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1.
Pons Jaume; Planas Antoni; Querol Enrique 《Protein engineering, design & selection : PEDS》1995,8(9):939-945
Bacillus 1,3-1,4-ß-glucanases possess a highly conserveddisulfide bridge connecting a ß-strand with a solventexposedloop lying on top of the extended binding site cleft The contributionof the disulfide bond and of both individual cysteines (Cys61and Cys90) in the Bacillus licheniformis enzyme to stabilityand activity has been evaluated by protein engineering methods.Reduction of the disulfide bond has no effect on kinetic parameters,has only a minor effect on the activity-temperature profileat high temperatures, and destabilizes the protein by less than0.7 kcal/mol as measured by equilibrium urea denatu ration at37°C. Replacing either of the Cys residues with Ala destabilizesthe protein and lowers the specific activity. C90A retains 70%of wild-type (wt) activity (in terms of Vmax), whereas C61Aand the double mutant C61AC90A have 10% of wt Vmax. Alarger change in free energy of unfolding is seen by equilibriumurea denaturation for the C61A mutation (loop residue, 3.2 kcal/molrelative to reduced wt) as compared with the C90A mutation (ß-strandresidue, 1.8 kcal/mol relative to reduced wt), while the doublemutant C61AC90A is 0.8 kcal/mol less stable than thesingle C61A mutant. The effects on stability are interpretedas a result of the change in hydrophobic packing that occursupon removal of the sulfur atoms in the Cys to Ala mutations 相似文献
2.
The consequences of engineering an extra disulfide bond in the Penicillium camembertii mono-and diglyceride specific lipase 总被引:2,自引:0,他引:2
Yamaguchi S.; Takeuchi K.; Mase T.; Oikawa K.; McMullen T.; Derewenda U.; McElhaney R.N.; Kay C.M.; Derewenda Z.S. 《Protein engineering, design & selection : PEDS》1996,9(9):789-795
The extracellular lipase from Penicillium camembertii has uniquesubstrate specificity restricted to mono- and diglycerides.The enzyme is a member of a homologous family of lipases fromfilamentous fungi. Four of these proteins, from the fungi Rhizomucormiehei, Humicola lanuginosa, Rhizopus delemar and P.camembertii,have had their structures elucidated by X-ray crystallography.In spite of pronounced sequence similarities the enzymes exhibitsignificant differences. For example, the thermo-stability ofthe P.camembertii lipase is considerably lower than that ofthe H.lanuginosa enzyme. Since only the P.camembertii enzymelacks the characteristic long disulfide bridge, correspondingto Cys22Cys268 in the H.lanuginosa lipase, we have engineeredthis disulfide into the former enzyme in the hope of obtaininga significantly more stable fold. The properties of the doublemutant (Y22C and G269C) were assessed by a variety of biophysicaltechniques. The extra disulfide link was found to increase themelting temperature of the protein from 51 to 63°C. However,no difference is observed under reducing conditions, indicatingan intrinsic instability of the new disulfide. The optimal temperaturefor catalytic activity decreased by 10°C and the optimumpH was shifted by 0.7 units to more acidic. 相似文献
3.
Stabilization of lysozyme against irreversible inactivation by alterations of the Asp-Gly sequences 总被引:3,自引:0,他引:3
Tomizawa Hideyuki; Yamada Hidenori; Hashimoto Yoshio; Imoto Taiji 《Protein engineering, design & selection : PEDS》1995,8(10):1023-1028
Site-directed mutagenesis was performed at Asp-Gly (4849,6667, 101102) and Asn-Gly (103104) sequencesof hen egg-white lysozyme to protect the enzyme against irreversiblethermoinactivation. Because the lysozyme inactivation was causedby the accumulation of multiple chemical reactions, includingthe isomerization of the Asp-Gly sequence and the deamidationof Asn [Tomizawa et al.(1994) Biochemistry, 33, 1303213037],the suppression of these reactions by the substitution of Glyto Ala, or the introduction of a sequence of human-type lysozyme,was attempted and the mutants (where each or all labile sequenceswere replaced) were prepared. The substitution resulted in thereversible destabilization from 1 to 2 kcal/mol per substitution.The destabilization was caused by the introduction of ß-carbonto the constrained position that had conformational angles withinthe allowed range for the Gly residue. Despite the decreasein the reversible conformational stability, the mutants hadmore resistance to irreversible inactivation at pH 4 and 100°C.In particular, the rate of irreversible inactivation of themutant, which was replaced at four chemically labile sequences,was the latest and corresponded to 18 kcal/mol of the reversibleconformational stability. Therefore, replacement of the chemicallylabile sequence was found to be more effective at protectingenzymes against irreversible thermoinactivation than at strengtheningreversible conformational stability. 相似文献
4.
