contrasting with H275Y/Q222R mutant whose viral titers were significantly reduced. All studied mutations reduced the cell surface NA activity compared to WT with the maximum reduction being obtained for the H275Y/Q222R mutant. Comparable infectivity and transmissibility were seen between the WT and the H275Y mutant in ferrets whereas the H275Y/Q222R mutant was associated with significantly lower lung viral titers. In conclusion, the Q222R reversion mutation compromised Bris07-like H1N1 virus in vitro and in vivo. Thus, the R222Q NA mutation present in the WT virus may have facilitated the emergence of NAI-resistant Bris07 variants.     

Thermodynamic analyses of the catalytic pathway of F1-ATPase from Escherichia coli. Implications regarding the nature of energy coupling by F1-ATPases

Thermodynamic properties of 12 different F1-ATPase enzymes were analyzed in order to gain insights into the catalytic mechanism and the nature of energy coupling to delta mu H+. The enzymes were normal soluble Escherichia coli F1, a group of nine beta-subunit mutant soluble E. coli F1 enzymes (G142S, K155Q, K155E, E181Q, E192Q, M209I, D242N, D242V, R246C), and both soluble and membrane-bound bovine heart mitochondrial F1. Unisite activity was studied by use of Gibbs free energy diagrams, difference energy diagrams, and derivation of linear free energy relationships. This allowed construction of binding energy diagrams for both the unisite ATP hydrolysis and ATP synthesis reaction pathways, which were in agreement. The binding energy diagrams showed that the step of Pi binding is a major energy-requiring step in ATP synthesis, as is the step of ATP release. It is suggested that there are two major catalytic enzyme conformations, and ATP- and an ADP-binding conformation. The effects of the mutations on the rate-limiting steps of multisite as compared to unisite activity were correlated, suggesting a direct link between the rate-limiting steps of the two types of activity. Multisite activity was analyzed by Arrhenius plots and by study of relative promotion from unisite to multisite rate. Changes in binding energy due to mutation were seen to have direct effects on multisite catalysis. From all the data, a model is derived to describe the mechanism of ATP synthesis. ATP hydrolysis, and energy coupling to delta mu H+ in F1F0-ATPases.     

Enhancement of thermostability of fungal deglycating enzymes by directed evolution

Fructosyl peptide oxidases are valuable for the determination of glycoproteins such as hemoglobin A1c. For practical use in clinical diagnosis, we applied directed evolution to improve the thermostability of these enzymes. After two rounds of random mutagenesis and high-throughput screening, six thermostabilizing amino acid substitutions were identified. Therefore, site-directed and cassette mutageneses were applied to combine these six stabilizing mutations. The simultaneous mutants showed that the stabilizing effect of the amino acid replacement was cumulative. The sextuple mutant enzyme, R94K/G184D/F265L/N272D/H302R/H388Y, had a half-life of thermal inactivation at 50°C that was 79.8-fold longer than that of the parental fructosyl peptide oxidase. The thermostable variants also showed increased tolerance to digestion by a protease. The sextuple mutant enzyme did not lose its activity on incubation with neutral protease, while the wild-type enzyme almost completely lost its activity. Furthermore, three amino acid substitutions were introduced into another fructosyl peptide oxidase with a different substrate specificity. The half-life of inactivation at 50°C was 3.61-fold longer than that of the parent enzyme. These engineered fructosyl peptide oxidases will be useful for industrial application to clinical diagnosis.     

Critical role of the residue size at position 87 in H2O2- dependent substrate hydroxylation activity and H2O2 inactivation of cytochrome P450BM-3

Replacement of phenylalanine 87 with alanine or glycine (mutant F87A or F87G) greatly increased the H2O2-supported substrate hydroxylation activity of cytochrome P450BM-3, whose original H2O2-supported activity is hardly detectable. On the other hand, replacement of phenylalanine 87 with valine (mutant F87V) did not. In the oxidation of p-nitrophenoxydodecanoic acid (12-pNCA), the turnover numbers of the mutant F87A in the presence of NADPH and O2, or H2O2 were 493 and 162 nmol/min/nmol, respectively. The H2O2-supported F87A hydroxylation activity was further confirmed with free fatty acids as substrates. Moreover, the stability of F87A in H2O2 solutions also largely increased. The order of the stability of the wild type (WT), F87A, and their substrate (12-pNCA)-binding complexes in H2O2 solutions listed from high to low was F87A, WT, F87A substrate-binding complex, and WT substrate-binding complex. We propose that the free space size in the vicinity of the heme iron significantly influences P450BM-3 H2O2-supported activity and H2O2 inactivation.     

Catalytic ability and stability of two recombinant mutants of D-amino acid transaminase involved in coenzyme binding.

Of the major amino acid side chains that anchor pyridoxal 5'-phosphate at the coenzyme binding site of bacterial D-amino acid transaminase, two have been substituted using site-directed mutagenesis. Thus, Ser-180 was changed to an Ala (S180A) with little effect on enzyme activity, but replacement of Tyr-31 by Gln (Y31Q) led to 99% loss of activity. Titration of SH groups of the native Y31Q enzyme with DTNB proceeded much faster and to a greater extent than the corresponding titration for the native wild-type and S180A mutant enzymes. The stability of each mutant to denaturing agents such as urea or guanidine was similar, i.e., in their PLP forms, S180A and Y31Q lost 50% of their activities at a 5-15% lower concentration of urea or guanidine than did the wild-type enzyme. Upon removal of denaturing agent, significant activity was restored in the absence of added pyridoxal 5'-phosphate, but addition of thiols was required. In spite of its low activity, Y31Q was able to form the PMP form of the enzyme just as readily as the wild-type and the S180A enzymes in the presence of normal D-amino acid substrates. However, beta-chloro-D-alanine was a much better substrate and inactivator of the Y31Q enzyme than it was for the wild-type or S180A enzymes, most likely because the Y31Q mutant formed the pyridoxamine 5-phosphate form more rapidly than the other two enzymes. The stereochemical fidelity of the Y31Q recombinant mutant enzyme was much less than that of the S180A and wild-type enzymes because racemase activity, i.e., conversion of L-alanine to D-alanine, was higher than for the wild-type or S180A mutant enzymes, perhaps because the coenzyme has more flexibility in this mutant enzyme.