Harpers

THE EXTRACELLULAR MATRIX / 537in the Y position may also be posttranslationally modifiedto hydroxylysine through the action of lysyl hydroxylase,an enzyme with similar cofactors. Some ofthese hydroxylysines may be further modified by theaddition of galactose or galactosyl-glucose through anO-glycosidic linkage, a glycosylation site that isunique to collagen.Collagen types that form long rod-like fibers in tissuesare assembled by lateral association of these triplehelical units into a “quarter staggered” alignment suchthat each is displaced longitudinally from its neighborby slightly less than one-quarter of its length (Figure48–1, upper part). This arrangement is responsible forthe banded appearance of these fibers in connective tissues.Collagen fibers are further stabilized by the formationof covalent cross-links, both within and betweenthe triple helical units. These cross-links form throughthe action of lysyl oxidase, a copper-dependent enzymethat oxidatively deaminates the ε-amino groups ofcertain lysine and hydroxylysine residues, yielding reactivealdehydes. Such aldehydes can form aldol condensationproducts with other lysine- or hydroxylysinederivedaldehydes or form Schiff bases with theε-amino groups of unoxidized lysines or hydroxylysines.These reactions, after further chemical rearrangements,result in the stable covalent cross-linksthat are important for the tensile strength of the fibers.Histidine may also be involved in certain cross-links.Several collagen types do not form fibrils in tissues(Table 48–2). They are characterized by interruptionsof the triple helix with stretches of protein lacking Gly-X-Y repeat sequences. These non-Gly-X-Y sequencesresult in areas of globular structure interspersed in thetriple helical structure.Type IV collagen, the best-characterized example ofa collagen with discontinuous triple helices, is an importantcomponent of basement membranes, where itforms a mesh-like network.Collagen Undergoes ExtensivePosttranslational ModificationsNewly synthesized collagen undergoes extensive posttranslationalmodification before becoming part of amature extracellular collagen fiber (Table 48–3). Likemost secreted proteins, collagen is synthesized on ribosomesin a precursor form, preprocollagen, which containsa leader or signal sequence that directs thepolypeptide chain into the lumen of the endoplasmicreticulum. As it enters the endoplasmic reticulum, thisleader sequence is enzymatically removed. Hydroxylationof proline and lysine residues and glycosylation ofhydroxylysines in the procollagen molecule also takeplace at this site. The procollagen molecule containsTable 48–3. Order and location of processing ofthe fibrillar collagen precursor.Intracellular1. Cleavage of signal peptide2. Hydroxylation of prolyl residues and some lysylresidues; glycosylation of some hydroxylysyl residues3. Formation of intrachain and interchain S–S bonds in extensionpeptides4. Formation of triple helixExtracellular1. Cleavage of amino and carboxyl terminal propeptides2. Assembly of collagen fibers in quarter-staggered alignment3. Oxidative deamination of ε-amino groups of lysyl andhydroxylysyl residues to aldehydes4. Formation of intra- and interchain cross-links via Schiffbases and aldol condensation productspolypeptide extensions (extension peptides) of 20–35kDa at both its amino and carboxyl terminal ends, neitherof which is present in mature collagen. Both extensionpeptides contain cysteine residues. While theamino terminal propeptide forms only intrachain disulfidebonds, the carboxyl terminal propeptides formboth intrachain and interchain disulfide bonds. Formationof these disulfide bonds assists in the registration ofthe three collagen molecules to form the triple helix,winding from the carboxyl terminal end. After formationof the triple helix, no further hydroxylation of prolineor lysine or glycosylation of hydroxylysines cantake place. Self-assembly is a cardinal principle in thebiosynthesis of collagen.Following secretion from the cell by way of theGolgi apparatus, extracellular enzymes called procollagenaminoproteinase and procollagen carboxyproteinaseremove the extension peptides at the amino andcarboxyl terminal ends, respectively. Cleavage of thesepropeptides may occur within crypts or folds in the cellmembrane. Once the propeptides are removed, thetriple helical collagen molecules, containing approximately1000 amino acids per chain, spontaneously assembleinto collagen fibers. These are further stabilizedby the formation of inter- and intrachain cross-linksthrough the action of lysyl oxidase, as described previously.The same cells that secrete collagen also secrete fibronectin,a large glycoprotein present on cell surfaces,in the extracellular matrix, and in blood (see below). Fibronectinbinds to aggregating precollagen fibers andalters the kinetics of fiber formation in the pericellularmatrix. Associated with fibronectin and procollagen in