5.
Jimenez M.A.; Arnold J.R.P.; Andrews J.; Thomas J.A.; Roberts G.C.K.; Birdsall B.; Feeney J. 《Protein engineering, design & selection : PEDS》1989,2(8):627-631
The complex of Lactobacillus casei dihydrofolate reductase withthe substrate folate and the coenzyme NADP* has been shown toexist in solution as a mixture of three slowly interconvertingconformations whose proportions are pH-dependent and which differin the orientation of the pteridine ring of the substrate inthe binding site. The Asp26 Asn mutant of L. casei dihydrofolatereductase has been prepared by oligonucleotide-directed mutagenesisand studied by one-and two-dimensional 1H-NMR spectroscopy.NMR studies of the mutant enzymefolateNADP* complexshow that this exists to > 90% in a single conformation overthe pH* range 57.1. The single conformation observedcorresponds to conformation I (the methotrexate-likeconformation) of the wild-type enzymefolateNADP*complex. These observations demonstrate that Asp26 is the ionizablegroup controlling the pH-dependence of the conformational equilibriumseen in the wild-type enzyme. 相似文献
6.
Peter W.; Dunkel R.; Stouten P.F.W.; Vriend G.; Beato M.; Suske G. 《Protein engineering, design & selection : PEDS》1992,5(4):351-359
The progesteronebinding protein uteroglobin has beenexpressed in Escherichia coli in an unfused, soluble form. likemature uteroglobin from rabbit endometrium (UG), the E.coliproduceduteroglobin (UG1) dimerizes in vitro, forms an antiparalleldimer with Cys3Cys69' and Cys69Cys3' disulfidebonds and binds progesterone under reducing conditions. In orderto analyze the dimerization and the reduction dependence ofprogesterone binding in more detail, we separately replacedcysteine 3 and cysteine 69 by serines. Under reducing conditions,both uteroglobin variants (UGl3Ser and UGl69Ser)bind progesterone with the same affinity as the wildtypesuggesting that both cysteine residues are not directly involvedin progesterone binding. In contrast to the wildtypeprotein, both cysteine variants also bind progesterone withhigh affinity in the absence of reducing agents. In addition,UGl-3Ser and UGl-69Ser both form covalently linked homodimers.Thus, unnatural Cys6969' and Cys33' disulfidebonds exist in UG13Ser and UG169Ser, respectively.These data together with computer models based on X-ray diffractiondata strongly support the idea that progesterone reaches itsbinding site located in an internal hydrophobic cavity via ahydrophobic tunnel along helices 1 and 4. Under nonreducingconditions the tunnel is closed by two disulfide bridges (Cys3Cys69'(and Cys69Cys3') that lie in the most flexible regionof the dimer. Reduction or replacement of a cysteine residueenables conformational changes that open the channel allowingprogesterone to enter. 相似文献
7.
