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PubMed1997

The role of the thioredoxin and glutaredoxin pathways in reducing protein disulfide bonds in the Escherichia coli cytoplasm.

In Escherichia coli, two pathways use NADPH to reduce disulfide bonds that form in some cytoplasmic enzymes during catalysis: the thioredoxin system, which consists of thioredoxin reductase and thioredoxin, and the glutaredoxin system, composed of glutathione reductase, glutathione, and three glutaredoxins. These systems may also reduce disulfide bonds which form spontaneously in cytoplasmic proteins when E. coli is grown aerobically. We have investigated the role of both systems in determining the thiol-disulfide balance in the cytoplasm by determining the ability of protein disulfide bonds to form in mutants missing components of these systems. We find that both the thioredoxin and glutaredoxin systems contribute to reducing disulfide bonds in cytoplasmic proteins. In addition, these systems can partially substitute for each other in vivo since double mutants missing parts of both systems generally allow substantially more disulfide bond formation than mutants missing components of just one system. Some of these double mutants were found to require the addition of a disulfide reductant to the medium to grow well aerobically. Thus, E. coli requires either a functional thioredoxin or glutaredoxin system to reduce disulfide bonds which appear after each catalytic cycle in the essential enzyme ribonucleotide reductase and perhaps to reduce non-native disulfide bonds in cytoplasmic proteins. Our results suggest the existence of a novel thioredoxin in E. coli.

Bacterial Proteins↗

PubMed2000

Synthesis, 1H NMR structure, and activity of a three-disulfide-bridged maurotoxin analog designed to restore the consensus motif of scorpion toxins.

Maurotoxin (MTX) is a 34-residue toxin that has been isolated from the venom of the chactidae scorpion Scorpio maurus palmatus. The toxin displays an exceptionally wide range of pharmacological activity since it binds onto small conductance Ca(2+)-activated K(+) channels and also blocks Kv channels (Shaker, Kv1.2 and Kv1.3). MTX possesses 53-68% sequence identity with HsTx1 and Pi1, two other K(+) channel short chain scorpion toxins cross-linked by four disulfide bridges. These three toxins differ from other K(+)/Cl(-)/Na(+) channel scorpion toxins cross-linked by either three or four disulfide bridges by the presence of an extra half-cystine residue in the middle of a consensus sequence generally associated with the formation of an alpha/beta scaffold (an alpha-helix connected to an antiparallel beta-sheet by two disulfide bridges). Because MTX exhibits an uncommon disulfide bridge organization among known scorpion toxins (C1-C5, C2-C6, C3-C4, and C7-C8 instead of C1-C4, C2-C5, and C3-C6 for three-disulfide-bridged toxins or C1-C5, C2-C6, C3-C7, and C4-C8 for four-disulfide-bridged toxins), we designed and chemically synthesized an MTX analog with three instead of four disulfide bridges ([Abu(19),Abu(34)]MTX) and in which the entire consensus motif of scorpion toxins was restored by the substitution of the two half-cystines in positions 19 and 34 (corresponding to C4 and C8) by two isosteric alpha-aminobutyrate (Abu) derivatives. The three-dimensional structure of [Abu(19), Abu(34)]MTX in solution was solved by (1)H NMR. This analog adopts the alpha/beta scaffold with now conventional half-cystine pairings connecting C1-C5, C2-C6, and C3-C7 (with C4 and C8 replaced by Abu derivatives). This novel arrangement in half-cystine pairings that concerns the last disulfide bridge results mainly in a reorientation of the alpha-helix regarding the beta-sheet structure. In vivo, [Abu(19),Abu(34)]MTX remains lethal in mice as assessed by intracerebroventricular injection of the peptide (LD(50) value of 0. 25 microg/mouse). The structural variations are also accompanied by changes in the pharmacological selectivity of the peptide, suggesting that the organization pattern of disulfide bridges should affect the three-dimensional presentation of certain key residues critical to the blockage of K(+) channel subtypes.

Amino Acid Sequence↗

PubMed2000

Disulfide bonds of GM2 synthase homodimers. Antiparallel orientation of the catalytic domains.

