Which of the following trace elements is important for conversion of procollagen to collagen –
Correct Answer: Cu
Description: Ans. is 'b' i.e., Cu o Lysyl oxidase is involved in stabilization of collagen cross-linking, wrhen tropocollagen molecules assemble to form collagen fibrils.o Copper is a cofactor for lysyl oxidase;Biosynthesis of collageno The collagens are synthesized and secreted by specilized cells called fibroblasts0. Following steps occur in collagen synthesisPolypeptide chain synthesisSynthesis of polypeptide chains (a-chains) of collagen occurs in ribosomes on the rough endoplasmic reticulum (RER). Collagen is synthesized as preprocollagen, which contains a leader or signal sequence along with main polypeptide chain.Post-translational modificationNewly synthesized collagen (polypeptide chains) undergoes extensive post-translational modification.In endoplasmic reticulum:- Signal (leader) sequence of preprocollagen directs the polypeptide chain into the lumen of RER from the ribosomes. As it enters the RER, signal sequence is enzymatically removed. Hydroxylation of proline and lysine residues occurs to form hydroxyproline and hydroxylysine. Then glycosylation of hydroxylysine0 takes place in procollagen molecule. Then, three pro-a-chains assemble together to form triple helix0, which is helped by formation of intrachain and interchain disulfide (S-S) bonds in extension peptides (propeptides), (extension peptides are additional amino acid residues (100-300) on both amino and carboxy terminals). Winding starts from carboxy terminal end. After formation of triple helix, no hydroxylation of proline or lysine and glycosylation of hydroxy lysines take place.Golgi-Apparatus:- Procollagen is then transported to golgi apparatus w'here it is packed in the secretory vesicles and then secreted out of the fibroblasts.C) Extracellular (out side the fibroblasts) :- Following secretion from the fibroblast, extracellular enzymes called procollagen aminopeptidase and procollagen carboxypeptidase remove extension peptides (propeptides) from amino-and carboxy-terminal, respectively. By removal of extension peptides, procollagen is converted to tropocollagen (mature collagen monomer). Then, these triple helical tropocollagen molecules spontaneously assemble to form collagen fibrils. These tropocollagen molecules (which have aggregated into collagen fibrils) now become extensively cross linked0 covalently to produce mature collagen fiber. These cross-linked are further stabilized by lysyl oxidase that oxidatively deaminates the amino group of same lysine and hydroxylysine residues. The tensile strength of collagen is provided by these covalent cross-links between and within triple helical units (tropocollagen).
Category:
Biochemistry
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