Insulin molecule

Insulin and Amylin. Insulin is a member of a family of related peptides, the insulin-like growth factors (IGFs), including IGF-I and IGF-II (60) and amylin (75), a 37-amino acid peptide that mimics the secretory pattern of insulin. Amylin is deficient ia type 1 diabetes meUitus but is elevated ia hyperinsulinemic states such as insulin resistance, mild glucose iatolerance, and hypertension (33). Insulin is synthesized ia pancreatic P cells from proinsulin, giving rise to the two peptide chains, 4. and B, of the insulin molecule. IGF-I and IGF-II have stmctures that are homologous to that of proinsulin (see INSULIN AND OTHER ANTIDIABETIC DRUGS). 

Insulin is composed of two peptide chains covalently linked by disulfide bonds (Figures 5.17 and 6.35). This monomer of insulin is the active form that binds to receptors in target cells. However, in solution, insulin spontaneously forms dimers, which themselves aggregate to form hexamers. The surface of the insulin molecule that self-associates to form hexamers is also the surface that binds to insulin receptors in target cells. Thus, hexamers of insulin are inactive. 

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.

However, if a triplet genetic code system really did exist around 3.5 billion years ago, an RNA strand containing about 100 nucleotides would only have been able to code for a maximum of 33 amino acids. With 33 amino acids, the polypeptide formed would have been only two thirds as long as the insulin molecule, and it is doubtful whether such a chain length would have sufficed for an active replication system. 

Gitman, A.G., Kahane, L., and Loyter, A. (1985a) Use of virus-attached antibndies or insulin molecules to mediate fusion between Sendai virus envelopes and neuraminidase-treated cells. Biochemistry 24, 2762-2768. 

Gitman, A.G., Graessmann, A., and Loyter, A. (1985b) Targeting of loaded Sendai virus envelopes by covalently attached insulin molecules to virus receptor-depleted cells Fusion-mediated microinjection of ricin A and simian 40 DNA. Proc. Natl. Acad. Sci. USA 82, 7209-7313. 

AII insulins available in the United States are made by human recombinant DNA technology. An insulin analog is a modified human insulin molecule that imparts particular pharmacokinetic advantages. 

The three-dimensional structure of insulin remained recalcitrant in spite of the knowledge of its primary sequence. The early crystals had been found by Scott (1936) to contain zinc which could be replaced by other divalent metals. The zinc atom is not heavy enough to be unambiguously distinguishable. Eventually it proved possible to introduce uranyl acetate and uranyl fluoride into the insulin molecule and to obtain the three-dimensional structure, first at 2.8 A resolution and then at 1.9 A (see Blundell, Dodson, Hodgkin, and Mercola, 1972). 

Another degradation reaction observed in suspension was the formation of covalent insulin dimers [134][136], These involve isopeptide links between two insulin molecules, that result from a transamidation reaction mainly between the B-chain N-terminus of one insulin molecule, and one of the four amide side chains in the A-chain (principally AsnA21) of the second insulin molecule. 

Physical properties of the protein structure should be considered in designing strategies to achieve stable formulations because they can often yield clues about which solution environment would be appropriate for stabilization. For example, the insulin molecule is known to self-associate via a nonspecific hydrophobic mechanism66 Stabilizers tested include phenol derivatives, nonionic and ionic surfactants, polypropylene glycol, glycerol, and carbohydrates. The choice of using stabilizers that are amphiphilic in nature to minimize interactions where protein hydrophobic surfaces instigate the instability is founded upon the hydro-phobic effect.19 It has already been mentioned that hydrophobic surfaces prefer... 

Human insulin molecule. Refer to Table A2.1 for the names of amino... 

The significance of Sanger s work is immense. It proved for the first time that the structure of a protein is unique that is, aU molecules of bovine insulin, for example, possess the same sequence of amino acids along the polypeptide chains. This sequence has no obvious order, but it is unique. This singular finding requires that there is a genetic code information encoded in a molecule which specifies the sequence of amino acids in the insulin molecule and, for that matter, in all protein molecules. 

An apparently more effective method for prolonging the half-life of insulin in the blood is to add substituents at the end of the A- or B-chain (or both) that alter the chemical properties of the molecule and delay its breakdown in the body. A product known as HOE 901 (insulin glargine) has two glycine residues added to one end of the B-chain and the A21 asparagine residue replaced with another glycine residue. These changes modify the acidity of the insulin molecule, reducing the rate at which it is absorbed and metabolized in the body. 

The higher molecular mass contaminants in conventional insulin preparations include various proteases. Such preparations are generally maintained in solution at acidic pH values (often as low as pH 2.5-3.5). This minimizes the risk of proteolytic degradation of the insulin molecules, as contaminant proteases are inactive at such pH values. 

Upon s.c. injection, the insulin experiences an increase in pH towards more neutral values and, consequently, appears to precipitate in the subcutaneous tissue. It resolubilizes very slowly and hence a greatly prolonged duration of release into the bloodstream is noted. Consequently, a single daily injection supports the maintenance of acceptable basal blood insulin levels, and insulin molecules are still detected at the site of injection more than 24 h after administration. 

Insulin Lispro was the first recombinant fast-acting insulin analogue to gain marketing approval (Table 8.3). It displays an amino acid sequence identical to native human insulin, with one alteration — an inversion of the natural proline lysine sequence found at positions 28 and 29 of the insulin jS-chain. This simple alteration significantly decreased the propensity of individual insulin molecules to self-associate when stored at therapeutic dose concentrations. The dimerization constant for Insulin Lispro is 300 times lower than that exhibited by unmodified human insulin. Structurally, this appears to occur as the change in sequence disrupts the formation of inter-chain hydrophobic interactions critical to self-association. 

Insulin Preparations. Since diabetes mellitus is a defect of one or more of insulin production, secretion, or action, the administration of insulin replacement as a treatment for diabetes in the 1920s was a landmark discovery. Historically, most commercial insulin came from either bovine or porcine sources. Beef insulin differs from human insulin by three amino acid substitutions pork insulin differs by only one residue. For many years, standard insulin preparations were 70% beef and 30% porcine. However, the biosynthesis of human insulin has now displaced the animal insulins, especially bovine insulin which was more antigenic. Mass production of human insulin by recombinant DNA methods is achieved by inserting the human proinsulin gene into either E. coli or yeast and treating the resulting proinsulin to yield the human insulin molecule. Insulin preparations may be divided into four major types ... 

The short-acting insulins are dispensed with the insulin molecules in solution, thereby enabling a rapid onset of action. The intermediate- and long-acting insulins are dispensed as turbid suspensions such that mobilization of the insulin molecule from the... 

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