Insulin formation from proinsulin

FIGURE 23-5 Insulin. Mature insulin is formed from its larger precursor preproinsulin by proteolytic processing. Removal of a 23 amino acid segment (the signal sequence) at the amino terminus of preproinsulin and formation of three disulfide bonds produces proinsulin. Further proteolytic cuts remove the C peptide from proinsulin to produce mature insulin, composed of A and B chains. The amino acid sequence of bovine insulin is shown in Figure 3-24. 

The polypeptide chains of all proteins are synthesized by the process described above. This mechanism gives rise to primary polypeptide chains, which are often further modified—for example, by cleavage into smaller peptides, by stmctural modification of selected amino acid residues, by splicing of the polypeptide chain, or by the formation of covalent bonds between polypeptide chains. Some of these secondary modifications are related to the correct folding of polypeptide chains and to the production of active enzymes or peptide hormones from inactive precursors (e.g., insulin from proinsulin). Also, the transport of proteins within the cell or the secretion of extracellular proteins is often linked to structural changes in polypeptide chains either during or after completion of synthesis. 

Fig. 7.1. The formation of insulin and glucagon from their respective precursors, pre-proinsulin and pre-proglucagon in domestic fowl.

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