The essential tryptophan residues of pig kidney aminoacylase

The tryptophan residues in pig kidney aminoacylase (N-acylamino acid amido hydrolase, EC 3.5.1.14) have been modified by N-bromosuccinimide (NBS) at low pH. The modification of eight tryptophan residues as measured by spectrophotometric and spectrofluorimetric methods leads to complete loss of enzymatic activity. The decreases in absorption at 280 nm and fluorescence emission at 337 nm indicate the modification of tryptophan residues. Both the inactivation and tryptophan residual modification are monophasic, first-order reactions. Quantitative treatment of the data (Tsou, C. L., Sci. Sin., 1962, 11, 1535-1558) shows that among the tryptophan residues modified, two are essential for aminoacylase catalytic activity. Kördel and Schneider (Hoppe-Seyler's Physiol. Chem. 1976, 357, 1109-1115) reported that the modification of tryptophan residues led to inactivation of aminoacylase, and suggested that tryptophan residues are essential for enzymatic activity. We have now shown that eight tryptophan residues can be modified by N-bromosuccinimide and that two of them are essential for the catalytic activity of this enzyme.

Reduction in the amide hydrogen exchange rates of an anti-lysozyme Fv fragment due to formation of the Fv-lysozyme complex

The Fv fragment of the monoclonal antibody D1.3 was expressed in bacteria. Standard triple resonance techniques were used to obtain the NMR resonance assignments for 211 out of 215 backbone 15N/NH atoms for D1.3 Fv. Using these assignments, hydrogen exchange rates are measured for 82 amide hydrogen atoms in D1.3 Fv free and bound to hen egg-white lysozyme. Upon binding to antigen, exchange rates are decreased for residues throughout the Fv. Many of these residues are located remote from the site of interaction with the antigen. These changes are larger than previously observed for the antigen portion of the complex. Evidently, the beta-sheet structure of the Fv propagates the effects of binding more efficiently than the antigen. These effects are compared between the three different polypeptide chains that make up the complex. These data suggest that reduced dynamics are a general feature of antibody binding to antigen.

Characterisation of a xylanolytic amyloglucosidase of Termitomyces clypeatus

A xylanolytic amyloglucosidase of Termitomyces clypeatus was characterised with respect to other amyloglucosidases. The enzyme contained high alpha-helix destabilising amino acids but no sulphur amino acid. It contained high threonine and serine, analogous to other raw starch hydrolysing enzymes. Both xylanase and amyloglucosidase activities were gradually lost with the progress of tryptophan oxidation by NBS and total inactivation occurred after oxidation of 4-5 tryptophan residues. In the presence of substrates (either starch or xylan), complete inactivation of either activities was not noticed even after oxidation of 7.7 mol of tryptophan residues. Inactivation by HNBB was not possible in the absence of any denaturant. Only 4.9 mol of tryptophan could be modified in the presence of 5 M urea which resulted in only 42% inhibition of activity. Thus modified enzyme had higher Vm/Km and lower pH optima in comparison to those of native enzyme. It was suggested that tryptophan was present at the substrate binding site and not at the active site. No such change in activity was noticed after modification of tyrosine, lysine or arginine residues. HPGPLC analysis of both dilute and concentrated enzyme solution indicated that the enzyme existed as an equilibrium mixture of protomer-oligomer. Perhaps for this reason molar mass of NAI modified enzyme appeared to be almost half of that modified by NAI in presence of substrate. Arrhenius plot of the enzyme also indicated reversible oligomerisation as a function of temperature.

An essential tryptophan residue of green crab (syclla serrata) alkaline phosphatase

The tryptophan residues in green crab (scylla serrata) alkaline phosphatase (EC 3.1.3.1) have been modified by N-bromosuccinimide (NBS). The modification of five tryptophan residues leads to complete loss of enzymatic activity. With the increase of NBS concentration, both the absorption at 278 nm and the fluorescence emission intensity at 335 nm of the modified enzyme decreased markedly indicating the modification of tryptophan residues. Quantitative treatment of the data (Tsou, Sci. Sinica 1962, 11, 1535-1558) shows that among the tryptophan residues modified, one is essential for its catalytic activity. The presence of the substrate markedly protects the modification of tryptophan residues as well as the inactivation, suggesting that the essential tryptophan residue is situated at the active site of this enzyme.