McDowall Sharon; Heeswijck Robyn van; Hoogenraad Nicholas 《Protein engineering, design & selection : PEDS》1990,4(1):73-77
We have investigated the putative carbamylphosphate- and ornithine-bindingdomains in ornithine transcarbamylase from rat liver using site-directedmutagenesis. Arg60, present in the phosphate-binding motif X-Ser-X-Arg-Xand therefore implicated in the binding of the phosphate moietyof carbamylphosphate has been replaced with a leucine. Thisresults in a dramatic reduction of catalytic activity, althoughthe enzyme is synthesized in cells stably transfected with themutant clone and imported, correctly processed and assembledinto a homotrimer in mitochondria. The sole cysteine residue(Cys271) has been implicated in ornithine binding by the chemicalmodification studies of Marshall and Cohen in 1972 and 1980(J. Biol. Chem., 247, 16541668, 16691682; 255,72917295, 72967300). Replacement of this residuewith serine did not eliminate enzyme activity but affected theMichaelis constant for ornithine (Kb, increasing it 5-fold from0.71 to 3.7 mM and reduced the kcat at pH 8.5 by 20-fold. Thesechanges represent a loss in apparent binding energy for theenzyme - ornithine complex of 2.9 kcal/mol, suggesting thatCys271 is normally involved in hydrogen bonding to the substrate,ornithine. The cysteine to serine substitution also caused thedissociation constant (Kä for the competitive inhibitor,L-norvaline to be increased 10-fold, from 12 to 120 µM.The small loss in binding energy and relatively high residualcatalytic activity of the mutant strongly suggests that a numberof other residues are involved in the binding of ornithine.The effect of replacement of Cys271 with serine was restrictedto the ornithine binding site of the enzyme since both the bindingconstant for carbamyl-phosphate (Kia) and Michaelis constant(Ka) were not appreciably different for mutant and wild-typeenzymes. The pH optimum of the wild-type enzyme (8.6) is increasedto > 9.6 in the Ser271 mutant. 相似文献
8.
Turner S.L.; Ford G.C.; Mountain A.; Moir A. 《Protein engineering, design & selection : PEDS》1992,5(6):535-541
The moderate thermophile Bacillus stearothermophilus was usedas a host in which to detect more thermostable variants of theB.pumilus chloramphenicol acetyltransferase (Cat-86) protein.Seventeen mutants were isolated and detected by their abilityto grow in the presence of chloramphenicol at a previously restrictivetemperature (58°C). The genes encoding these proteins weresequenced; all 17 mutants carried the same C to T transitionthat conferred an amino acid substitution of alanine by valineat position 203 of the protein sequence. The wild-type and onemutant Cat-86 protein were purified to homogeneity using affinitychromatography, and kinetic and thermal stability studies wereundertaken. Both enzymes had similar sp. act. in the regionof 215 U/mg, with Km values for chloramphenicol in the range13.815.4 µM and for acetyl CoA in the range 13.615.5µM. The A203V mutant shows greater stability than thewild-type Cat-86 protein at temperatures above 50°C andappears to pass through a transition state between 48 and 50°C. 相似文献
9.
Grunert Hans-Peter; Landt Olfert; Zirpel-Giesebrecht Maria; Backmann Jan; Heinemann Udo; Saenger Wolfram; Hahn Ulrich 《Protein engineering, design & selection : PEDS》1993,6(7):739-744
Using point mutated overproducing strains of E.coli, ribonucleaseT1 was prepared with the single substitutions Tyr24Trp, Tyr42Trp,Tyr45Trp or Trp59Tyr and the corresponding double substitutionsTyr24Trp/Trp59Tyr, Tyr42Trp/Trp59Tyr and Tyr45Trp/Trp59Tyr.Steady state kinetics of the transesterification reaction forthe two dinucleoside monophosphate substrates guanylyl-3', 5'-cytidineand guanylyl-3', 5'-adenosine indicate that the tryptophan canbe introduced in different positions within the ribonucleaseT1 molecule without abolishing enzymatic activity. The Trp59Tyrexchange even enhances catalysis of the cleavage reaction (kcat/Km)relative to the wild type enzyme and similar effects are foundwith single tyrosine to tryptophan substitutions. For the pHdependencies of the guanylyl-3', 5'-cytidine transesterificationreaction of wild type ribonuclease T1 and of the variants, typicallybell-shaped curves are observed with a plateau in the rangepH 4.57.0. Their shapes and slopes indicate that theenzymes are comparable in their macroscopic pKa, values. AtpH 7.5, the variant Tyr45Trp/Trp59Tyr shows a more than 3-foldhigher transesterification activity for guanylyl-3', 5'-adenosineand a 2-fold increase for guanylyl-3', 5'-cytidine comparedto the wild type enzyme, i.e. this variant catalyses the transesterificationof the substrate guanylyl-3', 5'-adenosine with the same orbetter efficiency as guanylyl-3', 5'-cytidine. 相似文献
10.
