Insulin proteolytic processing

Proinsulin is proteolytically processed in the coated secretory granules, yielding mature insulin and a 34-amino acid connecting peptide (C peptide, Figure 11.1). The C peptide is further proteolytically modified by removal of a dipeptide from each of its ends. The secretory granules thus contain low levels of proinsulin, C peptide and proteases, in addition to insulin itself. The insulin is stored in the form of a characteristic zinc-insulin hexamer, consisting of six molecules of insulin stabilized by two zinc atoms. 

Figure 11.1 Proteolytic processing of proinsulin, yielding mature insulin, as occurs within the coated secretory granules...

Many proteins are formed as inactive precursors and become activated by proteolysis. The inactive precursors are termed proenzymes, zymogens or - for hormones like e.g. insulin - prehormones. Processing to the active form occius in a cell- and tissue-specific way and usually requires a specific protease. Activation can also occur intramolecu-larly by autoproteolysis. In most cases, short sequences of the protease substrate serve as a recognition signal for the attack of the processing protease. Of the numerous examples of proteolytic processing of proteases only the digestive proteases will be discussed in more detail. 

Many other protein hormones are also synthesized as inactive precmsors. Examples are insulin, which is formed in a two-step proteolytic process from the precursor pre-pro-insulin. Another noteworthy example is that of pre-pro-opiomelanocortin, which is the precursor for eight peptide hormones and neuropeptides in the epiphyse. 

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. 

C Insulin is released from its precursor by sequential proteolytic processing steps. 

Conversion of proinsulin to insulin and C-peptide in secretory granules involves site-specific cleavages at the Arg-Arg and Lys-Arg sequences (Figure 22-6) these serve as signals for proteolytic processing of many other proteins. Cleavage occurs at the C-terminal end of each pair by trypsin-like enzymes and is followed by... 

Fig. 1.2 A schematic overview of the proteolytic processing of SREBPs. This pathway was described as a cholesterol-sensing process which applies to both SREBP-2 and SREBP-la. While SCAP and Insig are involved in the proteolytic cleavage of SREBP-lc the detailed mechanisms by which insulin regulates SREBP-lc processing are not known...

Nearly all peptide hormones are synthesized as inactive precursors and then converted to active hormones by proteolytic processing. Studies of the synthesis of insulin provided the first evidence of this phenomenon (see Figure 5.21). Insulin contains two polypeptide chains, of 21 and 30 residues, with two interchain disulfide bridges and one intrachain bridge (Figure 5.15). 

Insulin is synthesized as preproinsulin that is proteolytically processed in the P cells of the islets of Lang-erhans in the pancreas, to give proinsulin. After synthesis and folding, a section of the molecule (the C peptide) is excised, leaving the A and B peptides connected via disulfide bonds. Thus, native insulin, lacking the C peptide, lacks some of the information necessary to direct the folding process. 

Nearly all bacterial proteins are synthesized with a formyl-methionine as the first amino acid, as described above. Yet most proteins isolated from cell culture do not contain an N-terminal fMet or Met residue, indicating that the maturation process involves proteolytic removal of this amino acid. Some proteolytic events release a functional protein from a synthesized precursor, or proprotein. An example is insulin, which is released from its proinsulin precursor by excision of an internal 33-residue peptide. Another type of proteolytic processing is the removal of N-terminal signal peptides, which target some proteins for insertion into membranes or for secretion from the cell. 

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