Structure-function relationships in glucosidase I: amino acids involved in binding the substrate to the enzyme

As the enzyme that initiates the maturation phase of the oligosaccharide moiety of N-linked glycoproteins, glucosidase I controls the flux of carbohydrate during the biosynthesis of these proteins. In a previous study to elucidate the structure-function relationships, we reported the presence of a cysteine residue at or near the active site of the enzyme from the bovine mammary gland (Pukazhenthi,B.S., Muniappa,N. and Vijay,I.K., 1993, J. Biol. Chem., 268, 6445-6452). We have now extended this approach to identify the participation of an arginine and a tryptophan residue in the enzyme that may play an important role in binding the substrate. The data have been combined with the results of the previous study and the cDNA-derived sequence to propose a ERHLDLRCW motif in the active site of the enzyme in the rat mammary gland that is involved in binding the incipient glycoprotein substrate for processing.

Detection and comparison of specific hemin binding by Porphyromonas gingivalis and Prevotella intermedia

A radioligand assay was designed to detect and compare specific hemin binding by the periodontal anaerobic black-pigmenting bacteria (BPB) Porphyromonas gingivalis and Prevotella intermedia. The assay included physiological concentrations of the hemin-binding protein rabbit serum albumin (RSA) to prevent self-aggregation and nonspecific interaction of hemin with cellular components. Under these conditions, heme-starved P. intermedia cells (two strains) expressed a single binding site species (4,100 to 4,600 sites/cell) with a dissociation constant (Kd) of 1.0 x 10(-9) M. Heme-starved P. gingivalis cells (two strains) expressed two binding site species; the higher-affinity site (1,000 to 1,500 sites/cell) displayed a Kd of between 3.6 x 10(-11) and 9.6 x 10(-11) M, whereas the estimated Kd of the lower-affinity site (1.9 x 10(5) to 6.3 x 10(5) sites/cell) ranged between 2.6 x 10(-7) and 6.5 x 10(-8) M. Specific binding was greatly diminished in heme-replete cells of either BPB species and was not displayed by iron-replete Escherichia coli cells, which bound as much hemin in the absence of RSA as did P. intermedia. Hemin binding by BPB was reduced following treatment with protein-modifying agents (heat, pronase, and N-bromosuccinimide) and was blocked by protoporphyrin IX and hemoglobin but not by Congo red. Hemopexin also inhibited bacterial hemin binding. These findings indicate that both P. gingivalis and P. intermedia express heme-repressible proteinaceous hemin-binding sites with affinities intermediate between those of serum albumin and hemopexin. P. gingivalis exhibited a 10-fold-greater specific binding affinity and greater heme storage capacity than did P. intermedia, suggesting that the former would be ecologically advantaged with respect to heme acquisition.

Investigation into the catalytic role for the tryptophan residues within domain III of Pseudomonas aeruginosa exotoxin A