GM2 synthase is a homodimer in which the subunits are joined by lumenal domain disulfide bond(s). To define the disulfide bond pattern of this enzyme, we analyzed a soluble form by chemical fragmentation, enzymatic digestion, and mass spectrometry and a full-length form by site-directed mutagenesis. All Cys residues of the lumenal domain of GM2 synthase are disulfide bonded with Cys(429) and Cys(476) forming a disulfide-bonded pair while Cys(80) and Cys(82) are disulfide bonded in combination with Cys(412) and Cys(529). Partial reduction to produce monomers converted Cys(80) and Cys(82) to free thiols while the Cys(429) to Cys(476) disulfide remained intact. CNBr cleavage at amino acid 330 produced a monomer-sized band under nonreducing conditions which was converted upon reduction to a 40-kDa fragment and a 24-kDa myc-positive fragment. Double mutation of Cys(80) and Cys(82) to Ser produced monomers but not dimers. In summary these results demonstrate that Cys(429) and Cys(476) form an intrasubunit disulfide while the intersubunit disulfides formed by both Cys(80) and Cys(82) with Cys(412) and Cys(529) are responsible for formation of the homodimer. This disulfide bond arrangement results in an antiparallel orientation of the catalytic domains of the GM2 synthase homodimer.

Amino Acid Sequence↗

PubMed2002

Conformation of beta 2-microglobulin amyloid fibrils analyzed by reduction of the disulfide bond.

Beta2-microglobulin (beta2-m), a major component of dialysis-related amyloid fibrils, has an intrachain disulfide bond buried inside the native structure. We examined the conformation of beta2-m amyloid fibrils by analyzing the reactivity of the disulfide bond to a reducing reagent, dithiothreitol. Although the disulfide bond in the native structure was highly protected from reduction, the disulfide bonds in the amyloid fibrils prepared at pH 2.5 were progressively reduced at pH 8.5 by 50 mm dithiothreitol. Because beta2-m amyloid fibrils prepared under acidic conditions have been known to depolymerize at a neutral pH, we examined the relation between depolymerization and reduction of the disulfide bond. The results indicate that the disulfide bonds in the amyloid fibrils were protected from reduction, and the reduction occurred during depolymerization. On the other hand, the disulfide bonds of immature filaments, the thin and flexible filaments prepared under conditions of high salt at pH 2.5, were reduced at pH 8.5 more readily than those of amyloid fibrils, suggesting that the disulfide bonds are exposed to the solvent. Taken together, the disulfide bond once exposed to the solvent upon acid denaturation may be progressively buried in the interior of the amyloid fibrils during its formation.

Amyloid↗

PubMed2005

Metallothionein disulfides are present in metallothionein-overexpressing transgenic mouse heart and increase under conditions of oxidative stress.

Metallothionein (MT) releases zinc under oxidative stress conditions in cultured cells. The change in the MT molecule after zinc release in vivo is unknown although in vitro studies have identified MT disulfide bond formation. The present study was undertaken to test the hypothesis that MT disulfide bond formation occurs in vivo. A cardiac-specific MT-overexpressing transgenic mouse model was used. Mice were administered saline as a control or doxorubicin (20 mg/kg), which is an effective anticancer drug but with severe cardiac toxicity at least partially because of the generation of reactive oxygen species. A differential alkylation of cysteine residues in MT of the heart extracts was performed. Free and metal-bound cysteines were first trapped by N-ethylmaleimide and the disulfide bonds were reduced by dithiothreitol followed by alkylation with radiolabeled iodoacetamide. Analyses of the differentially alkylated MTs in the heart extract by high performance liquid chromatography, SDS-PAGE, Western blot, and mass spectrometry revealed that disulfide bonds were present in MT in vivo under both physiological and oxidative stress conditions. More disulfide bonds were found in MT under the oxidative stress conditions. The MT disulfide bonds were likely intramolecular and both alpha- and beta-domains were involved in the disulfide bond formation, although the alpha-domain appeared to be more easily oxidized than the beta-domain. The results suggest that under physiological conditions, the formation of MT disulfide bonds is involved in the regulation of zinc homeostasis. Additional zinc release from MT under oxidative stress conditions is accompanied by more MT disulfide bond formation.