Sierks Michael R.; Ford Clark; Reilly Peter J.; Svensson Birte 《Protein engineering, design & selection : PEDS》1989,2(8):621-625
Trpl20 of Aspergillus awamori glucoamylase has previously beenshown by chemical modification to be essential for activityand tentatively to be located near subsite 4 of the active site.To further test its role, restriction sites were inserted inthe cloned A.awamori gene around the Trpl20 coding region, andcassette mutagenesis was used to replace it with His, Leu, Pheand Tyr. All four mutants displayed 2% or less of the maximalactivity (kcat) of wild-type glucoamylase towards maltose andmaltoheptaose. MichaelLs constants (KM) of mutants decreased2- to 3-fold for maltose and were essentially unchanged formaltoheptaose compared with the wild type, except for a >3-fold decrease for maltoheptaose with the Trp120 Tyrmutant. This mutant also bound isomaltose more strongly andhad more selectivity for its hydrolysis than wild-type glucoamylase.A subsite map generated from malto-oligosaecharide substrateshaving 2 7 D-glucosyl residues indicated that subsites1 and 2 had greater affinity for D-glucosyl residues in theTrp120 Tyr mutant than in wild-type glucoamylase. Theseresults suggest that Trpl20 from a distant subsite is crucialfor the stabilization of the transition-state complex in subsites1 and 2. 相似文献
11.
Site-specific mutagenesis was employed to study structure-functionrelationships at the substrate binding site of rat tissue kallikrein.Four kallikrein mutants, the Pro219 deletion (P219del), the3438 loop Tyr-Tyr-Phe-Gly to Ile-Asn mutation [YYFG(3438)IN],the Trp215Gly exchange (W215G) and the double mutant with Tyr99Hisand Trp215Gly exchange (Y99H:W215G) were created by site-directedmutagenesis to probe their function in substrate binding. Themutant proteins were expressed in Esclzerichia coli at highlevels and analyzed by Western blot. These mutant enzymes werepurified to apparent homogeneity. Each migrated as a singleband on SDS-PAGE, with slightly lower molecular mass (36 kDa)than that of the native enzyme, (38 kDa) because of their lackof glycosylation. The recombinant kallikreins are immunologicallyidentical to the native enzyme, displaying parallelism withthe native enzyme in a direct radioimmunoassay for rat tissuekallikrein. Kinetic analyses of Km and kcat using fluorogenicpeptide substrates support the hypothesis that the Tyr99Trp215interaction is a major determinant for hydrophobic P2 specificity.The results suggest an important role for the 3438 loopin hydrophobic P3 affinity and further show that Pro219 is essentialto substrate binding and efficient catalysis of tissue kallikrein. 相似文献
12.
Allemann Rudolf K.; Presnell Scott R.; Benner Steven A. 《Protein engineering, design & selection : PEDS》1991,4(7):831-835
A comparison of the sequences of three homologous ribonucleases(RNase A, angiogenin and bovine seminal RNase) identifies threesurface loops that are highly variable between the three proteins.Two hypotheses were contrasted: (i) that this variation mightbe responsible for the different catalytic activities of thethree proteins; and (ii) that this variation is simply an exampleof surface loops undergoing rapid neutral divergence in sequence.Three hybrids of angiogenin and bovine pancreatic ribonuclease(RNase) A were prepared where regions in these loops taken fromangiogenin were inserted into RNase A. Two of the three hybridshad unremarkable catalytic properties. However, the RNase Amutant containing residues 6374 of angiogenin had greatlydiminished catalytic activity against uridylyl-(3' 5')-adenosine(UpA), and slightly increased catalytic activity as an inhibitorof translation in vitro. Both catalytic behaviors are characteristicof angiogenin. This is one of the first examples of an engineeredexternal loop in a protein. Further, these results are complementaryto those recently obtained from the complementary experiment,where residues 5970 of RNase were inserted into angiogenin[Harper and Vallee (1989) Biochemistry, 28, 18751884].Thus, the external loop in residues 6374 of RNase A appearsto behave, at least in part, as an interchangeable modulethat influences substrate specificity in an enzyme in a waythat is isolated from the influences of other regions in theprotein. 相似文献
13.
