Cysteine disulfide bond

Protein thermostability may be related to glycosylation and compactness but not cysteine disulfide bonds. 

FIGURE 3-7 Reversible formation of a disulfide bond by the oxidation of two molecules of cysteine. Disulfide bonds between Cys residues stabilize the structures of many proteins. 

Disulfide bond exchange. Disulfide linkages are important in determining protein tertiary structure. Disulfide bond formation and/or exchange may occur during metal-catalyzed oxidation of the cysteine residue. This may lead to protein aggregation due to the formation of intermo-lecular disulfide bonds. In addition to cysteine disulfide bond formation, cysteine is susceptible to oxidation (Fig. 134) (200) (See also discussion on thiol chemistry earlier in this chapter). 

Q.26.6 Proteins with subunit chains linked by cysteine-cysteine disulfide bonds (RS-SR) can be reduced to sulfhydryl (R-SH HS-R) containing unlinked chains by the addition of 3-mercaptoethanol or dithiothreitol. These reduced proteins will then run independently according to their molecular... 

The individual chains were then broken down into smaller components Acid was used to cleave the polypeptide backbone, performic acid was used to break the cysteine disulfide bonds, and proteolytic enzymes were used to hydrolyze the polypeptide at specific sites on the chain. The reaction products were separated from each other and their sequence determined. 

The defensins are a family of small antimicrobial peptides with three to four intramolecular cysteine disulfide bonds. They are found in mammals, insects, and plants and are divided on the basis of six conserved cysteine residues into two categories designated a- and jS-defensins (Ganz, 2003). 

Traditionally, antibodies have been tagged with radioisotopes, enzymes (e.g. horseradish peroxidase) or fluorochrome molecules. Thus the chemistry of antibody tagging has been well established and typically involves using accessible primary amines (e.g. lysines) and thiol groups (reduced cysteine disulfide bonds) for binding. The same methods are also used for coupling metal tags to antibodies. 

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... 

Several mucolytics reduce the viscosity of mucus by cleaving the disulfide bonds that maintain the gel stmcture. AJ-Acet l-L-cysteine [616-91 -1] (19), introduced in 1963, and mesna [19677-45-5] (20), developed in Europe in the early 1970s (20,21), are effective compounds in this class. Whereas most mucolytics must be adrninistered by aerosol, carbocysteine [638-23-6] (21), which contains a derivatized sulfhydryl group, has shown activity by the oral route (22,23). However, carbocysteine does not reduce mucus viscosity, as does acetylcysteine, but appears to have a direct action on mucus glycoprotein production (24). 

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. 

Insulin has 51 anino acids, divided between two chains. One of these, the A chain, has 21 amino acids the other, the B chain, has 30. The A and B chains are joined by disulfide bonds between cysteine residues (Cys-Cys). Figure 27.10 shows some of the information that defines the amino acid sequence of the B chain. 

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