Connecting peptide

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. 

Soluble polyethylene glycol (PEG) as polymer support was also successfully investigated. Precipitation after each reaction step in order to remove excesses of reagents and soluble byproducts seemed to work well. Another topic of great interest is direct glycosylation of solid support connected peptides in order to arrive finally at glycopeptides. (9-Glycosyl trichloroacetimidates also proved successful in this endeavor. 

Coupling of lipids to peptide antigens may be performed directly or via an amplifying dendrimer carrier system. A commonly investigated carrier is the multiple antigen peptide (MAP)-system, which connects peptide antigens through a branched polylysine core. 

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.)...

High resolution X-ray analysis of protein structures shows that the conformational categories of the connecting peptides which link the a-helices and -sheets are limited. Such well defined types of folding units, such as aa- and PP-hairpins, and aP- and Pa-arches, are referred to as supersecondary structures. One important step towards building a tertiary structure from secondary structures is to identify these supersecondary structure... 

The two lobes are joined by a short connecting peptide, which is the only covalent link between them. This peptide varies between different transferrins, both in length (7 to 14 residues—see Section III.C) and in conformation in lactoferrin, it is 12 residues long and forms a three-turn helix, whereas in serum transferrin it is 14 residues long and has a much less regular structure. 

Fig. 3. Domain organization of transferrins. The N-terminal lobe (above) is divided into domains N1 and N2, and the C-terminal lobe (below), into domains Cl and C2. The two lobes are related by a screw axis, a rotation of -180°, and a translation of 25 A. The two iron sites are identified with closed circles. The connecting peptide that joins the two lobes is helical in lactoferrin (solid line) and less regular in transferrin (dashed line).

Fig. 4. Ribbon diagram of human diferric lactoferrin, showing the organization of the molecule, with the N-lobe above and C-lobe below. The four domains (Nl, N2, Cl, C2), the interlobe connecting peptide (H), and the C-terminal helix (C) are indicated. The glycosylation sites in various transferrins are shown by triangles and numbered (1, human transferrin 2, rabbit transferrin 3, human lactoferrin 4, bovine lactoferrin 5 chicken ovotransferrin). The interdomain backbone strands in each lobe can be seen behind the iron atoms. Adapted from Baker et al. (82), with permission.

Sequence Variations in Interlobe Connecting Peptide Protein Sequence"... 

The one-letter code for amino acids is used (C, Cys P, Pro etc.). Alignment on either side of the connecting peptide iB based on the lactoferrin and transferrin 3D BtructureB. The last helix in the N-lobe and firBt /3-strand in the C-lobe are indicated. Cys and Pro residues are in boldface. 

Connecting peptide of insulin Cytosine triphosphate Extracellular fluid Essential fatty acid Endoplasmic reticulum Fructose- I bisphosphate Flavin adenine dinucleotide Free fatty acid Formiminoglutamtc acid Glucose transporter gene or protein... 

The insulin molecule consists of 51 amino acids arranged in two chains, an A chain (21 amino acids) and B chain (30 amino acids), that are linked by two disulfide bonds. Proinsulin is the insulin precursor that is first processed in the Golgi apparatus of the beta cell where it is processed and packaged into granules. Proinsulin, a single-chain 86-amino acid peptide, is cleaved into insulin and C-peptide, a connecting peptide. These are secreted in equimolar portions... 

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