Cystine linkages

Fig. 7.7. Some of the covalent links which occur in structural proteins. ABCD represents one protein, EFB C G another. The N-terminal amino acid A is linked to an o-quinone which is also linked to a lysyl residue F(=quinone tanning) BB represents two tyrosyl residues coupled by a biphenyl linkage CC represents two cysteinyl residues coupled by a cystine linkage (as in keratin) D, E and G are not cross-linked. (After Brunet, 1967.)...

The general properties of regenerated portein fibres include the wool like attributes of resilience, warmth and soft handle. The strength is lower than wool. These fibres do not contain cystine linkage, which results in more open structure. These fibres are far less successful than hoped and never seriously challanged wool. 1.7 Synthetic Fibres... 

R.NHj+HOOC.R = R.NH,6oC.R, and adjacent molecular chains are also united by cystine linkages ... 

A unique feature of keratin is the existence of cystine linkages CH CH... 

There is abundant evidence that oxidizing agents attack the cystine linkage, although the nature of the reaction is obscure. Hydrogen peroxide will, under certain conditions, oxidize all the sulphur with the formation of sulphuric acid or sulphates. Alexander, Hudson, and Fox Biochem. J., 1946, 50, 27) showed that peracetic acid and other peracids could oxidize all the disulphide groups. A product is obtained which, after acid hydrolysis. 

If a wool fibre be immersed under tension in water at 100 C for one hour it will become distorted permanently in other words, will have acquired a permanent set. Binding forces of a more powerful nature than hydrogen bonds must have been created. The explanation proposed by Speakman Nature, 1933, 132, 930) was that the water severs the cystine linkage with the formation of sulphenic acid and sulphide radicals ... 

Properties and Structure. a -Acid glycoprotein (a -AGP) has a molecular mass of about 41,000 and consists of a peptide chain having 181 amino acid residues and five carbohydrate units (14,15). Two cystine disulfide cross-linkages connect residues 5 and 147 and residues 72 and 164. The carbohydrate units comprise 45% of the molecule and contain siaUc acid, hexosamine, and neutral hexoses. In phosphate buffer the isoelectric point of the... 

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. 

The deamino-dicarba-analogues of various biologically active cystine-containing peptides, such as oxytocin/1-3 calcitonin/31 32 and somatostatin/33-35 have shown high biological activity and enhanced metabolic and chemical stability owing to the absence of reducible disulfide linkages. 

Subscript (ads) denotes adsorption via a thiolate linkage, while (ps) stands for a physisorbed and/or adsorbed state via different interactions. However, large dimensions of the studied molecules and their amphiphilic nature make the surface reaction mechanism more complex than in case of cystine/cysteine. Interfacial microstructure plays an important role in the determination of the surface behavior of the adsorbed molecules. From the study on the charge-transfer kinetics, the transfer coefficient a was calculated as slightly less than 0.50, while the rate constant (based on Laviron s derivations [105]) was of the order of 10 s k The same authors [106] have shown earlier that the adsorption rate constant of porcine pancreatic phospholipase A2 at mercury via one of its disulfide groups is of the order of 10 s h... 

Possibilities to cleave the disulfide linkage of the fluorocarbon cystine leading to partially watersoluble cysteine derivatives are given in the following reaction scheme ... 

The structure of wool is more complicated than that of silk fibroin (Figure 25-13) because wool, like insulin (Figure 25-8) and lysozyme (Figure 25-15), contains a considerable quantity of cystine, which provides —S—S— (disulfide) cross-links between the peptide chains. These disulfide linkages play... 

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