Crystal structure of two covalent nucleoside derivatives of ribonuclease A

Crystal structures of two forms of ribonuclease A with deoxynucleosides covalently bound to respectively His 12 and His 119 have been solved. One form, T-H12-RNase, has a deoxythymidine bound to N epsilon 2 of His 12, while the other one, U-H119-RNase, has a deoxyuridine bound to N delta 1 of His 119. The two crystal forms are nearly isomorphous, with two molecules in the asymmetric unit. However, the modified ribonucleases differ both in their enzymatic activities and in the conformation of the catalytic site and of the deoxynucleoside-histidine moiety. T-H12-RNase is characterized by complete loss of enzymatic activity; in this form the deoxynucleoside completely blocks the catalytic site and forms intramolecular contacts with residues associated with both the B1 and B2 sites. U-H119-RNase retains 1% of the activity of the unmodified enzyme, and in this form His 119 adopts a different orientation, corresponding to the alternate conformation reported for this residue; the deoxynucleoside-histidine moiety points out of the active site and does not form any contacts with the rest of the protein, thus allowing partial access to the catalytic site. On the basis of these structures, we propose possible mechanisms for the reactions of bromoacetamido nucleosides with ribonuclease A.     

Thermodynamics of protein-peptide interactions in the ribonuclease S system studied by titration calorimetry

Two fragments of pancreatic ribonuclease A, a truncated version of S-peptide (residues 1-15) and S-protein (residues 21-124), combine to give a catalytically active complex designated ribonuclease S. Residue 13 in the peptide is methionine. According to the X-ray structure of the complex of S-protein and S-peptide (1-20), this residue is almost fully buried. We have substituted Met-13 with seven other hydrophobic residues ranging in size from glycine to phenylalanine and have determined the thermodynamic parameters associated with the binding of these analogues to S-protein by titration calorimetry at 25 degrees C. These data should provide useful quantitative information for evaluating the contribution of hydrophobic interactions in the stabilization of protein structures.     

Chemical modification of methionine residues of the phosphatidylcholine transfer protein from bovine liver. A spin-label study

The role of methionine residues in the interaction of the phosphatidylcholine transfer protein from bovine liver with phospholipid vesicles was investigated by specific modification of these residues with iodoacetamide. The modified protein was digested with cyanogen bromide in order to determine which methionine residues had become resistant to this cleavage. Automated Edman degradation on the digest indicated that after 72 h of reaction, Met-1 was modified for 80%, Met-73 for 50%, Met-109 for 20%, whilst Met-173 and Met-203 were found to be unmodified. This distinct modification did not result in any loss of phosphatidylcholine transfer activity. The interaction of the phosphatidylcholine transfer protein with phospholipid vesicles was investigated by making use of electron spin resonance spectroscopy. The interaction of unmodified protein with vesicles composed of phosphatidylcholine/phosphatidic acid/spin-labeled phosphatidylethanolamine (79:16:5, mol%) or composed of phosphatidylserine/spin-labeled phosphatidylethanolamine (95:5, mol%), gave an increase of about 50% in the rotation correlation time. A similar increase was observed with the modified protein. This interaction was further investigated by labeling Met-1 and Met-73 in the transfer protein with iodoacetamidoproxyl spin-label. Spin-labeling did not inactivate the transfer protein. In addition, the electron spin resonance spectra of the spin-labeled protein were not affected upon addition of vesicles composed of phosphatidylcholine/phosphatidic acid (80:20, mol%). These experiments strongly suggest that Met-1 and Met-73 are not part of the site that interacts with the membrane.     

Heat capacity changes for protein-peptide interactions in the ribonuclease S system.

