Structure and Function of Some Proteins

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Submitted By ObanaC
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1. collagen Major structural protein (285 kDa) of extracellular matrix. An unusual protein in amino acid composition (very rich in glycine (30%), proline, hydroxyproline, lysine and hydroxylysine; no tyrosine or tryptophan), structure (a triple-helical arrangement of 95-kDa polypeptides giving a tropocollagen molecule, dimensions 300×0.5 nm), and resistance to peptidases. Most types are fibril-forming with a characteristic quarter-stagger overlap between molecules producing an excellent tension-resisting fibrillar structure. Type IV, characteristic of basal lamina, does not form fibrils. Many different types of collagen are now recognized. Some are glycosylated (glucose–galactose dimer on the hydroxylysine), and nearly all types can be cross-linked through lysine side chains. 2. elastin Glycoprotein (70 kDa) randomly coiled and crosslinked to form elastic fibres that are found in connective tissue. Like collagen, the amino acid composition is unusual with 30% of residues being glycine and with a high proline content. Cross-linking depends upon formation of desmosine from four lysine side groups. The mechanical properties of elastin are poorer in old animals. 3. keratins Group of highly insoluble fibrous proteins (of high _-helical content) which are found as constituents of the outer layer of vertebrate skin and of skin-related structures such as hair, wool, hoof and horn, claws, beaks and feathers. Extracellular keratins are derived from cytokeratins, a large and diverse group of intermediate filament proteins. 4. glucagon A polypeptide hormone (3485 Da) secreted by the A cells of the Islets of Langerhans in the pancreas in response to a fall in blood sugar levels. Induces hyperglycaemia. A family of structurally related peptides includes glucagon-like peptides 1 and 2 (encoded by the same gene); gastric inhibitory polypeptide; secretin; vasoactive…...

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