The role of the tryptophan residues in the substrate-binding and catalytic mechanism of an enzymatically active C-terminal fragment of Pseudomonas aeruginosa exotoxin A was studied by individually or jointly replacing these residues with phenylalanine. Substitution of W-466 decreased the ADP-ribosyltransferase and NAD(+)-glycohydrolase activities by 20- and 3-fold, respectively. In contrast, substitution of W-417 or W-558 with phenylalanine both resulted in a 3-fold decrease in ADP-ribosyltransferase activity with, however, only a decrease by 40% and 70% in NAD(+)-glycohydrolase activity, respectively. Simultaneous replacement of W-466 and W-558 resulted in a 200-fold decrease in ADP-ribosyltransferase and an 6-fold decrease in NAD(+)-glycohydrolase activities, suggesting that W-466 may play a minor role in the transfer of ADP-ribose to the eEF-2 protein. Chemical modification of the tryptophan residues in the wild-type toxin fragment by N-bromosuccinimide revealed the presence of a single residue important for enzymatic activity, W-466, with a minor contribution from W-558. Additionally, tryptophan residues, W-305 and W-417, were refractory to oxidation by N-bromosuccinimide, which likely indicated the buried nature of these residues within the protein structure. Titration of the wild-type toxin fragment with NAD+ resulted in the quenching of the intrinsic tryptophan fluorescence to 58% of the initial value. Titration of the various single and a double tryptophan replacement mutant protein(s) indicated that W-558 and W-466 are responsible for the substrate-induced fluorescence quenching, with the former being responsible for the largest fraction of the observed quenching in the wild-type toxin. Consequently, a molecular mechanism is proposed for the substrate-induced fluorescence quenching of both W-466 and W-558. Furthermore, molecular modeling of the recent crystal structures for both exotoxin A (domain III fragment) and diphtheria toxin, combined with a variety of previous results, has led to the proposal for a catalytic mechanism for the ADP-ribosyltransferase reaction. This mechanism features a SN1 attack (instead of the previously purported SN2 mechanism) by the diphthamide residue (nucleophile) of eukaryotic elongation factor 2 on the C-1 of the nicotinamide ribose of NAD+, which results in an inversion of configuration likely due to steric constraints within the NAD(+)-toxin-elongation factor 2 complex.

Discrimination between the four tryptophan residues of MM-creatine kinase on the basis of the effect of N-bromosuccinimide on activity and spectral properties

Rabbit muscle cytosolic creatine kinase (MM-CK) has been treated with N-bromosuccinimide, a reagent known to oxidize selectively the indole moiety of tryptophan residues of proteins in acidic conditions. Inactivation of the enzyme is achieved by modification of one residue per monomer. NBS treatment decreases the ultraviolet absorbance at 280 nm and the intrinsic fluorescence of the protein. From these data it can be deduced that the quantum yields of the four tryptophan residues of each monomer are different due to the more or less hydrophobic environment of each of them and that at least two of them are sufficiently close to Cys 282 to allow fluorescence energy transfer to an extrinsic fluorophore bound to this residue. The accessibility to iodide of the tryptophans has been evaluated during guanidinium chloride denaturation. These data allowed us to acquire a new insight into the environment, the contribution to intrinsic fluorescence and the role in enzymatic activity and fluorescence resonance energy transfer of the tryptophan residues of CK and to tentatively assign a position in the sequence to each of them.

Kinetics of inhibition of alkaline phosphatase from green crab (Scylla serrata) by N-bromosuccinimide

The inactivation of alkaline phosphatase from green crab (Scylla serrata) by N-bromosuccinimide has been studied using the kinetic method of the substrate reaction during modification of enzyme activity previously described by Tsou [(1988), Adv. Enzymol. Related Areas Mol. Biol. 61, 381-436]. The results show that inactivation of the enzyme is a slow, reversible reaction. The microscopic rate constants for the reaction of the inactivator with free enzyme and the enzyme-substrate complex were determined. Comparison of these rate constants indicates that the presence of substrate offers marked protection of this enzyme against inactivation by N-bromosuccinimide. The above results suggest that the tryptophan residue is essential for activity and is situated at the active site of the enzyme.

Purification, kinetic characterization and involvement of tryptophan residue at the NADPH binding site of xylose reductase from Neurospora crassa

Xylose reductase (XR) from Neurospora crassa was purified to homogeneity and was found to be specific to NADPH (nicotinamide adenine dinucleotide phosphate). The purified enzyme showed M(r) of 60 and 29 kDa by gel filtration and SDS-PAGE indicating the presence of two subunits. The kinetic mechanism of xylose reductase is 'iso-ordered bi bi'. Inactivation of XR by N-bromosuccinimide (NBS) was found to be biphasic with second-order rate constants of 2.5 x 10(2) and 80 M-1S-1 for the fast (kf) and slow phase (ks), respectively. NADPH protected 90% of XR activity against inhibition by NBS. The fluorescence and circular dichroism (CD) studies revealed that inactivation was not due to gross conformational change in the enzyme. Analysis of the modified Stern-Volmer plot indicated that 49% of the tryptophanyl fluorescence was available for quenching which was completely abolished in the presence of NADPH confirming the involvement of tryptophan at the coenzyme binding site. Experimental evidence presented here serves to implicate the involvement of a tryptophan residue at the low-affinity NADPH binding site and the nature of this site has been assessed by using the hydrophobic probe ANS.

