All of the following are the components of collagen except:
Ans. d. Desmosine (Ref: Harper 29/e p589-590 28/e p527, 530)Desmosine is the component of elastin. not the collagen.'A striking characteristic of collagen is the occurrence of glycine residues at every third position of the triple helical portion of the alpha chain. This is necessary because glycine is the only amino acid small enough to be accommodated in the limited space available down the central core of the triple helix. This repeating structure, represented as (Gly-X-Y)n as an absolute requirement for the formation of the triple helix.While X and Y can be any other amino acids, about 100 of the X positions are proline and about 100 of the Y positions are hydroxyproline. Proline and hydroxyproline confer rigidity on the Collagen molecule. Hydroxyproline is formed by the posttranslational hydroxylation of peptide-bound proline residues catalyzed by the enzyme prolyl hydroxylase, whose cofactors are ascorbic acid (vitamin C) and alpha-ketoglutarate. Lysines in the Y position may also be post-translationally modified to hydroxylysine through the action of lysyl hydroxylase, an enzyme with similar cofactors.Harper 28/e p527'The major cross-links formed in elastin are the desmosines, which result from the condensation of three of these lysine- derived aldehydes with an unmodified lysine to form a tetra functional cross-link unique to elastin. - Harper 28/e p530Fig. 14: Molecular features of Collagen structure from primary sequence upto the fibrilCollagenCollagen type I is composed of a triple helix structureQ and forms fibrils.Each polypeptide sub-unit or alpha chain is twisted into a left-handed polyproline helix of three residues per turnQ.Three of these alpha chains are then wound into a right-handed super helixQ.A striking characteristic of collagen is the occurrence of glycine residues at every third position of the triple helical portion of the alpha chain. This is necessary because glycine is the only amino acid small enough to be accommodated in the limited space available down the central core of the triple helixQ.This repeating structure, represented as (Gly-X-Y)n as an absolute requirement for the formation of the triple helixQ.While X and Y can be any other amino acids, about 100 of the X positions are Proline and about 100 of the Y positions are hydroxyprolineQ. Proline and hydroxyproline confer rigidity on the collagen moleculeQ.Hydroxyproline is formed by the posttranslational hydroxylation of peptide-bound proline residues catalyzed by the enzyme prolyl hydroxylase, whose cofactors are ascorbic acid (vitamin C) and alpha-ketoglutarate.Lysines in the Y position may also be post-translation ally modified to hydroxylysine through the action of Iysyl hydroxylase, an enzyme with similar cofactorsQ.Some of these hydroxylysines may be further modified by the addition of galactose or galactosyl-glucose through an O-glycosidic linkage, a glycosylation site that is unique to collagenQ.ElastinElastin is a connective tissue protein that is responsible for properties of extensibility and elastic recoil in tissues.Elastin is present in large amounts, particularly in tissues that require these physical properties, e.g., lung, large arterial blood vessels, and some elastic ligaments.In contrast to collagen, there appears to be only one genetic type of elastic.Elastin does not contain repeat Gly-X-Y sequences, triple helical structure, or carbohydrate moietiesQ.After secretion from the cell, certain lysyl residues of tropoelastin are oxidatively deaminated to aldehydes by Iysyl oxidase, the same enzyme involved in this process in collagen.However, the major cross-links formed in elastin are the desmosines, which result from the condensation of three of these lysine-derived aldehydes with an unmodified lysine to form a tetra functional cross-link unique to elastinQ.CollagenElastinMany different genetic typesQOne genetic typeQTriple helixQNo triple helix; random coil conformations permitting stretchingQ(Gly-X-Y)n repeating structureQNo (Gly-X-Y)n repeating structureQPresence of hydroxylysineQNo hydroxylysineQCarbohydrate-containingQNo carbohydrateQIntramolecular aldol cross-linksQIntramolecular desmosine cross-linksQPresence of extension peptides during biosynthesisQNo extension peptides present during biosynthesisQ
Glycine
Lysine
Proline
Desmosine
Membrane Structure & Function