Secretion of recombinant human IgE-Fc by mammalian cells and biological activity of glycosylation site mutants 总被引:5,自引:0,他引:5
Young Robert J.; Owens Raymond J.; Mackay Graham A.; Chan Christina M. W.; Shi Jianguo; Hide Michihiro; Francis David M.; Henry Alistair J.; Sutton Brian J.; Gould Hannah J. 《Protein engineering, design & selection : PEDS》1995,8(2):193-199
We have constructed an expression vector that leads to secretionof the whole Fc of human immunoglobulin E (hIgE-Fc) from mammaliancells at levels up to 100 mg/l of culture. Two surface glycosylationsites at Asn265 and Asn371 have been changed to glutamine, toobtain a more homogeneous preparation of hIgE-Fc for structuralstudies. Comparison of wild-type and mutant products revealedthat Asn371 is rarely glycosylated in Chinese hamster ovarycells. Both the double mutant and wild-type hIgEFc bind to thehigh-affinity IgE receptor, FcRI, with about the same affinityas myeloma IgE (Ka in the range 10101011 M1),and were able to sensitize isolated human basophils for anti-IgEtriggering of histamine release. However, only the double mutanthIgE-Fc approached the affinity of myeloma IgE for the low-affinityreceptor, FcRII (Ka = 7.3x107 M1), whereas the wild-type hIgE-Fc bound with a 10-fold lower affinity (Ka = 4.1x106M1). 相似文献
14.
Eijsink Vincent G.H.; Vriend Gerrit; van den Burg Bertus; van der Zee J.Rob; Veltman Oene R.; Stulp Ben K.; Venema Gerard 《Protein engineering, design & selection : PEDS》1992,5(2):157-163
A 10 residue ß-hairpin, which is characteristic ofthermostable Bacillus neutral proteases, was engineered intothe thermolabile neutral protease of Bacillus subtilis. Therecipient enzyme remained fully active after introduction ofthe loop. However, the mutant protein exhibited autocatalyticnicking and a 0.4°C decrease in thermostability. Two additionalpoint mutations designed to improve the interactions betweenthe enzyme surface and the introduced ß-hairpin resultedin reduced nicking and increased thermostability. After theintroduction of both additional mutations in the loopcontainingmutant, nicking was largely prevented and an increase in thermostabilityof 1.1°C was achieved. 相似文献
15.
Simons Guus; van den Heuvel Wim; Reynen Theo; Frijters Adri; Rutten Ger; Slangen Charles J.; Groenen Martien; de Vos Willem M.; Siezen Roland J. 《Protein engineering, design & selection : PEDS》1993,6(7):763-770
A cDNA clone containing the entire coding region for bovineß-casein A3 flanked by 53 base pairs of 5' non-codingand 358 base pairs of 3' non-coding sequences was isolated froma bovine mammary cDNA phagemid library. The coding segment formature ß-casein was subcloned into the T7 expressionsystem, in which the expression of recombinant ß-caseinwas controlled by the T7 gene 10 promoter and ribosome bindingsite. High level expression of Met-ß-casein to 20%of the total soluble proteins was obtained in Escherichia coliwithin 2 h after induction of T7 RNA-polymerase synthesis. Inan attempt to induce secretion the coding segment for matureß-casein was coupled to the ompA translations initiationsignal and signal peptide coding sequence but no secretion ofthe fusion protein and no processing of the signal peptide fromthe fusion protein was observed. Instead, the Met-ß-caseincould be isolated in asoluble form from E.coli cells after anosmotic shock, indicative of a periplasmic location. This proceduredid not lyse the cells. The protein was purified to homogeneityafter a pH 4.8 isoelectric precipitation followed by reversed-phasehigh-performance liquid chromatography. The ß-caseincDNA was altered to change the main chymosin cleavage siteinß-casein at position 192193 in two ways, namelyfrom LeuTyr to ProPro and to Leustop. Thesemutations were designed to prevent generation of the bitterpeptide ßcasein(193209) by chymosin cleavage.The mutant Met-ß-caseins were expressed in E.colito the same level as wild-type Met-ß-casein. Purifiedmutant Met-ß-casein(Prol92 Prol93) was no longerhydrolysed by chymosin at the 192193 bond. 相似文献
16.