Structure-function relationship of xylanase: fluorimetric analysis of the tryptophan environment

Involvement of one out of three tryptophan residues in the active site of the low-molecular-mass xylanase from Chainia has been demonstrated previously [Deshpande, Hinge and Rao (1990) Biochim. Biophys. Acta 1041, 172-177]. The work described here aims at: (i) deducing the structure-function relationship for the tryptophan residue involved at the active site (a) by correlating the effect of N-bromosuccinimide (NBS) on the fluorescence and activity, and (b) by assessing the ability of xylan to protect against decrease in fluorescence versus activity of NBS-treated enzyme; and (ii) probing into the environment of the tryptophan residues by studying the quenching of their fluorescence by various solute quenchers in the presence and absence of guanidine hydrochloride (Gdn.HCl). Complete inactivation of the NBS-treated enzyme occurs well before the loss of fluorescence. Full protection by xylan (0.5%) of the inactivation of enzyme by NBS compared with 30% protection for the decrease in fluorescence confirms the participation of a single tryptophan at the substrate-binding site of the xylanase. The xylanase exhibited a rather low fluorescence emission maximum at 310 nm. There was no shift in the emission maximum on treatment of the enzyme with Gdn.HCl (6.5 M), indicating the rigidity of the microenvironment around tryptophan residues. The quenching studies with acrylamide suggested the occurrence of both collisional as well as static quenching processes. The enzyme retained full activity as well as the characteristic emission maximum at 310 nm in the presence of acrylamide (100 mM), indicating that quenching of fluorescence by acrylamide is a physical process. Acrylamide was more efficient as a quencher than CsCl or KBr. Treatment of the enzyme with Gdn.HCl resulted in an increase in accessibility of the quenchers to the fluorophore as suggested by an increase in the Stern-Volmer quenching constants (K(SV)) of the solute quenchers. The analysis of K(SV) and V values of KBr and CsCl suggests that the overall tryptophan microenvironment in the xylanase from Chainia is slightly electronegative.

Chemical modification and inactivation of rat liver arginase by N-bromosuccinimide: reaction with His141

Treatment of rat liver arginase with N-bromosuccinimide results in modification of six tryptophan residues per enzyme molecule and is accompanied by loss of catalytic activity (E. Ber and G. Muzynska (1979) Acta Biochim. Pol. 26, 103-114). In order to probe the chemistry of N-bromosuccinimide inactivation and the role of tryptophan residues in catalysis, the two tryptophan residues of rat liver arginase, Trp122 and Trp164, have been separately mutated to phenylalanine using site-directed mutagenesis of the protein expressed in Escherichia coli. Both single Trp -> Phe mutant enzymes have kinetic parameters nearly identical to those for the wild-type enzyme. Treatment of native, wild-type, and each of the Trp -> Phe mutant enzymes with N-bromosuccinimide results in loss of absorbance at 280 nm and is accompanied by a loss of catalytic activity. However, treatment of the wild-type enzyme with N-bromosuccinimide in the presence of the arginase inhibitors NG-hydroxy-L-arginine or the combination of L-ornithine and borate protects against inactivation, even though tryptophan residues are modified. Treatment of the H101N and H126N mutant arginases with N-bromosuccinimide also results in loss of catalytic activity and modification of tryptophan residues. In contrast, the H141N mutant arginase is not inactivated by N-bromosuccinimide, indicating that His141 is the critical target for the N-bromosuccinimide inactivation of the enzyme.