Two fragments of pancreatic ribonuclease A, a truncated version of S-peptide (residues 1-15) and S-protein (residues 21-124), combine to give a catalytically active complex designated ribonuclease S. We have substituted the wild-type residue Met-13 with six other hydrophobic residues ranging in size from alanine to phenylalanine and have determined the thermodynamic parameters associated with binding of these analogues to S-protein by titration calorimetry in the temperature range 5-25 degrees C. The heat capacity change (delta Cp) associated with binding was obtained from a global analysis of the temperature dependences of the free energies and enthalpies of binding. The delta Cp's were not correlated in any simple fashion with the nonpolar surface area (delta Anp) buried upon binding.     

Methionine oxidation in human growth hormone and human chorionic somatomammotropin. Effects on receptor binding and biological activities

The oxidation of the methionine residues of human growth hormone (hGH) and human chorionic somatomammotropin (hCS) to methionine sulfoxide by hydrogen peroxide has been studied. The kinetics of oxidation of individual methionine residues has been measured by reverse-phase high pressure liquid chromatography tryptic peptide mapping. Met-170 is completely resistant to oxidation in both hormones. The other 3 methionine residues in hCS (Met-64, Met-96, and Met-179) have markedly different reaction rates. Oxidation of the methionine residues does not appear to cause gross conformational changes in either hGH or hCS, as judged by CD and 1H NMR spectroscopy. Oxidation of Met-14 and Met-125 in hGH has little effect on affinity of the hormone for lactogenic receptors or on its potency in the Nb2 rat lymphoma in vitro bioassay for lactogenic hormones. The oxidation of Met-64 and/or Met-179 in hCS reduces profoundly both its affinity for lactogenic receptors and its in vitro biological potency. It is inferred by induction that residues 64 and/or 179 are critical for the binding of both hGH and hCS to lactogenic receptors and the expression of lactogenic biological activity.     

alpha-Bromo-4-amino-3-nitroacetophenone, a new reagent for protein modification. Modification of the methionine-290 residue of porcine pepsin.

A new coloured reagent for protein modification, alpha-bromo-4-amino-3-nitroacetophenone (NH2BrNphAc), was synthesized. The reagent was found to alkylate specifically the methionine-290 residue of porcine pepsin below pH 3 at 37 degrees C, which lead to a 45% decrease of enzyme's activity towards haemoglobin. The effect of this reagent as well as that of other phenacyl bromides on the activity of pepsin appeared to be a result of steric hindrance caused by the attachment of bulky reagent residue to the edge of the cleft harbouring the enzyme active site. Only marginal reaction with the co-carboxy group of aspartic acid-315 was found under the above conditions. More pronounced esterification of carboxy groups (up to one residue per enzyme molecule) occurred when the pH was shifted to 5.2. The latter modification had no noticeable effect on enzyme activity, thus disproving a previously held assumption that pepsin inactivation by phenacyl bromide is due to the carboxy-group esterification. alpha-Bromo-4-amino-3-nitroacetophenone forms derivatives with characteristic u.v. spectra when it reacts with methionine, histidine, aspartic and glutamic acid residues, and may be recommended as a reagent for protein modification.     

Location of the antigenic determinants of bovine pancreatic ribonuclease.

Four antigenic regions of native bovine pancreatic ribonuclease have been located by using antibodies that react specifically with segments 1--13, 31--79, and 80--124. These antibodies were purified by affinity chromatography on columns to which these peptide segments were bound. Analysis of precipitin curves indicates that there are at least three antigenic determinants to which antibody molecules can bind simultaneously in the presence of excess antibodies. Analysis of binding data, however, for each purified specific antibody preparation, carried out by the method of Berzofsky et al. [Berzofsky, J. A., Curd, J. G., & Schechter, A. N. (1976) Biochemistry, 15, 2113], leads to an estimate of four for the number of antigenic determinants in ribonuclease; this estimate had also been made earlier by Stelos et al. [Stelos, P., Fothergill, J. E., & Singer, S. J. (1960) J. Am. Chem. Soc. 82, 6034]. We find that one determinant is associated with each of segments 1--13 and 80--124 and two with segment 31--79. No antigenic activity could be detected for segment 14--29 either in native ribonuclease or in the free fragment. These conclusions are based on (1) the use of specific peptides to isolate purified antibodies by affinity chromatography, (2) immunoprecipitation of an antigenic peptide from the peptic digest of ribonuclease, (3) competitive inhibition studies with various peptide and protein fragments [cyanogen bromide fragments 1--13, 31--79, and 80--124, the tryptic peptides 40--61 and 105--224, S-peptide, S-protein, and des(121--124)-RNase], and (4) comparison and evaluation of the published effects on antigenicity of chemical and enzymatic modifications and changes in sequence among homologous ribonucleases. These approaches provide evidence that the four antigenic determinants are localized around the alpha-helical portion of segment 1--10, somewhere in segment 40--61, at the beta bend in segment 63--75, and either at the beta bend or beta sheet in segment 87--104 of native ribonuclease.     