Animals↗

PubMed2000

Two pairs of conserved cysteines are required for the oxidative activity of Ero1p in protein disulfide bond formation in the endoplasmic reticulum.

In the major pathway for protein disulfide-bond formation in the endoplasmic reticulum (ER), oxidizing equivalents flow from the conserved ER-membrane protein Ero1p to secretory proteins via protein disulfide isomerase (PDI). Herein, a mutational analysis of the yeast ERO1 gene identifies two pairs of conserved cysteines likely to form redox-active disulfide bonds in Ero1p. Cys100, Cys105, Cys352, and Cys355 of Ero1p are important for oxidative protein folding and for cell viability, whereas Cys90, Cys208, and Cys349 are dispensable for these functions. Substitution of Cys100 with alanine impedes the capture of Ero1p-Pdi1p mixed-disulfide complexes from yeast, and also blocks oxidation of Pdi1p in vivo. Cys352 and Cys355 are required to maintain the fully oxidized redox state of Ero1p, and also play an auxiliary role in thiol-disulfide exchange with Pdi1p. These results suggest a model for the function of Ero1p wherein Cys100 and Cys105 form a redox-active disulfide bond that engages directly in thiol-disulfide exchange with ER oxidoreductases. The Cys352-Cys355 disulfide could then serve to reoxidize the Cys100-Cys105 cysteine pair, possibly through an intramolecular thiol-disulfide exchange reaction.

Alanine↗

PubMed2001

Identification of disulfide-linked peptides by isotope profiles produced by peptic digestion of proteins in 50% (18)O water.

Determination of the disulfide-bond arrangement of a protein by characterization of disulfide-linked peptides in proteolytic digests may be complicated by resistance of the protein to specific proteases, disulfide interchange, and/or production of extremely complex mixtures by less specific proteolysis. In this study, mass spectrometry has been used to show that incorporation of (18)O into peptides during peptic digestion of disulfide-linked proteins in 50% (18)O water resulted in isotope patterns and increases in average masses that facilitated identification and characterization of disulfide-linked peptides even in complex mixtures, without the need for reference digests in 100% (16)O water. This is exemplified by analysis of peptic digests of model proteins lysozyme and ribonuclease A (RNaseA) by matrix-assisted laser desorption/ionization-time of flight (MALDI-TOF) and electrospray ionization (ESI) mass spectrometry (MS). Distinct isotope profiles were evident when two peptide chains were linked by disulfide bonds, provided one of the chains did not contain the C terminus of the protein. This latter class of peptide, and single-chain peptides containing an intrachain disulfide bond, could be identified and characterized by mass shifts produced by reduction. Reduction also served to confirm other assignments. Isotope profiling of peptic digests showed that disulfide-linked peptides were often enriched in the high molecular weight fractions produced by size exclusion chromatography (SEC) of the digests. Applicability of these procedures to analysis of a more complex disulfide-bond arrangement was shown with the hemagglutinin/neuraminidase of Newcastle disease virus.

Amino Acid Sequence↗

PubMed1994

A novel strategy for the synthesis of the cysteine-rich protective antigen of the malaria merozoite surface protein (MSP-1). Knowledge-based strategy for disulfide formation.

The most promising antigen for a protective malaria vaccine is a cysteine-rich domain at the carboxyl terminus of the merozoite surface protein (MSP-1). Passive transfer of anti-MSP-1 antibody or immunization of MSP-1 against infection challenge confers protection in primate and rodent models. The antigen belongs to the three-disulfide epidermal growth factor (EGF) family based on the alignment of the six cysteines. In the K1 strain there are, however, only four cysteines corresponding to the four carboxyl cysteines of EGF. Furthermore, disulfide pairing would produce a non-EGF pattern. Because this cysteine-rich antigen is conformation-dependent, and reduction of the disulfide bonds abolishes antigenicity, we used a synthetic analog to investigate the probable disulfide pairing of this antigen. This paper describes the synthesis, folding and disulfide pairings of two 50-residue cysteine-rich peptides. One contains two disulfides (VK-50) derived from the native sequence of MSP-1 of the Thailand K1 strain (aa 1629-1679). The other contains an EGF-like, three-disulfide [Cys-9,14]VK-50 peptide. Both peptides were synthesized by a solid-phase method using Fmoc-chemistry. The crude peptide of VK-50 was folded, and the disulfide was oxidized by the DMSO method to obtain a structure with an expected disulfide pairing of 3-4, and 5-6. The specific pairing pattern of 1-3, 2-4 and 5-6 in [Cys 9,14]VK-50 corresponding to EGF in [Cys 9,14]VK-50 was obtained using a 'knowledge-based' (KB) strategy for their formation.(ABSTRACT TRUNCATED AT 250 WORDS)