The effect of N-bromosuccinimide on ferredoxin:NADP+ oxidoreductase

Treatment of spinach leaf ferredoxin:NADP+ oxidoreductase (FNR) with N-bromosuccinimide (NBS), under conditions where approximately one tryptophan residue per enzyme was modified, resulted in a loss of between 80 and 85% of the activity of the enzyme when electron transfer from NADPH to either ferredoxin or 2,6-dichlorophenol-indophenol was measured. Amino acid analysis revealed no detectable modification by NBS of any FNR amino acids other than tryptophan. Complex formation with ferredoxin, but not with NADP+, prevented both the inhibition of activity and the modification of tryptophan caused by the treatment with NBS. Modification of one FNR tryptophan residue had no significant effect on the Km values of the enzyme for either ferredoxin or NADPH or on the binding constants for the FNR complexes with either ferredoxin or NADP+. NBS treatment had only very small effects on the absorbance and circular dichroism spectra of FNR and did not significantly affect either the oxidation-reduction midpoint potential of the FAD prosthetic group of the enzyme or inhibit the reduction of the FAD group by NADPH. These results raise the possibility that a tryptophan residue may play a role in the electron transfer between the FAD of FNR and the enzyme substrate, ferredoxin.

Two tryptophans at the active site of UDP-glucose 4-epimerase from Kluyveromyces fragilis

Efficient fluorescence energy transfer from aromatic residues to the pyridine moiety of the bound coenzyme (NAD) of UDP-glucose 4-epimerase from Kluyveromyces fragilis had been reported earlier (Mukherji, S., and Bhaduri, A. (1992) J. Biol. Chem. 267, 11709-11713). We have employed N-bromosuccinimide (NBS) to identify tryptophan as the exclusive aromatic donor in the energy transfer. The characteristic UV absorption spectrum associated with Trp oxidation is observed during NBS modification of two of the four Trp residues of native epimerase along with concomitant inactivation of the enzyme. Excellent correlation between the observed inactivation and abolition of fluorescence energy transfer to coenzyme from Trp in epimerase upon treatment with NBS implicates the involvement of the same two tryptophans in both catalytic activity and fluorescence energy transfer. SDS-polyacrylamide gel electrophoresis and fluorescence data preclude gross structural/conformational changes in epimerase due to NBS oxidation. The susceptible tryptophans do not reside at the substrate binding site as substrates and UMP fail to protect against NBS modification. However, failure of sodium borohydride to reduce the bound NAD in the NBS-inactivated epimerase suggests that the reactive tryptophans are close to the coenzyme. Tryptophan fluorescence lifetime values of 1.9 and 3.9 ns for the native and 3.5 ns for the NBS-modified epimerase, complemented by a linear Stern-Volmer plot (effective Stern-Volmer constant = 2.85 M-1) of acrylamide quenching, suggest that the two key tryptophans are buried close to an intrinsic quencher, presumably NAD.

Purification and characterization of a raw-starch digesting amylase from a soil bacterium--Cytophaga sp

A newly isolated bacterium from soil, identified as Cytophaga sp. was found to produce raw-starch digesting amylase. The enzyme was purified from 24-hr cultured medium through ammonium sulfate fractionation, DEAE-Sepharose CL 6B ion exchange chromatography and Sephacryl S-200 gel filtration. The preparation was proved to be homogeneous by SDS-PAGE. The subunit molecular weight determined by SDS-PAGE was 59 KD. The optimum temperature was 50 degrees C on soluble starch and 60 degrees C on raw starch. The optimum pH was in the range of 4.5 to 6.5 on soluble starch and 6.5 to 9.5 on raw starch. In the presence of Mn+2, Cu+2 or Zn+2, the enzyme activity on either substrate was inhibited. Dinitrofluorobenzene, N-bromosuccinimide and trinitrobenzene sulfonic acid all showed inhibitory effect on the enzyme acting on both substrates.

