Sulfhydryl groups Cystine

Disulfides. As shown in Figure 4, the and h-chains of insulin are connected by two disulfide bridges and there is an intrachain cycHc disulfide link on the -chain (see Insulin and other antidiabetic drugs). Vasopressin [9034-50-8] and oxytocin [50-56-6] also contain disulfide links (48). Oxidation of thiols to disulfides and reduction of the latter back to thiols are quite common and important in biological systems, eg, cysteine to cystine or reduced Hpoic acid to oxidized Hpoic acid. Many enzymes depend on free SH groups for activation—deactivation reactions. The oxidation—reduction of glutathione (Glu-Cys-Gly) depends on the sulfhydryl group from cysteine. 

There are four disulfide bonds in short-chain (Type I) neurotoxins. This means that there are eight half-cystines. However, all Hydrophiinae toxins have nine halfcystines with one cysteine residue. An extra cysteine residue can be readily detected from the Raman spectrum as the sulfhydryl group shows a distinct S-H stretching vibration at 2578 cm" Some Laticaudinae toxins do not have a free cysteine residue as in the cases of L. laticaudata and L. semifasciata toxins. In long toxins (Type II) there are five disulfide bonds (Table III). 

One of the most convenient ways of generating sulfhydryl groups is by reduction of indigenous disulfides. Many proteins contain cystine disulfides that are not critical to structure or activity. 

THE SULFHYDRYL GROUP OF CYSTEINE AND THE SYNTHESIS OF PEPTIDES CONTAINING CYSTINE... 

The cyanide nitroprusside test determines the presence of free sulfhydryl or disulfide compounds in urine samples [1, 3,4]. During the first step of the assay, cyanide reduces any disulfides that are present to free sulfhydryl compounds. In the second step, a reddish color reaction results when the free sulfhydryl groups complex with nitroprusside. A positive result is most usually due to cystine in the urine. Familial cystinuria is among the most common aminoacidurias. Disulfides are also excreted in other metabolic disorders such as homo cystinuria and ji-m e reap lol ac la le - cy s lei ne disulfiduria. Both will also produce positive results according to the following reaction RSH + Na2Fe(CN)5NO (sodium nitroprusside) — chromophore + NO. 

Disulfide bond The side chain of cysteine contains a sulfhydryl group (-SH), which is an important component of the active site of many enzymes. In proteins, the -SH groups of two cysteines can become oxidized to form a dimer, cystine, which contains a covalent cross-link called a disulfide bond (-S-S-). (See p. 19 for a further discussion of disulfide bond formation.)... 

One erf the most convenient ways erf generating sulfhydryl groups is by reduction of indigenous disulfides. Many proteins contain cystine disulfides that are not critical to structure or activity. In some cases, mild reducing conditions can free one or more —SH groups for conjugation or modification purposes. The creation erf free sulf-hydryls in this manner allows for site-directed modification at a limited number of locations within the protein molecule. 

Thioredoxin is a protein of 12,000 Da that can donate two electrons by the oxidation of two cysteine sulfhydryl groups to cystine oxidized thioredoxin is reduced by NADPH ... 

Disulfides — A disulfide bond (R-S-S-R) is a strong covalent bond formed by the oxidation of two sulfhydryl groups (R-S-H). An amino acid that commonly forms S-S bonds in proteins is cysteine. When two cysteines are bonded by an S-S bond, the resulting molecule between the two protein chains is called cystine. The presence of disulfide bonds helps to maintain the tertiary structure of the protein. Industrial production of L-cysteine is based on the electrochemical - reduction of L-cystine in acidic - electrolytes using lead or silver -> cathodes. 

Streptomycin was inactivated by reducing and oxidizing agents. The bacteriostatic-activity of this antibiotic for Escherichia coli was reduced in the presence of cysteine, sodium thioglycolate, stannous chloride, sodium bisulfite, sodium hydrosulfide, sodium formate and sodium thiosulfate. Cysteine was the most active. Denkelwater, Cook and Tishler found that the cysteine inactivation of streptomycin could be reversed by iodine presumably cystine was formed during this process. Rake and Donovick inactivated streptomycin with semi-carbazide hydrochloride in order to test the sterility of concentrated streptomycin solutions. The inactivation of streptomycin by compounds containing sulfhydryl groups has been discussed by Cavallito. ... 

The half-cystine residues of many proteins are present in the form of disulfide bridges. Some proteins contain both sulfhydryl and disulfide groups. Since sulfhydryl groups are readily oxidized, and since disulfide interchange complicates the determination of half-cystine pairing in proteins containing several disulfide bonds, it is customary... 

The sulfhydryl groups of two cysteines may form a disulfide, producing cystine... 

Sulfhydryl group functional in the activity of many enzymes is — responsible for disulfide bridges in peptides and proteins cystine is dicysteine, Gys-S-S Cys homocysteine... 

Cleavage of disulfide bonds occurs before hydrolysis of the protein into peptides. Disulfide bonds may be cleaved oxidatively, or they may be reduced and alkylated. Treatment of the native protein with performic acid, a powerful oxidizing agent, breaks disulfide bonds and converts cystine residues to cysteic acid (Figure 3-11). Reduction of the disulfide linkage by thiols, such as d-mercaptoethanol, yields reactive sulfhydryl groups. These groups may be stabilized by alkylation with iodoacetate or ethyleneimine to yield the carboxymethyl or aminoethyl derivative, respectively. 

The purest available specimens of albumin are now known to be homogeneous (F15), except in one unique respect. The native protein appears to contain only 0.6-0.8 mole SH per mole albumin, i.e., in approximately 3 of 10 molecules there is no detectable reduced sulfhydryl group. Two explanations for this phenomenon have been put forward. In the first, the absent SH is thought to be the result of a sulfhydryl-disulfide exchange in the presence of cystine ... 

Three types of oxidation-reduction (redox) centers are found in biology protein side chains, small molecules, and redox cofactors. The first class is frequently overlooked by mechanistic enzymologists. The sulfhydryl group of cysteine is easily oxidized to produce a dimer, known as cystine ... 

Probably the amino acid side chains of proteins most often modified are the c-amino group of lysine and the sulfhydryl group of cysteine, or its oxidized product, the disulfide group of cystine. 

Paramyosin has no cystine residues and a low number of cysteine residues (11, 46, 47), whereas LMM Fr. I (35) has half-cystines and shows no trace of free sulfhydryl groups by the p-chloromercuribenzoate titration. Tropomyosin also has at least one disulfide bond (11) which may link the two chains of the coiled coil (65). 

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