Cystine residues

Peptide chain with two cystine residues (R = -CHgSH)... 

The side groups of the amino acids vary markedly in size and chemical nature and play an important role in the chemical reactions of the fiber. For example, the basic groups (hisidine, arginine, and lysine) can attract acid (anionic) dyes, and in addition the side chains of lysine and hisidine are important sites for the attachment of reactive dyes. The sulfur-containing amino acid cysteine plays a very important role, because almost all of the cysteine residues in the fiber are linked in pairs to form cystine residues, which provide a disulfide bridge —S—S— between different polypeptide molecules or between segments of the same molecules as shown ... 

The correct pairing of half-cystine residues is shown to be dependent upon specific noncovalent bonds 17). With this finding in mind, oxidation of a pair of associating thiols (7 and 2) was chosen as a model reaction. Thiol 7 has the same group as cysteine side chain (HSCH2), 2 being a derivative of cysteamine. 

D. B. Volkin and A. M. Klibanov, Thermal destruction processes in proteins involving cystine residues, J. Biol. Chem, 262, 2945 (1987). 

Permonosulphuric acid treatment confers only a modest shrink-resist effect which usually needs to be improved by a subsequent additive treatment. It has been suggested [300] that the most likely mechanism for inhibiting felting by permonosulphuric acid treatment is the removal of degraded protein from below the exocuticle, producing a modified surface with a reduced differential friction. The direct formation from cystine residues of low concentrations of Bunte salts has been confirmed, as indicated in Scheme 10.42. 

The iodoacetyl group of both isomers reacts with sulfhydryls under slightly alkaline conditions to yield stable thioether linkages (Figure 9.7). They do not react with unreduced disulfides in cystine residues or with oxidized glutathione (Gorman et al., 1987). The thioether bonds will be hydrolyzed under conditions necessary for complete protein hydrolysis prior to amino acid analysis. 

Cysteine residue Cystine residue Histidine residue... 

Dichromate anions are readily absorbed under acidic conditions by wool that has been dyed with chrome dyes. The chromium(VI) on the fibre is then gradually reduced by the cystine residues in wool keratin to chromium(III) cations, which react with the dye ligands to form a stable complex. In this way the cystine disulphide bonds are destroyed, resulting in oxidative degradation of the wool fibres [71]. 

Sokolovsky, M., and A. Patchornik Nonenzymatic cleavage of peptide bonds The hall cystine residues in bovine pancreatic ribonuclease. J. Amer. chem. Soc. 86, 1859—1860 (1964). 

BgK K+ channel-acting toxin from sea anemone Bunodosoma granulifera C (Cys) cysteine (reduced form) or half-cystine residue (oxidized form)... 

The first reaction is p-elimination in cysteine, serine, phosphoserine, and threonine residues due to attack by hydroxide ion, leading to the formation of very reactive dehydroalanine (DHA). In a cystine residue, this results in rupturing of the disulfide bond and liberation of a sulfide ion and free sulfur (Figure 13.4). Nucleophilic additions of the s-amino group of the protein-bound lysine to the double bond of DHA residue causes crosslinking of the polypeptide chain. After hydrolysis, a mixture of L-lysino-L-alanine and L-lysino-D-alanine, with probably a small proportion of dl and dd isomers,... 

Some of the multiple-cystine peptides are resistant to enzymatic and/or chemical cleavage, and thus the more simple methods described in Section 6.1.6.2 cannot be applied. In these cases assignment of their disulfide connectivities is generally attempted by NMR structural analysis. Thereby, if the two half-cystine residues are located on opposite sides of the NMR-derived 3D structure, a disulfide bond between them is improbable. 

Peptide bonds are cleaved in a nonselective, but not in a completely random manner. Based on anchimeric side-chain assistance, steric factors, and bond strains, acid-labile peptide bonds are predicted to include sites containing Asp, Glu, Ser, Thr, Asn, Gin, Gly, and ProJ22l The disulfide topologies of circulin B and cyclopsychotride, backbone-cyclized peptides with three disulfide bonds, were determined by partial hydrolysis for 5 hours.[22 Occasionally, the bond between adjacent half-cystine residues is cleaved due to the nonselective nature of the mechanism of partial acid hydrolysis.[21] By this procedure, in all cases, a complex mixture of peptide fragments is produced which requires careful chromatographic separation by RP-HPLC for subsequent analysis by mass spectrometry (see Section 6.1.6.2.7). 

Adjacent half-cystine residues are present in many peptides and proteins. In most of the cases they form two disulfide bonds with other cysteines in the molecule, unless the peptide bond between them is cis (see Section 6.1.5.1 ).t40 41 Specific enzymes that cleave the Cys-Xaa bond have not yet been discovered, although there are a few reports of cleavage of the Cys-Cys bond by enzymes such as elastase and pepsinJ42-43 For peptides with Cys-Cys bonds the cleavage method in Section 6.1.6.2.4 is recommended. 

Chen and Abruna [104] have studied, using ac and dc cyclic voltammetry, interfacial interaction between the adsorbed porcine pancreatic phospholipase Ai and mercury. The authors have proposed reaction mechanism based on the interaction of cystine residues (disulfide) with mercury. They have found that surface reactions are complex and that several factors influence their mechanism. Their results and observations agree with the reaction pathway postulated for the... 

N 069 " Rotational Isomeric State Treatment of the Cystine Residue. Configuration Partition Function and... 

The absence of half-cystine residues in collagens with chain compositions [ai(I)]2a2 and [ 2(11)13 exclude cystine disulfide bridges from participation in cross-linking in these collagens. However, half-cystine residues have been identified in [ai(III)]3 collagen and disulfide bridges may serve as cross-links in this type of collagen191). 

Breaking Disulfide Bonds Disulfide bonds interfere with the sequencing procedure. A cystine residue (Fig. 3-7) that has one of its peptide bonds cleaved by the Edman procedure may remain attached to another polypeptide strand via its disulfide bond. Disulfide bonds also interfere with the enzymatic or chemical cleavage of the polypeptide. Two approaches to irreversible breakage of disulfide bonds are outlined in Figure 3-26. 

FIGURE 3-26 Breaking disulfide bonds in proteins. Two common methods are illustrated. Oxidation of a cystine residue with performic acid produces two cysteic acid residues. Reduction by dithiothreitol to form Cys residues must be followed by further modification of the reactive —SH groups to prevent re-formation of the disulfide bond. Acetylation by iodoacetate serves this purpose. 

Formation of a disulfide bond by the oxidation of two cysteine residues, producing one cystine residue. 

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