Disulfide intrachain links

Formation of Disulfide Cross-Links After folding into their native conformations, some proteins form intrachain or interchain disulfide bridges between Cys residues. In eukaryotes, disulfide bonds are common in proteins to be exported from cells. The cross-links formed in this way help to protect the native conformation of the protein molecule from denaturation in the extracellular environment, which can differ greatly from intracellular conditions and is generally oxidizing. 

FIGURE 1.31 Typical chemical groupings in a sulfin-vulcanized natural rubber network, (a) Monosulfide crosslink (b) disulfide cross-link (c) polysulfide cross-link (x=3-6) (d) parallel vicinal cross-links ( = 1-6) attached to adjacent main-chain atoms (e) cross-links attached to common or adjacent carbon atoms (f) intrachain cyclic monosulfid (g) intrachain cyclic disulfide (h) conjugated diene (i) pendant sulfide group terminated by moietyZ derived from accelerator. 

Fig. 4.9. Sequences in the Ch regions of human y chains (four subclasses) and of the rabbit y chain. The numbering system of protein Eu (IgGl) is used (2). References for human IgGl (y,) are 2, 59, 78, and 92 for IgG2 (y ) 79, 89, 90, and 168 IgG3 (yi) 80, 167, and 168 IgG4 (74) 88 and 167 rabbit IgG 71, 61-63, 169. The symbols L or H adjacent to a Cys group indicate a disulfide bond linking that Cys to an L or H chain, respectively. Other Cys residues are involved in intrachain disulfide loops. The symbol, CHO, represents carbohydrate. Allotype-related substitutions, shown in the table, are discussed in Chapter 9.

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. 

FIGURE 5.6 Bovine pancreatic ribonuclease A contains 124 amino acid residues, none of which are tryptophan. Four intrachain disulfide bridges (S—S) form cross-links in this... 

Figure 12.5 Proteolytic cleavage of prothrombin by factor Xa, yielding active thrombin. Although prothrombin is a single-chain glycoprotein, thrombin consists of two polypeptides linked by what was originally the prothrombin intrachain disulfide bond. The smaller thrombin polypeptide fragment consists of 49 amino acid residues, and the large polypeptide chain contains 259 amino acids. The N-terminal fragment released from prothrombin contains 274 amino acid residues. Activation of prothrombin by Xa does not occur in free solution, but at the site of vascular damage...

This enzyme (RNase A) is a single chain protein of 124 amino acid residues, cross-linked by four intrachain disulfide bonds. Limited proteolysis of the enzyme cuts a single peptide bond between residues 20 and 21 (Richards and Vithayathil, 1959). The derived protein, RNase S, retains enzymic activity although the N-terminal peptide of 20 amino acids (S-peptide) is no longer covalently attached to the balance of the molecule (S-protein). Removal of S-peptide from... 

For the synthesis of bombyxin IV (53) 192 regioselective formation of three disulfide bonds was achieved by exploiting the differentiated acid-stability of the 5-Trt vs the 5-tBu protection in 49 that allowed the air-mediated intramolecular disulfide formation in the A-chain (Scheme 23). For subsequent activation of the third cysteine residue from the precursor S-tBu derivative the rather drastic conditions of TfOH/TFA (1%) were applied in presence of di(2-pyridyl) disulfide which despite the strong acidity allowed a concomitant deprotection/ activation of this residue to give 50. Subsequent reaction of 50 with the B-chain derivative 51 established the intrachain heterodisulfide cross-link in 52 which on oxidation gave bombyxin IV (53). 

Figure 17. Schematic diagrams of some representative topologically chiral proteins.79 (a) Condensed schematic drawing of the L subunit of the quinoprotein TV-MADH. The looped line represents the polypeptide backbone with N and C terminals. Cysteine (or half-cystine) residues are numbered, and their a-carbons are indicated by filled circles. Intrachain disulfide bonds are shown as dashed lines joining a pair of filled circles. The heavy line symbolizes an intrachain cofactor link, (b) Chromatium high potential iron protein (HiPIP), one of several Fe4S4 cluster-containing proteins, (c) Toxin II from the scorpion Androctonus australis Hector. Reprinted with permission from C. Liang and K. Mislow, J. Math. Chem. 1994,15,245. Copyright 1994, Baltzer Science Publishers.

Figure 2. Diagram showing the structure of rabbit IgG (which exists as a single major subclass). The heavy (H) and light (L) chains are composed of variable (V) and constant (C) domains and are linked by inter- and intrachain disulfide bonds...

On the basis of these data, the following model of the subunit structure is proposed the snail hemagglutinin consists of 6 identical, polypeptide chains (subunits), each containing one intrachain disulfide bond and a carbohydrate binding-site. Furthermore, two subunits are linked by an intrachain, disulfide bond to form subunit dimers of molecular weight 26,000, and three dimers (mol. wt. 26,000) are held together by noncovalent interactions.569... 

Disulfide (-S-S-) bonds play an important role in the structure and function of proteins. IgG molecules are comprised of two heavy and two light chains linked by interchain disulfide bonds. In addition, intrachain disulfide bonds are also present in IgGs. Alkyl thiols have been used to demonstrate the structural and functional role played by disulfide bonds. Reduction of IgGs with alkyl thiols under denaturing conditions results in separation of light and heavy chains. 

Notice that insulin consists of two chains (shown in blue and yellow) linked by two interchain disulfide bonds. The a chain (blue) also has an intrachain disulfide bond. [Drawn from IB2F.pdb. 

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