Functional roles of Trp337 and Glu632 in Clostridium glucoamylase, as determined by chemical modification, mutagenesis, and the stopped-flow method

Chemical modification of glucoamylase (EC 3.2.1.3) from Clostridium sp. G0005 (CGA) with N-bromosuccinimide (NBS) was carried out in the presence or absence of an inhibitor, acarbose. CGA lost its catalytic activity through NBS oxidation in the absence of acarbose. The absorbance change at 280 nm suggested that acarbose protects about 2 Trp residues from NBS oxidation. We performed peptide mapping analysis to identify the protected Trp residues, and Trp321, Trp337, Trp433, and Trp569 were identified as candidates to be protected by acarbose. These 4 Trp residues were replaced by site-directed mutagenesis with Phe. The Trp337-->Phe mutant showed very weak catalytic activity, so Trp337 is proposed as an important residue for the catalytic activity. Further, we constructed a Glu632-->Gln mutant. Glu632 is the putative catalytic base. The presteady-state kinetics of the Trp337-->Phe and Glu632-->Gln mutants and the wild-type CGA were investigated using maltotriose as a substrate. The reaction of wild-type CGA can be explained as one involving three intermediates. On the other hand, the two mutants' reactions are explained by a two-step mechanism lacking the third intermediate. Trp337 and Glu632 appear to be crucial for the formation of the third intermediate in the wild-type reaction, which precedes the transition state.

Purification and characterization of catalase from goat (Capra capra) lung

Catalase plays a major role in the protection of tissues from toxic effects of H2O2 and partially reduced oxygen species. In the present study catalase was extracted and purified 330-fold from goat lung by acetone fractionation and successive chromatographies on DEAE-cellulose, Sephadex G-200, Blue Sepharose CL-6B and Ultrogel AcA-34. The purified enzyme was almost homogeneous as judged by polyacrylamide gel electrophoresis and FPLC. The molecular weight and Stokes' radius of the purified enzyme were 339 kDa and 127 +/- 2 A. The enzyme had 11 sulfhydryl groups and 15 tryptophan groups per mol of the enzyme. A broad pH optimum in the range 5.2 to 7.8 was obtained. Sulfhydryl group binding agents, thiol reagents and N-Bromosuccinimide inhibited the enzyme activity. The kinetic data show no cooperativity between the substrate binding sites. Tryptophan, indole acetic acid, cysteine, formaldehyde and sodium azide inhibited the enzyme non-competitively with Ki values of 1.5, 1.6, 6.7, 0.55 and 0.0017 mM, respectively.

Involvement of tryptophan(s) at the active site of polyphosphate/ATP glucokinase from Mycobacterium tuberculosis

The glucokinase (EC 2.7.1.63) from Mycobacterium tuberculosis catalyzes the phosphorylation of glucose using inorganic polyphosphate (poly(P)) or ATP as the phosphoryl donor. The nature of the poly(P) and ATP sites was investigated by using N-bromosuccinimide (NBS) as a probe for the involvement of tryptophan in substrate binding and/or catalysis. NBS oxidation of the tryptophan(s) resulted in fluorescence quenching with concomitant loss of both the poly(P)- and ATP-dependent glucokinase activities. The inactivation by NBS was not due to extensive structural changes, as evidenced by similar circular dichroism spectra and fluorescence emission maxima for the native and NBS-inactivated enzyme. Both phosphoryl donor substrates in the presence of xylose afforded approximately 65% protection against inactivation by NBS. The Km values of poly(P) and ATP were not altered due to the modification by NBS, while the catalytic efficiency of the enzyme was decreased, suggesting that the essential tryptophan(s) are involved in the catalysis of the substrates. Acrylamide quenching studies indicated that the tryptophan residue(s) were partially shielded by the substrates against quenching. The Stern-Volmer quenching constant (KSV) of the tryptophans in unliganded glucokinase was 3.55 M-1, while KSV values of 2.48 and 2.57 M-1 were obtained in the presence of xylose+poly(P)5 and xylose+ATP, respectively. When the tryptophan-containing peptides were analyzed by peptide mapping, the same peptide was found to be protected by xylose+poly(P)5 and xylose+ATP against oxidation by NBS. The two protected peptides were determined to be identical by N-terminal sequence analysis and amino acid composition.(ABSTRACT TRUNCATED AT 250 WORDS)

