Location of 3-hydroxyproline residues, which could participate in the formation of supramolecular assemblies, have been identified in collagens I, II, III, and V/XI ( Weiss et al. The triple-helical sequences are comprised of Gly-X-Y repeats, X and Y being frequently proline and 4-hydroxyproline, respectively. 2005), involving lysine and aspartate ( Fallas et al.
The triple helix is stabilized by the presence of glycine as every third residue, a high content of proline and hydroxyproline, interchain hydrogen bonds, and electrostatic interactions ( Persikov et al. The three α chains of fibril-forming collagens are three left-handed polyproline II helices twisted in a right-handed triple helix with a one-residue stagger between adjacent α chains. The three α chains can be either identical to form homotrimers (e.g., collagen II) (Table (Table1) 1) or different to form heterotrimers (e.g., collagen IX) (Table (Table1). 2005).Ī Common Structural Motif: The Triple HelixĬollagen α chains vary in size from 662 up to 3152 amino acids for the human α1(X) and α3(VI) chains respectively ( Ricard-Blum et al. Membrane collagens exist in two different forms, a transmembrane form and a soluble one, released by shedding, that regulates cell behavior ( Franzke et al. Several collagens (IX, XII, XIV, XV, XVIII) carry glycosaminoglycan chains (chondroitin sulfate and/or heparan sulfate chains) and are considered also as proteoglycans.Īdditional levels of functional diversity are due (1) to the proteolytic cleavage of several collagen types to release bioactive fragments displaying biological activities of their own, and (2) to the exposure of functional cryptic sites because of conformational changes induced in collagens by interactions with extracellular proteins or glycosaminoglycans, multimerization, denaturation, or mechanical forces ( Ricard-Blum and Ballut 2011). Splicing events are sometimes specific to a tissue and/or a developmental stage, and splicing variants modulate collagen functions. The use of two alternative promoters gives different forms of α1(IX) and α(XVIII) chains, and alternative splicing contributes to the existence of several isoforms of α1(II), α2(VI), α3(VI), α1(VII), α1(XII), α1(XIII), α1(XIV), α1(XIX), α1(XXV), and α1(XXVIII) chains.
There is an increase in α1(V) and a decrease in α2(XI) during postnatal maturation of cartilage ( Wu et al. Indeed collagen XI is comprised of three α chains assembled into a heterotrimer (Table (Table1), 1), but the α(XI) chain forms type V/XI hybrid collagen molecules by assembling with the α1(V) chain in vitreous ( Mayne et al. Beyond the existence of 28 collagen types, further diversity occurs in the collagen family because of the existence of several molecular isoforms for the same collagen type (e.g., collagens IV and VI) and of hybrid isoforms comprised of α chains belonging to two different collagen types (type V/XI molecules) (Table (Table1). Signal-anchor for type II membrane proteinīeyond Collagen Types: Increased Molecular Diversity of the Collagen FamilyĬollagens consist of three polypeptide chains, called α chains, numbered with Arabic numerals. Pulmonary surfactant-associated protein D Pulmonary surfactant-associated proteins A1 and A2 Α1(VI), α2(VI), α3(VI), α4(VI) b, α5(VI) c, α6(V)Ĭomplement C1q (subcomponent subunits A, B, C)Īcetylcholinesterase collagenic tail peptide (collagen Q)
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