Intramolecular disulfide bonds

Streptokinase has a molecular weight of about 47,000 with a single chain of 415 amino acids there are no intramolecular disulfide bonds (64). The complete nucleotide sequence of the gene encoding the RNA for this protein has been reported (65,66). 

The Sema domain consisting of about 500 amino acids is characterized by highly conserved cysteine residues that form intramolecular disulfide bonds. Crystal structures have revealed that the Sema domain folds in the manner of the (3 propeller topology which is also found in integrins or the low-density lipoprotein (LDL) receptors. Sema domains are found in semaphorins, plexins and in the receptor tyrosine kinases Met and Ron. 

Like the other pectate lyases, PelZ folding appears to be mainly based on p-sheet structures. Moreover, PelZ possesses only one cysteine residue and thus can not present any intramolecular disulfide bond The folding process of this protein may be different from that of... 

In lipoic acid (6), an intramolecular disulfide bond functions as a redox-active structure. As a result of reduction, it is converted into the corresponding dithiol. As a prosthetic group, lipoic acid is usually covalently bound to a lysine residue (R) of the enzyme, and it is then referred to as lipoamide. Lipoamide is mainly involved in oxidative decarboxylation of 2-0X0 acids (see p. 134). The peptide coenzyme glutathione is a similar disulfide/ dithiol system (not shown see p. 284). 

Oxidation by molecular oxygen most likely occurs via a radical mechanism 10-13 and the reaction rates are generally slow unless traces of metal ions are present which are known to drastically affect the reaction rate 14 thus making control of the air-oxidation reaction rather difficult. The rates can also be significantly enhanced by adding charcoal to induce a surface-assisted catalysis of the intramolecular disulfide bond formation 15 Nonetheless the difficult control of this oxidation procedure can lead to partial oxidation of Met and Trp residues when peptides are exposed for longer periods of time to air oxygen 16 ... 

Oxidative Refolding of Peptides with Two Intramolecular Disulfide Bonds... 

Of this family of peptides containing two intramolecular disulfide bonds the most studied in terms of oxidative refolding are a-conotoxins with two adjacent cysteine residues, i.e. with m = 0, the bee venom toxins, for example apamin and mast cell degranulating peptide, and snake venom toxins, exemplified by sarafotoxins, and endothelins, mammalian peptide hormones with the characteristic Cys-(Xaa)rCys/Cys-(Xaa)3-Cys motif (Scheme 2). With m = 0 or 1 all these peptides are expected to show a weak tendency to form the isomer 3 with a disulfide bond between two proximal cysteine residues. This was fully confirmed by oxidative refolding experiments. 

Results similar to those reported for co-conotoxin GVIA were obtained with the Na+, K+-ATPase inhibitor-I (10) J67 a 49 amino acid residue peptide containing four intramolecular disulfide bonds (Scheme 5)J681 Even in this case the addition of redox reagents to the oxidation buffer had a minor effect on the product distribution. 

As exemplarily shown in the case of charybdotoxin, a 37-residue peptide with three intramolecular disulfide bonds,[70] operating in redox buffer was crucial for efficient formation of the correct disulfide bonds.[71] When the reduced peptide was oxidized in 0.1 M NHtOAc buffer (pH 8.0) at 0.11 mM concentration in the presence of redox reagents (peptide/GSH/GSSG ratio of 1 60 6), the main product was the native peptide contaminated... 

Elafin (12), an elastase-specific inhibitor isolated from human skin, is a 57-residue peptide containing four intramolecular disulfide bonds (Scheme 7).181,821... 

A similar optimization of reaction conditions was required for oxidation of PLTX-II (13) (Scheme 8). This presynaptic calcium channel blocker is a 44-residue peptide containing five intramolecular disulfide bonds and an O-palmitoylated Thr amide at the carboxy-termi-nusJ83 ... 

Although it is a general rule to operate at high dilution (10-4-10 5M) in oxidative folding reactions to avoid formation of intermolecular disulfide-bonded species, p.-conotoxin GIIIB offers an opposite example (Scheme 10). This toxin belongs to a family of conotoxins from marine snails consisting of 22 amino add residues with three intramolecular disulfide bonds.185 ... 

John SA, Revel JP Connexon integrity is maintained by non-covalent bonds Intramolecular disulfide bonds link the extracellular domains in rat connexin 43. Biochem Biophys Res Commun 1991 178 1312-1318. 

Gliadins 35 Soluble in aqueous solutions of alcohol (e.g., 70% ethanol or 55% isopropanol) Primarily globular proteins (MW 30,000-40,000) stabilized by intramolecular disulfide bonds Rich in Glu, Pro, and hydrophobic amino acids, and low content of basic amino acids Glu and Asp in primarily amide form low net charge... 

Thrombopoietin, a 65-85 kDa glycosylated protein, is constitutively expressed by a variety of organs and cell types. Hepatocytes appear to be the major source of human thrombopoietin, and patients with cirrhosis and thrombocytopenia have low serum thrombopoietin levels. Recombinant thrombopoietin is produced by expression in human cells the recombinant product contains two intramolecular disulfide bonds and a number of carbohydrate side chains. 

The protein encoded by the HFE gene is a 343-amino acid protein consisting of a 22-amino acid signal peptide, a large extracellular domain, a single transmembrane domain, and a short cytoplasmic tail (Fig. 31-2). As Figure 31-2 shows, the extracellular domain includes three loops (oij, a2, and a3) with intramolecular disulfide bonds within the second and third loops, a structure similar to other MHC class I proteins (Feder et al., 1996). 

Figure 8-30 Schematic Diagram of Bonds Within and Between Polypeptide Chains in Dough. Solid lines represent covalent bonds, dotted lines other bonds. (1) Intramolecular disulfide bond, (2) free sulfhydryl group, (3) intermolecular disulfide bond, (4) ionic bond, (5) van der Waals bond, (6) interpeptide hydrogen bond, (7) side chain hydrogen bond. Source From A.H. Bloksma, Rheology of Wheat Flour Dough, J. Texture Studies, Vol. 3, pp. 3-17,1972.

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