Structural changes in the protease domain of prothrombin upon activation as assessed by N-bromosuccinimide modification of tryptophan residues in prethrombin-2 and thrombin

Increases in intrinsic fluorescence (delta I), reflecting changes in tryptophan environments, occur upon bond cleavages necessary for prothrombin (II) activation to thrombin (IIa) by prothrombinase. Cleavage at Arg274-Thr275 (numbering based on bovine prothrombin sequence, with chymotrypsinogen numbering in braces) between the amino-terminal fragment 1.2 and protease (Pre2) domains of prothrombin yields delta I = 5%, and cleavage within the Pre2 domain at Arg323-Ile324 to form IIa yields delta I = 35%, while cleavage at both yields delta I = 25%. Since the change in fluorescence upon activation of prothrombin can be largely attributed to a change within the Pre2 domain, the susceptibilities of each of the 9 Trp residues of IIa and its immediate precursor Pre2 to oxidation by N-bromosuccinimide (NBS) were compared. Pre2 and IIa were titrated with increasing amounts of NBS (0.5-5 equiv of NBS/TRP), aliquots were removed and fully digested with trypsin, and tryptophan-containing peptides were separated and quantitated by RP-HPLC with fluorescence detection. Tryptic digests yielded 9 tryptophan-containing peptides, which were identified by amino acid composition. Tryptophan residues in IIa and Pre2 displayed a 10-fold range of sensitivity to modification. Tryptophans 337 and 360 (W29, W51) were modified less readily in IIa than in Pre2, while residues 373, 542, and 550 (W60D, W207, W215) were modified more readily, and other residues were equally susceptible. Residues 360 and 373 (W29, W60D) flank the active site histidine. From the crystal structure, residues 373 and 550 (W60D, W215) are implicated in substrate binding.(ABSTRACT TRUNCATED AT 250 WORDS)

Chemical modification of alpha-subunit tryptophan residues in Schizosaccharomyces pombe mitochondrial F1 adenosine 5'-triphosphatase: differential reactivity and role in activity

Chemical modification of mitochondrial F1-ATPase from Schizosaccharomyces pombe by the tryptophan-specific reagent N-bromosuccinimide (NBS) at pH 5.0 in the presence of 20% glycerol produced a characteristic lowering in both enzyme absorbance at 280 nm and intrinsic fluorescence at 332 nm that varied with NBS/F1 molar ratio up to a value of 130. Fluorometric titration of tryptophans and correlation to residual ATPase activity showed that modification of three reactive residues among the seven present on alpha- and epsilon-subunits did not markedly modify the enzyme activity but efficiently released endogenous ATP and abolished the fluorescence quenching related to GDP or ATP binding to the catalytic site. Additional modification of one, less reactive, tryptophan altered both negative cooperativity of ATP hydrolysis and sensitivity to azide inhibition and produced a nearly complete inactivation at high NBS/F1 molar ratio. NBS-induced inactivation of F1 was favored by catalytic-site saturation with GDP or low ATP concentration and on the contrary was prevented by noncatalytic-site saturation with ADP or high ATP concentration. When reactive tryptophans were selectively modified by NBS in the presence of ADP, and subunits were isolated after guanidine hydrochloride dissociation by one-step purification on reversed-phase HPLC, the absorbance of alpha-subunit at 280 nm was decreased, whereas that of epsilon-subunit was unchanged. Cyanogen bromide cleavage of alpha-subunit and fragments separation by reversed-phase HPLC showed that one peptide of 3 kDa apparent molecular mass had decreased absorbance. N-Terminal sequencing allowed its identification to fragment 255-282 that contains tryptophan257.

Modification of tryptophan 149 of inorganic pyrophosphatase from Escherichia coli

1. The inorganic pyrophosphatase from Escherichia coli was almost completely inactivated on chemical modification of Trp-149 with N-bromosuccinimide (NBS). 2. The presence of a complex of Mg2+ and a substrate analogue, iminodiphosphate (PNP), provided considerable protection against the inactivation, whereas Mg2+ or PNP alone afforded only slight protection.

