The opposite page presents models of insulin, a small protein. The biosynthesis and function of this important hormone are discussed elsewhere in this book (pp.160,388). Monomeric insulin consists of 51 amino acids, and with a molecular mass of 5.5 kDa it is only half the size of the smallest enzymes. Nevertheless, it has the typical properties of a globular protein.
Large quantities of pure insulin are required for the treatment of diabetes mellitus (see p. 160). The annual requirement for insulin is over 500 kg in a country the size of Germany. Formerly, the hormone had to be obtained from the pancreas of slaughtered animals in a complicated and expensive procedure. Human insulin, which is produced by overexpression in genetically engineered bacteria, is now mainly used (see p. 262).
A. Structure of insulin O
There are various different structural levels in proteins, and these can be briefly discussed again here using the example of insulin.
The primary structure of a protein is its amino acid sequence. During the biosynthesis of insulin in the pancreas, a continuous pep-tide chain with 84 residues is first synthesi-zed—proinsulin (see p.160). After folding of the molecule, the three disulfide bonds are first formed, and residues 31 to 63 are then proteolytically cleaved releasing the so-called C peptide. The molecule that is left over (1) now consists of two peptide chains, the A chain (21 residues, shown in yellow) and the B chain (30 residues, orange). One of the disulfide bonds is located inside the A chain, and the two others link the two chains together.
Secondary structures are regions of the peptide chain with a defined conformation (see p. 68) that are stabilized by H-bonds. In insulin (2), the a-helical areas are predominant, making up 57% of the molecule; 6% consists of p-pleated-sheet structures, and 10% of p-turns, while the remainder (27%) cannot be assigned to any of the secondary structures.
The three-dimensional conformation of a protein, made up of secondary structural elements and unordered sections, is referred to as the tertiary structure. In insulin, it is compact and wedge-shaped (B). The tip of the wedge is formed by the B chain, which changes its direction at this point.
Quaternary structure. Due to non-covalent interactions, many proteins assemble to form symmetrical complexes (oligomers). The individual components of oligomeric proteins (usually 2-12) are termed subunits or monomers. Insulin also forms quaternary structures. In the blood, it is partly present as a dimer. In addition, there are also hexamers stabilized by Zn2+ ions (light blue) (3), which represent the form in which insulin is stored in the pancreas (see p. 160).
B. Insulin (monomer) O
The van der Waals model of monomeric insulin (1) once again shows the wedge-shaped tertiary structure formed by the two chains together. In the second model (3, bottom), the side chains of polar amino acids are shown in blue, while apolar residues are yellow or pink. This model emphasizes the importance of the "hydrophobic effect" for protein folding (see p. 74). In insulin as well, most hydrophobic side chains are located on the inside of the molecule, while the hydrophilic residues are located on the surface. Apparently in contradiction to this rule, several apolar side chains (pink) are found on the surface. However, all of these residues are involved in hydrophobic interactions that stabilize the dimeric and hexameric forms of insulin.
In the third model (2, right), the colored residues are those that are located on the surface and occur invariably (red) or almost invariably (orange) in all known insulins. It is assumed that amino acid residues that are not replaced by other residues during the course of evolution are essential for the protein's function. In the case of insulin, almost all of these residues are located on one side of the molecule. They are probably involved in the binding of the hormone to its receptor (see p. 224).
i— A. Structure of insulin
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