Amino Acid Sequence↗

PubMed1987

Structural disulfide bonds in the Bacillus thuringiensis subsp. israelensis protein crystal.

We examined disulfide bonds in mosquito larvicidal crystals produced by Bacillus thuringiensis subsp. israelensis. Intact crystals contained 2.01 X 10(-8) mol of free sulfhydryls and 3.24 X 10(-8) mol of disulfides per mg of protein. Reduced samples of alkali-solubilized crystals resolved into several proteins, the most prominent having apparent molecular sizes of 28, 70, 135, and 140 kilodaltons (kDa). Nonreduced samples contained two new proteins of 52 and 26 kDa. When reduced, both the 52- and 26-kDa proteins were converted to 28-kDa proteins. Furthermore, both bands reacted with antiserum prepared against reduced 28-kDa protein. Approximately 50% of the crystal proteins could be solubilized without disulfide cleavage. These proteins were 70 kDa or smaller. Solubilization of the 135- and 140-kDa proteins required disulfide cleavage. Incubation of crystals at pH 12.0 for 2 h cleaved 40% of the disulfide bonds and solubilized 83% of the crystal protein. Alkali-stable disulfides were present in both the soluble and insoluble portions. The insoluble pellet contained 12 to 14 disulfides per 100 kDa of protein and was devoid of sulfhydryl groups. Alkali-solubilized proteins contained both intrachain and interchain disulfide bonds. Despite their structural significance, it is unlikely that disulfide bonds are involved in the formation or release of the larvicidal toxin.

Bacillus thuringiensis↗

PubMed2006

Furin cleavage potentiates the membrane fusion-controlling intersubunit disulfide bond isomerization activity of leukemia virus Env.

The membrane fusion protein of murine leukemia virus is a trimer of a disulfide-linked peripheral-transmembrane (SU-TM) subunit complex. The intersubunit disulfide bond is in SU linked to a disulfide bond isomerization motif, CXXC, with which the virus controls its fusion reaction (M. Wallin, M. Ekström, and H. Garoff, EMBO J. 23:54-65, 2004). Upon receptor binding the isomerase rearranges the intersubunit disulfide bond into a disulfide bond isomer within the motif. This facilitates SU dissociation and fusion activation in the TM subunit. In the present study we have asked whether furin cleavage of the Env precursor potentiates the isomerase to be triggered. To this end we accumulated the late form of the precursor, gp90, in the cell by incubation in the presence of a furin-inhibiting peptide. The isomerization was done by NP-40 incubation or by a heat pulse under alkylation-free conditions. The cells were lysed in the presence of alkylator, and the precursor was immunoprecipitated, gel isolated, deglycosylated, and subjected to complete trypsin digestion. Disulfide-linked peptide complexes were separated by sodium dodecyl sulfate-tricine-polyacrylamide gel electrophoresis under nonreducing conditions. This assay revealed the size of the characteristic major disulfide-linked peptide complex that differentiates the two isomers of the disulfide bond between Cys336 (or Cys339) and Cys563, i.e., the bond corresponding to the intersubunit disulfide bond. The analyses showed that the isomerase was five- to eightfold more resistant to triggering in the precursor than in the mature, cleaved form. This suggests that the isomerase becomes potentiated for triggering by a structural change in Env that is induced by furin cleavage in the cell.

Disulfides↗

PubMed1993

Location of intrachain disulfide bonds in the VP5* and VP8* trypsin cleavage fragments of the rhesus rotavirus spike protein VP4.