Evidence for the involvement of tryptophan 38 in the active site of glutathione S-transferase P

Glutathione S-transferase P (GST-P) exists as a homodimeric form and has two tryptophan residues, Trp28 and Trp38, in each subunit. In order to elucidate the role of the two tryptophan residues in catalytic function, we examined intrinsic fluorescence of tryptophan residues and effect of chemical modification by N-bromosuccinimide (NBS). The quenching of intrinsic fluorescence was observed by the addition of S-hexylglutathione, a substrate analogue, and the enzymatic activity was totally lost when single tryptophan residue was oxidized by NBS. To identify which tryptophan residue is involved in the catalytic function, each tryptophan was changed to histidine by site-directed mutagenesis. Trp28His GST-P mutant enzyme showed a comparable enzymatic activity with that of the wild type one. Trp38His mutant neither was bound to S-hexylglutathione-linked Sepharose nor exhibited any GST activity. These findings indicate that Trp38 is important for the catalytic function and substrate binding of GST-P.

Involvement of tryptophan residue(s) in the specific binding of agonists/antagonists to 5-HT3 receptors in NG108-15 clonal cells

Chemical modification of the 5-HT3 receptors in membranes from NG108-15 hybridoma cells was achieved using protein modifying reagents specific for various amino acid residues: N-bromosuccinimide for tryptophan, dithiothreitol for cystine, sodium tetrathionate for cysteine, 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide and N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline for aspartic and glutamic acids, diethylpyrocarbonate for histidine, tetranitromethane for tyrosine and 2,3-butanedione for arginine. Among all the reagents tested, N-bromosuccinimide produced the largest alteration in the specific binding of [3H]zacopride onto 5-HT3 receptors. A significant reduction in Bmax (approximately 50%) with no change in Kd were noted on [3H]zacopride specific binding to membranes which were incubated with 40 microM N-bromosuccinimide for 60 min at 25 degrees. The occupancy of 5-HT3 receptor binding sites by various 5-HT3 agonists and antagonists (phenylbiguanide, ondansetron, granisetron, MDL 72222) prevented, at least partially, any subsequent reduction in [3H]zacopride specific binding by N-bromosuccinimide treatment. However, neither m-chloro-phenylbiguanide, among the agonists, nor zacopride, among the antagonists, were able to prevent the effect of N-bromosuccinimide, suggesting that variations might exist in the molecular mechanisms implicated in the binding of 5-HT3 ligands to the recognition site on 5-HT3 receptors. Nevertheless, these data support the suggestion that tryptophan residue(s) are probably involved in the binding of agonists and antagonists onto 5-HT3 receptors in NG108-15 cell membranes.

Modification of a single tryptophan of the inorganic pyrophosphatase from thermophilic bacterium PS-3: possible involvement in its substrate binding

The effect of N-bromosuccinimide (NBS) on the activity of the inorganic pyrophosphatase (PPiase) from thermophilic bacterium PS-3 was studied. The enzyme was almost completely inactivated on chemical modification with NBS, depending upon the concentration of NBS. The presence of a complex of Mg2+ and a substrate analogue, imidodiphosphate (PNP), provided extensive protection against the inactivation, whereas Mg2+ or PNP alone showed no protective effect. Amino acid analysis of the NBS-modified enzyme after hydrolysis with 6 M HCl indicated no change in the amino acid composition. However, the magnetic circular dichroism (MCD) bands around 293 nm due to the tryptophan residue and the optical density at 280 nm, decreased concomitantly with modification by NBS. These results strongly suggested that the tryptophan residue at position 143, which is the only tryptophan residue per subunit in the thermophilic PPiase (Ichiba, T., Takenaka, O., Samejima, T. and Hachimori, A. (1990) J. Biochem. 108, 572-578), might be involved in the active site or be located in the vicinity of the active site. The circular dichroism (CD) spectrum in the far ultraviolet region showed no significant alteration during the modification, indicating that the polypeptide chain backbone of the enzyme remained unaltered. However, the modification considerably altered the CD bands in, the near ultraviolet region, indicating that a conformational change occurred in the vicinity of the active site in the enzyme molecule.