Point mutation of alanine (31) to valine prohibits the folding of reduced lysozyme by sulfhydryl-disulfide interchange

In the preceding paper in this issue, we described the overproduction of one mutant chicken lysozyme in Escherichia coli. Since this lysozyme contained two amino acid substitutions (Ala31----Val and Asn106----Ser) in addition to an extra methionine residue at the NH2-terminus, the substituted amino acid residues were converted back to the original ones by means of oligonucleotide-directed site-specific mutagenesis and in vitro recombination. Thus, four kinds of chicken lysozyme [Met-1Val31Ser106-, Met-1Ser106-, Met-1Val31- and Met-1 (wild type)] were expressed in E. coli. From the results of folding experiments of the reduced lysozymes by sulfhydryl-disulfide interchange at pH 8.0 and 38 degrees C, followed by the specific activity measurements of the folded enzymes, the following conclusions can be drawn: (i) an extra methionine residue at the NH2-terminus reduces the folding rate but does not affect the lysozyme activity of the folded enzyme; (ii) the substitution of Asn106 by Ser decreases the activity to 58% of that of intact native lysozyme without changing the folding rate; and (iii) the substitution of Ala31 Val prohibits the correct folding of lysozyme. Since the wild type enzyme (Met-1-lysozyme) was activated in vitro without loss of specific activity, the systems described in this study (mutagenesis, overproduction, purification and folding of inactive mutant lysozymes) may be useful in the study of folding pathways, expression of biological activity and stability of lysozyme.     

Role of Methionine in Biosynthesis of Prodigiosin by Serratia marcescens

Methionine alone did not allow biosynthesis of prodigiosin (2-methyl-3-amyl-6-methoxyprodigiosene) in nonproliferating cells (NPC) of Serratia marcescens strain Nima. However, when methionine was added to NPC synthesizing prodigiosin in the presence of other amino acids, the lag period for synthesis of prodigiosin was shortened, an increased amount of the pigment was formed, and the optimal concentrations of the other amino acids were reduced. Less prodigiosin was synthesized when addition of methionine was delayed beyond 4 h. The specific activity of prodigiosin synthesized by addition of (14)CH(3)-methionine was 40 to 50 times greater than that synthesized from methionine-2-(14)C or (14)COOH-methionine. NPC of mutant OF of S. marcescens synthesized norprodigiosin (2-methyl-3-amyl-6-hydroxyprodigiosene), and the specific activity of this pigment synthesized in the presence of (14)CH(3)-methionine was only 5 to 13 times greater than that synthesized from methionine-2-(14)C or (14)COOH-methionine. A particulate, cell-free extract of mutant WF of S. marcescens methylated norprodigiosin to form prodigiosin. When the extract was added to NPC of mutant OF synthesizing norprodigiosin in the presence of (14)CH(3)-methionine, the prodigiosin formed had 80% greater specific activity than the norprodigiosin synthesized in the absence of the extract. The C6 hydroxyl group of norprodigiosin was methylated in the presence of the extract and methionine. Biosynthesis of prodigiosin by NPC of strain Nima also was augmented by addition of S-adenosylmethionine. Various analogues of methionine such as norleucine, norvaline, ethionine, and alpha-methylmethionine did not affect biosynthesis of prodigiosin by NPC either in the presence or absence of methionine.     