Because the rotavirus spike protein VP4 contains conserved Cys residues at positions 216, 318, 380, and 774 and, for many animal rotaviruses, also at position 203, we sought to determine whether disulfide bonds were structural elements of VP4. Electrophoretic analysis of untreated and trypsin-treated rhesus rotavirus (RRV) and simain rotavirus SA11 in the presence and absence of the reducing agent dithioerythritol revealed that VP4 and its cleavage fragments VP5* and VP8* possessed intrachain disulfide bonds. Given that the VP8* fragments of RRV and SA11 contain only two Cys residues, those at positions 203 and 216, these data indicated that these two residues were covalently linked. Electrophoretic examination of truncated species of VP4 and VP4 containing Cys-->Ser mutations synthesized in reticulocyte lysates provided additional evidence that Cys-203 and Cys-216 in VP8* of RRV were linked by a disulfide bridge. VP5* expressed in vitro was able to form a disulfide bond analogous to that in the VP5* fragment of trypsin-treated RRV. Analysis of a Cys-774-->Ser mutant of VP5* showed that, while it was able to form a disulfide bond, a Cys-318-->Ser mutant of VP5* was not. These results indicated that the VP4 component of all rotaviruses, except B223, contains a disulfide bond that links Cys-318 and Cys-380 in the VP5* region of the protein. This bond is located between the trypsin cleavage site and the putative fusion domain of VP4. Because human rotaviruses lack Cys-203 and, hence, unlike many animal rotaviruses cannot possess a disulfide bond in VP8*, it is apparent that VP4 is structurally variable in nature, with human rotaviruses generally containing one disulfide linkage and animal rotaviruses generally containing two such linkages. Considered with the results of anti-VP4 antibody mapping studies, the data suggest that the disulfide bond in VP5* exists within the 2G4 epitope and may be located at the distal end of the VP4 spike on rotavirus particles.

Amino Acid Sequence↗

PubMed2002

Disulfide bond configuration of human cytomegalovirus glycoprotein B.

Glycoprotein B (gB) is the most highly conserved of the envelope glycoproteins of human herpesviruses. The gB protein of human cytomegalovirus (CMV) serves multiple roles in the life cycle of the virus. To investigate structural properties of gB that give rise to its function, we sought to determine the disulfide bond arrangement of gB. To this end, a recombinant form of gB (gB-S) comprising the entire ectodomain of the glycoprotein (amino acids 1 to 750) was constructed and expressed in insect cells. Proteolytic fragmentation and mass spectrometry were performed using purified gB-S, and the five disulfide bonds that link 10 of the 11 highly conserved cysteine residues of gB were mapped. These bonds are C94-C550, C111-C506, C246-C250, C344-C391, and C573-C610. This configuration closely parallels the disulfide bond configuration of herpes simplex type 2 (HSV-2) gB (N. Norais, D. Tang, S. Kaur, S. H. Chamberlain, F. R. Masiarz, R. L. Burke, and F. Markus, J. Virol. 70:7379-7387, 1996). However, despite the high degree of conservation of cysteine residues between CMV gB and HSV-2 gB, the disulfide bond arrangements of the two homologs are not identical. We detected a disulfide bond between the conserved cysteine residue 246 and the nonconserved cysteine residue 250 of CMV gB. We hypothesize that this disulfide bond stabilizes a tight loop in the amino-terminal fragment of CMV gB that does not exist in HSV-2 gB. We predicted that the cysteine residue not found in a disulfide bond of CMV gB, cysteine residue 185, would play a role in dimerization, but a cysteine substitution mutant in cysteine residue 185 showed no apparent defect in the ability to form dimers. These results indicate that gB oligomerization involves additional interactions other than a single disulfide bond. This work represents the second reported disulfide bond structure for a herpesvirus gB homolog, and the discovery that the two structures are not identical underscores the importance of empirically determining structures even for highly conserved proteins.

Cell Line↗

PubMed2002

Biological regulation through protein disulfide bond cleavage.