Ohkuri Takatoshi; Ueda Tadashi; Tsurumaru Miyako; Imoto Taiji 《Protein engineering, design & selection : PEDS》2001,14(11):829-833
We prepared two dissected fragments of hen lysozyme and examinedwhether or not these two fragments associated to form a native-likestructure. One (Fragment I) is the peptide fragment Asn59homoserine-105containing Cys64Cys80 and Cys76Cys94. The other(Fragment II) is the peptide fragment Lys1homoserine-58connected by two disulfide bridges, Cys6Cys127 and Cys30Cys115,to the peptide fragment Asn106Leu129. It was found thatthe Fragment I immobilized in the cuvette formed an equimolarcomplex with Fragment II (Kd = 3.3x104 M at pH 8 and25°C) by means of surface plasmon resonance. Moreover, fromanalyses by circular dichroism spectroscopy and ion-exchangechromatography of the mixture of Fragments I and II at pH 8under non-reducing conditions, it was suggested that these fragmentsassociated to give the native-like structure. However, the mutantFragment I in which Cys64Cys80 and Cys76Cys94are lacking owing to the mutation of Cys to Ala, or the mutantfragment in which Trp62 is mutated to Gly, did not form thenative-like species with Fragment II, because the mutant FragmentI derived from mutant lysozymes had no local conformation dueto mutations. Considering our previous results where the preferentialoxidation of two inside disulfide bonds, Cys64Cys80 andCys76Cys94, occurred in the refolding of the fully reducedFragment I, we suggest that the peptide region correspondingto Fragment I is an initiation site for hen lysozyme folding. 相似文献
17.
FK506-binding protein mutational analysis: defining the active-site residue contributions to catalysis and the stability of ligand complexes 总被引:2,自引:0,他引:2
DeCenzo Maureen T.; Park Steven T.; Jarrett Beth P.; Aldape Robert A.; Futer Olga; Murcko Mark A.; Livingston David J. 《Protein engineering, design & selection : PEDS》1996,9(2):173-180
The 12 kDa FK506-binding protein FKBP12 is a cis-trans peptidyl-prolylisomerase that binds the macrolides FK506 and rapamycin. Wehave examined the role of the binding pocket residues of FKBP12in proteinligand interactions by making conservativesubstitutions of 12 of these residues by site-directed mutagenesis.For each mutant FKBP12, we measured the affinity for FK506 andrapamycin and the catalytic efficiency in the cistranspeptidyl-prolyl isomerase reaction. The mutation of Trp59 orPhe99 generates an FKBP12 with a significantly lower affinityfor FK506 than wild-type protein. Tyr26 and Tyr82 mutants areenzymatically active, demonstrating that hydrogen bonding bythese residues is not required for catalysis of the cistranspeptidyl-prolyl isomerase reaction, although these mutationsalter the substrate specificity of the enzyme. We conclude thathydrophobic interactions in the active site dominate in thestabilization of FKBP12 binding to macrolide ligands and tothe twisted-amide peptidyl-prolyl substrate intermediate. 相似文献
18.
Sierks Michael R.; Ford Clark; Reilly Peter J.; Svensson Birte 《Protein engineering, design & selection : PEDS》1993,6(1):75-79
Fungal glucoamylases contain four conserved regions. One regionfrom the Aspergillus niger enzyme contains three key carboxylicacid residues, the general acid catalytic group, Glu179, alongwith Asp176 and Glu180. Three site-directed mutations, Leu177 His, Trp178 Arg and Asn182 Ala, wereconstructed near these acidic groups to reveal the functionof other conserved residues in this region. Leu177 and Trp178are strictly conserved among fungal glucoamylases, while anamide, predominantly Asn, always occurs at position 182. Substitutionsof Leu177 or Trp178 cause significant decreases in kcat withthe substrates tested. Similar increases in activation energiesobtained with Leu177 His with both -(1,4)- and -(1,6)-linkedsubstrates indicate Leu177 is located in subsite 1. KM valuesobtained with the Trp178 Arg mutation increase for an-(1,6)-linked substrate, but not for -(1,4)-linked substrates.Calculated differences in activation energy between substratesindicate Trp178 interacts specifically with subsite 2. The Asn182 Ala mutation did not change kcat or KM values, indicating thatAsn182 is not crucial for activity. These results support amechanism for glucoamylase catalytic activity consisting ofa fast substrate binding step followed by a conformational changeat subsite 1 to stabilize the transition state complex. 相似文献
19.
