Cysteine residues

Insulin has 51 ammo acids divided between two chains One of these the A chain has 21 ammo acids the other the B chain has 30 The A and B chains are joined by disul fide bonds between cysteine residues (Cys Cys) Figure 27 10 shows some of the infor matron that defines the ammo acid sequence of the B chain... 

Sanger also determined the sequence of the A chain and identified the cysteine residues involved m disulfide bonds befween fhe A and B chains as well as m fhe disulfide linkage wifhin fhe A chain The complefe insulin sfruefure is shown m Figure 27 11 The sfruefure shown is fhaf of bovine insulin (from cattle) The A chains of human insulin and bovine insulin differ m only fwo ammo acid residues fheir B chains are identical except for the ammo acid at the C terminus... 

The primary structure of a peptide is given by its ammo acid sequence plus any disulfide bonds between two cysteine residues The primary structure is determined by a systematic approach m which the protein is cleaved to smaller fragments even individual ammo acids The smaller fragments are sequenced and the mam sequence deduced by finding regions of overlap among the smaller peptides... 

Disulfide bridge (Section 27 7) An S—S bond between the sulfur atoms of two cysteine residues in a peptide or pro tein... 

For deterrnination of tryptophan, 4 M methanesulfonic acid hydrolysis is employed (18). For cystine, the protein is reduced with 2-mercaptoethanol, the resultant cysteine residue is carboxymethylated with iodoacetic acid, and then the protein sample is hydroly2ed. Also, a one-pot method with mercaptoethanesulfonic acid has been developed for tryptophan and cystine (19). 

Metallothioneins. The metaHothionekis, a group of low (<10,000) molecular weight proteins containing - 30% cysteine residues, are efficient... 

Calcitonins from several species have been characteri2ed and synthesi2ed. They are all single-chain 32-residue polypeptides (ca 3600 mol wt), although a disulfide link between the first and seventh cysteine residues results in a cycHc stmcture that is indispensable for activity (Eig. 6). 

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 ability to identify and quantify cyanobacterial toxins in animal and human clinical material following (suspected) intoxications or illnesses associated with contact with toxic cyanobacteria is an increasing requirement. The recoveries of anatoxin-a from animal stomach material and of microcystins from sheep rumen contents are relatively straightforward. However, the recovery of microcystin from liver and tissue samples cannot be expected to be complete without the application of proteolytic digestion and extraction procedures. This is likely because microcystins bind covalently to a cysteine residue in protein phosphatase. Unless an effective procedure is applied for the extraction of covalently bound microcystins (and nodiilarins), then a negative result in analysis cannot be taken to indicate the absence of toxins in clinical specimens. Furthermore, any positive result may be an underestimate of the true amount of microcystin in the material and would only represent free toxin, not bound to the protein phosphatases. Optimized procedures for the extraction of bound microcystins and nodiilarins from organ and tissue samples are needed. 

During the synthesis of peptides that contain 4-methoxybenzyl-protected cysteine residues, sulfoxide formation may occur. These sulfoxides, when treated with HF/ anisole, form thiophenyl ethers that cannot be deprotected therefore, the peptides should be subjected to a reduction step prior to deprotection. ... 

Two cysteine residues in different parts of the polypeptide chain but adjacent in the three-dimensional structure of a protein can be oxidized to form a disulfide bridge (Figure 1.4). The disulfide is usually the end product of air oxidation according to the following reaction scheme ... 

CH2SH + 1/2 O2 -CH2-S-S-CH2 + H2O Disulfide bonds form between the side chains of two cysfeine residues. Two SH groups from cysteine residues, which may be in different parts of the amino acid sequence but adjacent in the three-dimensional structure, are oxidized to form one S-S (disulfide) group. 

In contrast to the zinc-containing DNA-binding domains described so far, the GAL4 family contains a cluster of two zinc atoms liganded to six cysteine residues, and two of these cysteines (residues 11 and 28) are bound to... 

Lysozyme from bacteriophage T4 is a 164 amino acid polypeptide chain that folds into two domains (Figure 17.3) There are no disulfide bridges the two cysteine residues in the amino acid sequence, Cys 54 and Cys 97, are far apart in the folded structure. The stability of both the wild-type and mutant proteins is expressed as the melting temperature, Tm, which is the temperature at which 50% of the enzyme is inactivated during reversible beat denat-uration. For the wild-type T4 lysozyme the Tm is 41.9 °C. 

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