Site-directed mutagenesis of histidine-13 and histidine-114 of human angiogenin. Alanine derivatives inhibit angiogenin-induced angiogenesis

The roles of His-13 and His-114 in the ribonucleolytic and angiogenic activities of human angiogenin have been investigated by site-directed mutagenesis. Replacement of either residue by alanine (H13A and H114A) decreases enzymatic activity toward tRNA by at least 10,000-fold and virtually abolishes 10,000-fold and virtually abolishes angiogenic activity in the chick embryo chorioallantoic membrane assay. Both the H13A and H114A mutant proteins compete effectively with angiogenin in the latter assay; only a 5-fold molar excess of H13A over unmodified protein is required for complete inhibition. The His----Ala substitutions, however, do not have any significant effect on the interaction of angiogenin with human placental ribonuclease inhibitor, an extremely potent inhibitor of angiogenin (Ki approximately 7 x 10(-16 M) previously shown to interact with another active-site residue, Lys-40. The effects of more conservative replacements-glutamine at position 13 and asparagine at position 114--were also examined. While the enzymatic activity of the H114N mutant was at least 3300-fold less than for the unmodified protein, the H13Q derivative had only 300-fold reduced activity toward tRNA and cytidylyl(3'----5') adenosine. Both substitutions substantially decreased angiogenic activity. The parallel effects on ribonucleolytic and biological activities observed with all four mutant proteins provide strong evidence that the latter activity of angiogenin is dependent on a functional enzymatic active site. The capacity of the H13A and H114A derivatives to compete with angiogenin in the chorioallantoic membrane assay suggests several additional features of the biological mode of action of this protein.     

Active site labeling of the shikimate pathway enzyme, dehydroquinase. Evidence for a common substrate binding site within dehydroquinase and dehydroquinate synthase

Dehydroquinase, the third enzyme of the shikimate biosynthetic pathway, is inactivated by iodoacetate. Iodoacetate behaves as an affinity label for the Escherichia coli enzyme with a Ki of 30 mM and a limiting inactivation rate of 0.014 min-1 at pH 7.0 and 25 degrees C. Affinity labeling is mediated by the negative charge of the reagent since iodoacetamide does not inactivate the enzyme. 2.1-2.3 mol of carboxymethyl groups are incorporated per mol of protein monomer resulting in 90% inactivation of enzymic activity. The majority of the bound label (80%) is split equally between 2 methionine residues, Met-23 and Met-205, which were identified by sequencing radiolabelled peptide fragments isolated after proteolytic digestion. An equilibrium mixture of the substrate (dehydroquinate) and product (dehydroshikimate) substantially reduces the inactivation rate and specifically decreases the incorporation of label at both of these site, implicating them as being in or near the active site of the enzyme. Sequence alignments with other biosynthetic dehydroquinases show that of the 2 methionine residues only Met-205 is conserved. N-terminal alignments of all the available dehydroquinase sequences (both catabolic and biosynthetic classes) revealed that Met-23, although itself not conserved, resides within a cluster of conserved sequence which may constitute part of the dehydroquinate binding site. A consensus sequence was derived from these alignments and used to probe the protein sequence data banks. A related sequence was found in dehydroquinate synthase, the enzyme which precedes dehydroquinase in the shikimate pathway. These results suggest that we have identified part of the dehydroquinate binding site in both enzymes.     

