Insulin quaternary structure

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 Zn ions (light blue) (3), which represent the form in which insulin is stored in the pancreas (see p.l60). 

In many instances, one sees multisubunit proteins whose individual polypeptide chains are held together via disulfide bonds in addition to physical interactions. Examples are insulin and the immunoglobulins. However, the nature of their subunit interactions is not referred to as quaternary structure. 

The globular proteins sometimes consist of an assembly of a small number of identical or very similar sub-units, not linked together by covalent bonds. This is usually designated as the quaternary structure. Hemoglobin consists of 4 units of the type indicated in Figure 10.18b. Insulin readily forms a dimer from the two units in Figure 10.19. 

In some cases, several chains of amino acids, also referred to as polypeptide chains, form complexes. For example, insulin forms hexamers in the presence of zinc ions. The relative orientation of the polypeptide chains with respect to each other in such a complex is called the quaternary structure. 

For example, insulin is a protein that promotes the absorption of glucose out of the blood and into muscle cells where glucose is needed for energy. Insulin recognizes muscle cells because their surfaces contain insulin receptors, molecules that fit a specific portion of the insulin protein. If insulin were a different shape, it would not latch onto insulin receptors on muscle cells and therefore would not do its job. So the shape, or conformation, of proteins is crucial to their function. We can xmderstand protein structure by exploring it on four levels primary structure, secondary structure, tertiary structure, and quaternary structure (T Figure 19.7). 

These processes are shown for insulin in Figure 3.27. After the polypeptide synthesis (primary structure), the signaling protein is removed and the proinsulin is folded into its secondary structure using thiol-disulfide oxidation and transported into a vesicle. Here, acknowledgement proteolytic enzymes, known as prohormone convertases (PCI and PC2), remove the C peptide segment and the exoprotease carboxypep-tidase E produces the insulin molecule that has a molecular weight of 5,808 g/mol (Dalton). Six of these units will then make the quaternary structure shown in Figure 3.27. 

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