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Insulin consists of two polypeptide chains linked by disulphide bridges; the A‐chain contains 21 amino acids and the B‐chain contains 30 amino acids (Figure 5.6). In the circulation, insulin exists as a monomer of 6000 Da molecular weight. The tertiary (three‐dimensional) structure of monomeric insulin consists of a hydrophobic core buried beneath a surface that is hydrophilic, except for two non‐polar regions involved in the aggregation of the monomers into dimers and hexamers.

Figure 5.6 The primary structure (amino acid sequence) of human insulin. The highlighted residues are those that differ in porcine and bovine insulins, as shown in the inset.

In concentrated solution (such as in the insulin vial supplied by the pharmaceutical company for injection) and in crystals (such as in the insulin secretory granule), six monomers self‐associate with two zinc ions to form a hexamer (Figure 5.7). This is of therapeutic importance because the slow absorption of native insulin from the subcutaneous tissue partly results from the time taken for the hexameric insulin to dissociate into the smaller, more easily absorbed monomeric form.

Figure 5.7 The double zinc insulin hexamer composed of three insulin dimers in a threefold symmetrical pattern.

Insulin is synthesised in the β cells from a single amino acid precursor called proinsulin (Figure 5.8). Synthesis begins with the formation of an even larger precursor, preproinsulin, which is cleaved by protease activity to proinsulin. The gene for preproinsulin (and therefore the ‘gene for insulin’) is located on chromosome 11. Proinsulin is packaged into vesicles in the Golgi apparatus of the β cell; in the maturing secretory granules that bud off it, proinsulin is converted by enzymes into insulin and connecting peptide (C‐peptide).

Insulin and C‐peptide are released from the β cell when the secretory granules are transported (‘translocated’) to the cell surface and fuse with the plasma membrane (exocytosis) (Figure 5.9). Microtubules, formed of polymerised tubulin, probably provide the mechanical framework for granule transport, and microfilaments of actin, interacting with myosin and other motor proteins such as kinesin, may provide the mechanical force that propels the granules along the tubules. Although the actin cytoskeleton is a key mediator of biphasic insulin release, cyclic GTPases are involved in F‐actin reorganization in the islet β cell and play a crucial role in stimulus‐secretion coupling.

The ‘regulated pathway’, with almost complete cleavage of proinsulin to insulin, normally accounts for about 95% of the β cell insulin production. In certain conditions, however, e.g insulinoma and type 2 diabetes, an alternative ‘constitutive’ pathway operates, in which large amounts of unprocessed proinsulin and intermediate insulin precursors (‘split proinsulins’) are released directly from vesicles that originate in the endoplasmic reticulum (Figure 5.10).

Figure 5.8 Insulin biosynthesis and processing. Proinsulin is cleaved on the C‐terminal side of two dipeptides. The cleavage dipeptides are liberated, so yielding the ‘split’ proinsulin products and ultimately insulin and C‐peptide.

Figure 5.9 (a) Electron micrograph of insulin secretory granules in a pancreatic β cell and their secretion by exocytosis. Arrows show exocytosis occurring. Ca, capillary lumen; Is, interstitial space. (b) Freeze‐fracture views of β cells that reveal the secretory granules in the cytoplasm (asterisks) and the granule content released by exocytosis at the cell membrane (arrows). Magnification: ×52,000.

From Orci. Diabetologia 1974; 10: 163–187.

Figure 5.10 The regulated (normal) and constitutive (active in type 2 diabetes) pathways of insulin processing.