Reversible alkylation of an active site methionine residue in dehydroquinase

Iodoacetic acid inactivates dehydroquinase by simultaneously alkylating 2 methionine residues (Met-23 and Met-205), presumed to be active site residues (described in Kleanthous, C., Campbell, D. G., and Coggins, J. R. (1990) J. Biol. Chem. 265, 10929-10934). Although both sites are carboxymethylated to the same degree in the inactivated enzyme, the modification of Met-205 may be reversed by treatment with mercaptoethanol at alkaline pH, as shown by the stoichiometric loss of label from this site. This, in turn, leads to partial reactivation of the inactive enzyme. Alkylation of Met-23 is not reversible under these conditions. The chemistry of the cleavage reaction at Met-205 was investigated by isolating the cleavage product which was identified by mass spectrometry as the ammonium salt of 2-hydroxyethyl thioacetate. This result is consistent with nucleophilic attack by the thiolate anion of mercaptoethanol on the alpha-carbon of the carboxymethyl moiety, which restores the side chain of the methionine residue (Met-205) and liberates 2-hydroxyethyl thioacetate. The differential reactivity of the 2 carboxymethylated methionine residues toward mercaptoethanol is likely to be a reflection of their different microenvironments in the folded protein. This assertion is borne out by unfolding experiments which indicate that neither of the carboxymethylated methionine residues in dicarboxymethylated dehydroquinase is susceptible to mercaptoethanol cleavage if the protein is first denatured by either guanidine hydrochloride or urea. Furthermore, this denatured material refolds after removal of denaturant to yield protein with reactivation properties similar to untreated, dicarboxymethylated enzyme.     

Comparison of the antioxidant activity of albumin from various animal species

We measured the antioxidant activity of human, rat, bovine, rabbit, and guinea pig albumins against the superoxide, hydroxyl, and 1,1-diphenyl-2-picryl-hydrazyl (DPPH) radicals. The albumins of different animal species did not differ in antioxidant activity against superoxide. Human and rat albumins exhibited antioxidant activity against hydroxyl radicals, but bovine, rabbit, and guinea pig albumins showed weaker antioxidant activity than human and rat albumins. Human, rat, rabbit, and guinea pig albumins, but not bovine albumin, exhibited strong antioxidant activity against DPPH radicals. Human and rat albumins with strong antioxidant activity against hydroxyl radicals contained methionine-123 in domain 1, but bovine, rabbit, and guinea pig albumins did not. Rat, rabbit, and guinea pig albumins with strong antioxidant activity against DPPH radicals had methionine-264 in domain 2. Human albumin did not have methionine-264, but methionine-298 and methionine-329 in domain 2. Bovine albumin, with the weakest antioxidant activity against DPPH radicals, contained no methionine residues in domain 2. These results suggest that methionine residues in domain 1 or 2 influence the antioxidant activity of albumin.     

Methionine sulfoxide cytochrome c.

Cytochrome c has been chemically modified by methylene blue mediated photooxidation. It is established that the methionine residues of the protein have been specifically converted to methionine sulfoxide residues. No oxidation of any other amino acid residues or the cysteine thioether bridges of the molecule occurs during the photooxidation reaction. The absorbance spectrum of methionine sulfoxide ferricytochrome c at neutrality is similar to that of the unmodified protein except for an increase in the extinction coefficient of the Soret absorbance band and for the complete loss of the ligand sensitive 695 nm absorbance band in the spectrum of the derivative. The protein remains in the low spin configuration which implies the retention of two strong field ligands. Spin state sensitive spectral titrations and model studies of heme peptides indicate that the sixth ligand is definitely not provided by a lysine residue but may be methionine-80 sulfoxide coordinated via its sulfur atom. Circular dichroism spectra indicate that the heme crevice of methionine sulfoxide ferri- and ferrocytochrome c is weakened relative to native cytochrome c. The redox potential of methionine sulfoxide cytochrome c is 184 mV which is markedly diminished from the 260 mV redox potential of native cytochrome c. The modified protein is equivalent to native cytochrome c as a substrate for cytochrome oxidase and is not autoxidizable at neutral pH but is virtually inactive with succinate-cytochrome c reductase. These results indicate that the major role of the methionine-80 in cytochrome c is to preserve a closed hydrophobic heme crevice which is essential for the maintainance of the necessary redox potential.     

