Proinsulin structure

Figure 42-12. Structure of human proinsulin. Insulin and C-peptide molecules are connected at two sites by dipeptide links. An initial cleavage by a trypsin-like enzyme (open arrows) followed by several cleavages by a car-boxypeptidase-like enzyme (solid arrows) results in the production of the heterodimeric (AB) insulin molecule (light blue) and the C-peptide.

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. 

The IGFs (also termed somatomedins ), constitute a family of two closely related (small) polypeptides IGF-I and IGF-II. As the names suggest, these growth factors bear a strong structural resemblance to insulin (or, more accurately, proinsulin). Infusion of IGF-I decreases circulating... 

Structurally insulin is a small peptide, with a molecular mass of around 5500 and composed of two subunits, denoted a and (3 chains. Insulin is synthesized as a single peptide, Proinsulin and stored within the pancreatic p-cells. At the moment of secretion, pro-insulin is cleaved, releasing C-peptide and functional insulin in to the blood circulation (Figure 4.22). 

IGF-1 and -2 display identical amino acid residues at 45 positions, and exhibit in excess of 60% sequence homology. Both display A and B domains, connected by a short C domain — similar to proinsulin. However, unlike in the case of proinsulin, the IGF s C domain is not subsequently removed. The predicted tertiary structure of both IGFs closely resemble that of proinsulin. The overall amino acid homology displayed between insulin and the two IGFs is in excess of 40%. 

This concept of reversible chemical crosslinking of the chains to drastically decrease the enthalpic penalty of the folding process and to exploit the highly favored disulfide loop formation in the A-chain with m = 4, has been further developed into artificial peptide linker chains that can be excised selectively by enzymatic processing, to allow for bioexpression of the artificial proinsulins on an industrial scale. 95,96 However, to apply this approach rationally to other double-stranded cystine peptides, knowledge about their three-dimensional structure is essential. 

Insulin growth factor (IGF) and the IGF-I receptor are structurally comparable to insulin and the insulin receptor. Recombinant human IGF-I has 54% identity to proinsulin. Insulin and IGF-I can bind to both receptors, but insulin can only transfer 1% of the IGF message on the IGF receptor and IGF-I only 1% of the insulin message on the insulin receptor. 

The names and structures of the twenty natural amino acids are given in Table 1.13. The International Union of Pure and Applied Chemistry (IUPAC) uses the three-letter abbreviations shown in the second column of Table 1.13 to describe amino acids. These are widely used in biological circles as well but are inappropriate when long peptide or protein sequences need to be described. For example, when proinsulin is cleaved, it forms the biologically important peptide insulin and another peptide usually called C-peptide. The human peptide consists of a linear chain of 31 amino acids that have the sequence, from amino to carboxyl, H2N-Glu-Ala-Glu-Asp-Leu-Gln-Val-Glu-Gln-Glu-Leu-Gly-Gly-Gly-Pro-Gly-Ala-Gly-Ser-Leu-Gln-Pro-Leu-Ala-Leu-Glu-Gly-Ser-Leu-Gln-OH. This is readily comprehensible to most chemists because the abbreviations are typically the first three letters of the amino acid. Thus, alanine is Ala and arginine is Arg. Aspartic acid and asparagine cannot both be named Asp so the latter is distinguished as Asn. 

Insulin-like growth factor-1 (IGF-1) is a peptide, 7649 Da, with structural homology to proinsulin. It binds preferentially to IGF-1 receptors on the surface of target cells. It has a lower affinity for insulin receptors. The receptors for insulin and IGF-1 share some structural and functional homology. IGF-1 is produced in the liver and in other organs in response to growth hormone action. IGF-1 that reenters the circulation is bound to... 

Figure 16.12 Structure of proinsulin, dark blocks representing the amino acids found in mature insulin. C peptide is the same as connecting peptide. (Reproduced by permission from Skillman T. Diabetes mellitus. In Kaplan A, Pesce AJ, eds. Clinical Chemistry. St. Louis CV Mosby, 1984, p. 526.)...

IGF-I and II have a proinsulin-like primary structure. Their major site of production is the liver, from which they are secreted continuously rather than being stored. Thus serum serves as their reservoir where they are bound non-covalently to specific carrier proteins [48]. 

Structures of human proinsulin and insulin. Insulin is derived from proinsulin by cleavage at the dipeptides Arg-Arg and Lys-Arg to give A and B chains held together by disulfide bonds. In the pig, B30 is Ala. In the cow, A8 is Ala, AlO is Val, and B30 is Ala. Bovine and porcine insulins are used extensively in clinical practice. 

IGF-1 Insulin-like growth factor-1, also called somatomedin C a 70-amino acid peptide that shares structural homology with proinsulin. IGF-1 is synthesized and released by the hver in response to growth hormone binding to receptors on the hepatocyte. 

RIO. Rinderknecht, E., and Humbel, R. E., The amino acid sequence of insulin-like growth factor I and its structural homology with proinsulin. /. Biol. Chem. 253, 2769-2776 (1978). 

Insulin, Inutral Optisulin long. Polypeptide hormone produced in the beta cells of the islets of Laager-bans situated in the pancreas of all vertebrates. Synthesized in vivo via a single chain polypeptide precursor, proinsulin, native conformation has been established. Secreted directly into the bloodstream where it regulates carbohydrate metabolism, influences the synthesis of protein and of RNA, and the formation and storage of neutral lipids. The first protein for which chemical structure and precise mol wt were determined Sanger, Tuppy, Biochem. J. 49, 463, 481 (1951) ... 

Some forms of diabetes have a genetic origin. Mutations in insulin structure can render the hormone inactive, and other mutations cause defects in the conversion of preproinsulin or proinsulin to the active hormone. In these cases, treatment involves administration of insulin. 

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