Francescutti D.; Baldwin J.; Lee L.; Mutus B. 《Protein engineering, design & selection : PEDS》1996,9(2):189-194
The catalytic properties of glutathione reductase for its substrate,glutathione disulfide, were altered following a 60 s exposureto a 100-fold molar excess of peroxynitrite; the KM value wasincreased by {small tilde}2.5-fold and the Vmax value was decreasedby {small tilde}1.7-fold. The kinetic alterations are thoughtto result from nitrotyrosine formation as the intrinsic Tyrfluorescence is diminished. The UV-visible spectrum of glutathionereductase exhibited absorbance at {small tilde}423 nm, characteristicof nitrotyrosine. In addition, the presence of nitrotyrosinehas been detected by Western immunoblots with an anti-nitrotyrosineantibody. The peroxynitrite-induced inactivation is not observedin the presence of excess glutathione disulfide. However, excessNADPH offered no protection against peroxynitriteinduced inactivation.These observations suggest that the modification of {small tilde}1.8Tyr per subunit, at or near the glutathione disulfide bindingdomain, probably results in the observed catalytic alterations.To test this hypothesis, the two tyrosines closest to the glutathionedisulfide binding domain (Tyr114 and Tyr106), as indicated bythe X-ray crystallographic data [Karplus and Schulz (1989) J.Biol. Chem, 210,163180], were each converted to nitrotyrosinesby molecular modeling and the structure energy was minimized.These theoretical calculations indicate that the bond lengthsbetween Tyr114-O and the Gly-N and Cys II-N of glutathione disulfidebound to glutathione reductase (Karplus and Schulz, 1989) increasedby 3.0 and 4.3 Å, respectively, upon nitration. In thecase of Tyr106, the OCys II-N distance also increasesby {small tilde}1.6 Å. The loss of these hydrogen bondingcontacts is likely to result in the observed catalytic alterationsupon reaction with peroxy-nitrite. 相似文献
20.
Wang Xing-guo; Britton K.Linda; Baker Patrick J.; Martin Stephen; Rice David W.; Engel Paul C. 《Protein engineering, design & selection : PEDS》1995,8(2):147-152
Two residues, K89 and S380, thought to interact with the -carboxylgroup of the substrate L-glutamate, have been altered by site-directedmutagenesis of clostridial glutamate dehydrogenase (GDH). Thesingle mutants K89L and S380V and the combined double mutantK89L/S380V were constructed. All three mutants were satisfactorilyoverproduced in soluble form. However, only the K89L mutantwas retained by the dye column normally used in purifying thewild-type enzyme. All three mutant enzymes were purified tohomogeneity and tested for substrate specificity with 24 aminoacids. The single mutant S380V showed no detectable activity.The alternative single mutant K89L showed an activity towardsL-glutamate that was decreased nearly 2000-fold compared withwild-type enzyme, whereas the activities towards the monocarboxylicsubstrates -aminobutyrate and norvaline were increased 2- to3-fold. A similar level of activity was obtained with methionine(0.005 U/mg) and norleucine (0.012 U/mg), neither of which giveany activity with the wild-type enzyme under the same conditions.The double mutant showed decreased activity with all substratescompared with the wild-type GDH. In view of its novel activities,the K89L mutant was investigated in greater detail. A strictlylinear relationship between reaction velocity and substrateconcentration was observed up to 80 mM L-methionine and 200mM L-norleucine, implying very high Km values. Values of kcat/Km,for L-methionine and L-norleucine were 6.7x102 and 0.15s1M1, respectively. Measurements with dithiobisnitrobenzoicacid showed that the mutant enzymes all reacted with a stoichiometryof one -SH group per subunit and all showed protection by coenzyme,indicating essentially unimpaired coenzyme binding. With glutamateor 2-oxoglutarate as substrate the Km values for the vestigialactivity in the mutant enzyme preparations were strikingly closeto the wild-type Km values. Both for wild-type GDH and K89L,L-glutamate gave competitive product inhibition of 2-oxoglutaratereduction but did not inhibit the reduction of 2-oxocaproatecatalysed by K89L enzyme. This suggests that the low levelsof glutamate/2-oxoglutarate activity shown by the mutant enzymeare due to trace contamination. Since stringent precautionswere taken, it appears possible that this reflects the levelof reading error during overexpression of the mutant proteins.CD measurements indicate that the S380V mutant has an alteredconformation, whereas the K89L enzyme gave an identical CD spectrumto that of wild-type GDH; the spectrum of the double mutantwas similar, although somewhat altered in intensity. The resultsconfirm the key role of K89 in dicarboxylate recognition byGDH. 相似文献