Crystal structure at 1.5-A resolution of Pyrus pyrifolia pistil ribonuclease responsible for gametophytic self-incompatibility

The crystal structure of the Pyrus pyrifolia pistil ribonuclease (S(3)-RNase) responsible for gametophytic self-incompatibility was determined at 1.5-A resolution. It consists of eight helices and seven beta-strands, and its folding topology is typical of RNase T(2) family enzymes. Based on a structural comparison of S(3)-RNase with RNase Rh, a fungal RNase T(2) family enzyme, the active site residues of S(3)-RNase assigned were His(33) and His(88) as catalysts and Glu(84) and Lys(87) as stabilizers of an intermediate in the transition state. Moreover, amino acid residues that constitute substrate binding sites of the two RNases could be superimposed geometrically. A hypervariable (HV) region that has an S-allele-specific sequence comprises a long loop and short alpha-helix. This region is far from the active site cleft, exposed on the molecule's surface, and positively charged. Four positively selected (PS) regions, in which the number of nonsynonymous substitutions exceeds that of synonymous ones, are located on either side of the active site cleft, and accessible to solvent. These structural features suggest that the HV or PS regions may interact with a pollen S-gene product(s) to recognize self and non-self pollen.     

Characterization of phospholipases A2 of Naja melanoleuca snake venom modified at the N-terminal region

In phospholipase A2 from Naja melanoleuca snake venom all four lysines were converted into the epsilon-amidinated derivatives without reaction of the alpha-amino group. The amidinated phospholipase (AMPA) showed high enzymatic activity. Starting from AMPA, chemical modification reactions were carried out at the alpha-amino function. This group was blocked with a tert-butyloxycarbonyl or a phenylthiocarbamyl group. Furthermore the polypeptide chain was shortened by one residue by removing the N-terminal asparagine, resulting in the formation of des-Asn1-AMPA. The native enzyme was shortened by eight residues by cyanogen bromide cleavage at the single methionine residue. Although all modified proteins show a reduced affinity for monomeric lipids, they are easily saturated with micellar substrate analogs. Whereas the removal of the N-terminal octapeptide abolished all enzymatic activity the other modified enzymes possess a low (1%), but measurable enzymatic activity. It is concluded that chemical modifications in the N-terminal region give rise to a distortion of the active site, thus reducing the activity of the lipid-bound enzyme.     

The semisynthesis of fragments corresponding to residues 66-104 of horse heart cytochrome c.

We describe the N epsilon-acetimidylation of horse heart cytochrome c with retention of biological activity, the cleavage of the modified protein by CNBr, the separation of the fragments, and their further side-chain protection. We describe the manipulation of the amino acid sequences of the fragments by stepwise semisynthetic methods. We have prepared fragments corresponding to residues 66-78 and 66-79 of the protein, as well as the [Asp66] analogue of fragment 66-79. We have prepared the natural sequence and the [o-fluoro-Phe82] analogue of the fragment corresponding to residues 81-104 of the protein, and the [N epsilon-trifluoroacetyl-Lys79], the [N epsilon-dinitrophenyl-Lys79] and the [S-acetamidomethyl-Cys79] analogues of fragment 79-104, and the [N epsilon-Cbz-Lys81] analogue of fragment 80-104. We have coupled back the fragments of natural sequence to form a semisynthetic fragment corresponding to residues 66-104 of the protein. Modified fragments were also coupled to give analogues of the 66-104-residue sequence. In every case the homoserine residue representing methionine-80 was removed from the C-terminus of the 66-80-residue fragment and replaced by methionine on the N-terminus of the 81-104 residue fragment during the preparation of the fragments for coupling. The semisynthetic fragments are ready for specific deprotection and further coupling. We have coupled one such fragment to the (1-65)-peptide to produce semisynthetic [Hse65]cytochrome c. The product has satisfactory characteristics on chemical analysis, and on assay of its biological activity.     