It is thought that disulfide bonds in secreted proteins are inert because of the oxidizing nature of the extracellular milieu. We have suggested that this is not necessarily the case and that certain secreted proteins contain one or more disulfide bonds that can be cleaved and that this cleavage is central to the protein's function. This review discusses disulfide bond cleavage in the secreted soluble protein, plasmin. Cleavage of plasmin disulfide bond(s) triggers peptide bond cleavage and formation of the tumour angiogenesis inhibitor, angiostatin. Tumour cells secrete phosphoglycerate kinase which facilitates cleavage of the plasmin disulfide bond(s). Phosphoglycerate kinase is not a conventional disulfide bond reductase. We propose that phosphoglycerate kinase facilitates cleavage of a particular plasmin disulfide bond by hydroxide ion, which results in formation of a sulfenic acid and a free thiol. The free thiol is then available to exchange with another nearby disulfide bond resulting in formation of a new disulfide and a new free thiol. The reduced plasmin is then susceptible to discreet proteolysis which results in release of angiostatin.

Amino Acid Sequence↗

PubMed1990

Inactivation of (Na+,K+)-ATPase by beta-mercaptoethanol. Differential sensitivity to reduction of the three beta subunit disulfide bonds.

The relative rates of reduction of disulfide bonds in sheep kidney (Na+,K+)-ATPase were determined in order to test the hypothesis that cleavage of a single disulfide bond in the beta subunit is responsible for loss of ATPase activity upon treatment with high concentrations of beta-mercaptoethanol at elevated temperatures (Kawamura, M., and Nagano, K. (1984) Biochim. Biophys. Acta 774, 188-192). This was accomplished by peptide mapping of tryptic digests of the alpha and beta subunits after treatment with beta-mercaptoethanol. Alkylation with 4-(aminosulfonyl)-7-fluoro-2,1,3-benzoxadiazole of all free sulfhydryls and all newly produced sulfhydryls resulting from reduction of disulfides by beta-mercaptoethanol introduced a fluorophore into those residues. The protective effect of NaCl and KCl on ATPase activity was examined with regard to the ability of those salts to protect the disulfides from reduction. The results indicated that although the disulfides in the beta subunit exhibit differential stabilities to reduction, the reduction of no single disulfide in the beta subunit can be quantitatively correlated with the loss of ATPase activity and the ability to bind ouabain. The kinetics of inhibition of ouabain binding were identical to the kinetics of inhibition of ATPase activity, suggesting a delocalized denaturation rather than reduction of a single disulfide bond as the mechanism of beta-mercaptoethanol induced inhibition. NaCl and KCl only partially protected all three beta disulfides from reduction, but completely eliminated the loss of ATPase activity and ouabain binding sites induced by beta-mercaptoethanol. The results suggest a possible folding model for the beta subunit. The alpha subunit was found to contain three disulfide bonds, but these bonds were not reduced by the beta-mercaptoethanol treatment used to inactivate the ATPase activity.

Adenosine Triphosphate↗

PubMed1990

Effects of carbon disulfide on cardiovascular function after acute and subacute exposure of rats.

Epidemiologic studies provided evidence that increased heart disease mortality after carbon disulfide exposure is not only related to carbon disulfide-caused coronary sclerosis but they suggest that there would be reversible, direct cardiotoxic effects. In order to investigate such direct toxic effects of carbon disulfide on the cardiovascular system, in this study carbon disulfide was administered acutely and subacutely to rats. Blood pressure and heart rate in normotensive conscious unrestrained rats were not markedly influenced by both acute and subacute carbon disulfide administration. Depressant effects of carbon disulfide administration on intracardiac impulse generation and conduction as well as on contractile force were observed in urethane-anesthetized rats. The appearance of several forms of aconitine-induced arrhythmias was delayed after subacute treatment with carbon disulfide. In the coronary occlusion model, subacute carbon disulfide treatment reduced the survival rate accompanied by marked influences on arrhythmia development. Our results show that short term carbon disulfide exposure alters cardiac function in rats under physiological and pathophysiological conditions.

Aconitine↗

PubMed1986

The crystallographically determined structures of atypical strained disulfides engineered into subtilisin.

The geometries of two disulfide bridges genetically engineered into subtilisin have been characterized by x-ray crystallography to determine the structural and energetic constraints involved in introducing disulfide bonds into proteins. Both disulfide bridges (Cys-24-Cys-87 and Cys-22-Cys-87) exhibit atypical sets of dihedral angles compared to those for other reported disulfide structures in proteins. The geometric trends for naturally occurring disulfides in protein crystal structures are examined. Comparison of the disulfide-containing mutant protein structures with the wild-type structure shows that, in both cases, disulfide incorporation is accommodated by relatively minor changes in local main-chain conformation. The Cys-22-Cys-87 disulfide has two high energy dihedral angles (X2 = 121 degrees, X2' = 143 degrees). Both disulfides produce short non-bonded contacts with the main-chain.

