Insulin disulphide bridges

Insulin is built up of two polypeptide chains. A of 21 amino-acids and B of 30 amino-acids, linked by two disulphide bridges. 

A small number of proteins, and again insulin is an example, are synthesized as pro-proteins with an additional amino acid sequence which dictates the final three-dimensional structure. In the case of proinsulin, proteolytic attack cleaves out a stretch of 35 amino acids in the middle of the molecule to generate insulin. The peptide that is removed is known as the C chain. The other chains, A and B, remain crosslinked and thus locked in a stable tertiary stiucture by the disulphide bridges formed when the molecule originally folded as proinsulin. Bacteria have no mechanism for specifically cutting out the folding sequences from pro-hormones and the way of solving this problem is described in a later section. 

Unlike polysaccharides, proteins do not have branched chains, but several chains may be linked together via disulphide bridges rather than peptide bonds. The primary structure of ox insulin is shown in Fig. S.A2. The protein consists of two peptide chains which are linked via the formation of the disulphide bridges. Disulphide bridges are formed by the condensation of the thiol groups of two cysteine residues. 

There are distinct receptors for both IGF-I and IGF-II. The IGF-I receptor is similar in structure to that for the insulin receptor, having a disulphide bridge-linked subunit (a-j8)2 structure [49-52]. The a subunit has a molecular mass of 130 kDa which is capable of binding IGF-I. The 95 kDa j8 subunit of the IGF-I receptor, like that for the insulin receptor, exhibits a tyrosyl kinase activity. In marked contrast, however, the IGF-II receptor is a monomeric protein of molecular mass 220 kDa [53,54] with no known intrinsic activity. 

Insulin is a polypeptide with two peptide chains (A chain, 21 amino acids and B chain, 30) linked by two disulphide bridges. The basic structure having metabolic activity is common to all mammalian species but there are minor species differences, which result in the development of antibodies in all patients treated with animal insulins, as well as to unavoidable impurities in the preparations, minimal though these now are. 

The insulin molecule consists of two chains, the A-chain with 21 amino acids and the B-chain with 30 amino acids (Fig. 12). They are interconnected by two intermolecular disulphide bridges between amino acids A7 and B7 and A20 and B19. A third disulphide bridge connects amino acids 6 and 11 on chain A, giving an intramolecular loop. It is synthesized as a single-chain precursor, preproinsulin, which is converted to proinsulin after the molecule has been translocated to the endoplasmic reticulum. There, the C-peptide, which connects the A- and B-chains, is cut away forming the active insulin (Briggs and Gierasch, 1986 Bailyes etal., 1993). The most often used insulins in therapeutics (Fig. 12), bovine, porcine and human insulin, exhibit differences in their amino acid sequences bovine insulin contains Ala instead of Thr in position 8 and Val instead of lie in position 10 of the A-chain, and both bovine and porcine insulin differ from human insulin by an Ala instead of Thr in position 30 of the B-chain. 

Insulin is a polypeptide containing SI amino acid.s arranged in two chains (A and Bl linked by disulphide bridges. A precursor, called proinsulin, is hydrolysed inside storage granules to fonn insulin and a residual C-peptide. The granules store insulin as crystals containing zinc and insulin. 

The amino acid sequences of four avian insulins have been determined, namely domestic fowl, turkey, duck and goose. Those of domestic fowl and turkey are identical but differ from that of the duck and goose at three positions (Fig. 7.2). There are also differences in the size of the C-peptide, which is 28 residues in the domestic fowl and 26 in the duck. Another feature that is evident from the proinsulin sequence (Fig. 7.2) is that the C-peptide is less conserved than the A- and B-peptides. tW C-peptide simply acts as a link between the A- and B-peptides to enable the disulphide bridges to form. Domestic fowl insulin is more potent than bovine insulin in ellidting changes in metabolism in the domestic fowl. This has been attributed to the six differences in the amino acid sequences of the two proteins. 

Both insulin and IGFs appear to have evolved from a common ancestral molecule, since they have about 60% of amino acids identical (Epple Brinn, 1987). IGFs are single chain polypeptides of 70 amino acid residues, with three disulphide bridges as in proinsulin (Fig. 7.2). IGF-I is synthesised as a 153 residue primary translation product (Kajimoto Rotwein, 1989). A 48 amino acid residue peptide is cleaved from the N-terminus, and a 35 amino acid residue peptide from the C-terminus. The complete amino acid sequence of IGF-I derived from the nucleotide sequence of cDNA has been determined (Fawcett Bulfield, 1990). The amino acid sequence has also been determined directly (Ballard et /.,... 

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