Purification and characterization of a potent homodimeric guanine-specific ribonuclease from fresh mushroom (Pleurotus tuber-regium) sclerotia

From the fresh sclerotia of the mushroom Pleurotus tuber-regium, a potent homodimeric ribonuclease exhibiting a molecular weight of 29 kDa in FPLC-gel filtration and sodium dodecyl sulfate-polyacrylamide gel electrophoresis was isolated. The protein was unadsorbed on DEAE-cellulose and adsorbed on Affi-gel blue gel, CM-cellulose and Mono S. It manifested strong ribonucleolytic activity toward Poly G, slight activity toward Poly U and Poly A, and minimal activity toward Poly C. Its optimal pH was determined to be 6.5 when yeast transfer RNA was used as substrate. Its ribonucleolytic activity was resistant to heating at 100 degrees C for 30 min but was inhibited by a number of salts. The protein inhibited cell-free translation in a rabbit reticulocyte lysate with an IC50 of 0.09 nM. Three out of the four amino acid residues at the active site (positions 38-41) of P. ostreatus ribonuclease, YNNF, were also found at positions 17-20 in the P. tuber-regum RNase. However, unlike P. ostreatus RNase, no cysteine residues were detected in the N-terminal sequence.     

Reversible blocking of half-cystine residues of proteins and an irreversible specific deamidation of asparagine-67 of S-sulforibonuclease under mild conditions

For use in protein-folding studies, a rapid procedure for the preparation of octa-S-sulforibonuclease A (SO3-RNase A) with 2-nitro-5-(sulfothio)benzoate is described. The modification is specific for thiols and disulfide bonds. The modified protein was characterized and found to be enzymatically inactive and predominantly conformationally disordered. In the absence of thiols, the modified sulfhydryl groups were found to be stable over the pH range of 2-9. However, when the modified protein is incubated at neutral to slightly alkaline conditions for prolonged periods of time or at elevated temperatures, it undergoes a further (irreversible) modification that decreases its net charge at pH 8.0. Evidence is presented that demonstrates that this additional modification is due to the specific deamidation of asparagine-67. When incubated with an excess of reduced and oxidized glutathiones for 24 h at pH 8.2 and 25 degrees C, the reversible sulfo blocking group was removed, and essentially quantitative (94%) native enzymatic activity was regenerated from both SO3-RNase A and its deamidated derivative (SO3-RNase B). Although the two fully active refolded species differ in their elution behavior on ion-exchange chromatography, they are indistinguishable by many other methods. The significance of this finding for studies of the folding of RNase A is discussed.     

Mycobacterium tuberculosis expresses methionine sulphoxide reductases A and B that protect from killing by nitrite and hypochlorite

Methionine sulphoxide reductases (Msr) reduce methionine sulphoxide to methionine and protect bacteria against reactive oxygen intermediates (ROI) and reactive nitrogen intermediates (RNI). Many organisms express both MsrA, active against methionine-( S )-sulphoxide, and MsrB, active against methionine-( R )-sulphoxide. Mycobacterium tuberculosis (Mtb) expresses MsrA, which protects Δ msrA-Escherichia coli from ROI and RNI. However, the function of MsrA in Mtb has not been defined, and it is unknown whether Mtb expresses MsrB. We identified MsrB as the protein encoded by Rv2674 in Mtb and confirmed the distinct stereospecificities of recombinant Mtb MsrA and MsrB. We generated strains of Mtb deficient in MsrA, MsrB or both and complemented the mutants. Lysates of singly deficient strains displayed half as much Msr activity as wild type against N -acetyl methionine sulphoxide. However, in contrast to other bacteria, single mutants were no more vulnerable than wild type to killing by ROI/RNI. Only Mtb lacking both MsrA and MsrB was more readily killed by nitrite or hypochlorite. Thus, MsrA and MsrB contribute to the enzymatic defences of Mtb against ROI and RNI.