Bacillus subtilis↗

PubMed1984

Role of a disulfide bond in the gamma subunit in activation of the ATPase of chloroplast coupling factor 1.

The relationship between activation of the latent ATPase activity of isolated chloroplast coupling factor 1 (CF1) and reduction of a disulfide in the gamma subunit has been assessed. The sulfhydryl residues involved in the disulfide bond are distinct from residues normally accessible to maleimide modification during incubation of thylakoids in the dark or the light. Dithiothreitol-induced activation is time dependent, and correlates with reduction of the disulfide. Sulfhydryl residues exposed during activation can be reoxidized to disulfide by incubation with iodosobenzoate , with a concomitant loss of ATPase activity. Activation and deactivation are reversible, but deactivation is prevented by treatment of the reduced enzyme with N-ethylmaleimide. Heat activation does not reduce the disulfide bond unless dithiothreitol is present during activation. Prior heating of CF1, which partially activates the enzyme, renders the disulfide more susceptible to subsequent dithiol reduction. The activity obtained when heat and dithiothreitol are used together is approximately equal to the sum of the partial activations obtained with heat or dithiothreitol alone. Iodosobenzoate has no effect on heat-activated CF1. Enzyme activated by heating in the presence of dithiothreitol can be partially deactivated, consistent with reversal of the activity attributable to the dithiol effect. Fluorescence polarization of anilinonaphthylmaleimide bound to the reduced enzyme indicates that the sulfhydryl residues involved in the disulfide are in a less rigid environment than the other two sulfhydryl residues in the gamma subunit. Polarization of anilinonaphthylmaleimide bound to these sulfhydryls is reduced by heat treatment of CF1. The increased susceptibility of the disulfide to reduction upon heat treatment, and the activation of ATPase activity with or without disulfide bond cleavage are indicative of conformational changes within the gamma subunit that occur during the conversion of CF1 from a latent to an active ATPase. In addition the results are consistent with at least two distinct conformational forms of CF1 that can hydrolyze ATP.

Disulfides↗

PubMed1981

Assignment of disulfide bonds in the beta subunit of human chorionic gonadotropin.

The assignment of five disulfide bonds in the alpha subunit of human chorionic gonadotropin (hCG) using partial reduction and S-[14C]carboxymethylation has been reported earlier (Mise, T., and Bahl, O. P. (1980) J. Biol. Chem. 255, 8516-8522). Employing a similar approach, we have determined the locations of six disulfide bonds in hCG-beta. Two partially reduced and S-[14C]carboxymethylated hCG-beta derivatives, DS1.4-hCG-beta and DS3.4-hCG-beta in which on the average 1.4 and 3.4 disulfide bonds were modified, respectively, were prepared. The 14C-labeled derivatives were then completely reduced and S-carboxymethylated with nonradioactive iodoacetic acid and subjected to hydrolysis with trypsin. The radioactive peptides were purified by gel filtration and high voltage paper electrophoresis. The tryptic peptides containing two or more S-[14C]carboxymethylcysteines were further degraded using various proteolytic enzymes such as thermolysin, carboxypeptidase A and Y, cathespin C, and subtilisin to obtain individual S-[14C]carboxymethylcysteine-containing peptides. From the specific radioactivities of S-[14C]carboxymethylcysteines in DS3.4-hCG-beta, four out of six disulfide bonds, 9-90, 26-110, 34-88, and 93-100 were assigned. Similar data from DS1.4-hCG-beta gave the locations of the other two disulfide bonds, 23-72 and 38-57, while confirming the locations of four disulfide bonds derived from the radioactivity distribution in DS3.4-hCG-beta. Thus, all six disulfide bonds in hCG-beta have been located. The results of controlled reduction and S-[14C]alkylation also indicate that disulfide bond 93-100 is the most reactive, followed by disulfide bond 26-110, and that the least reactive among all is 34-88.

Amino Acid Sequence↗