Residues from collagen

The alkaline treatment of collagen causes hydrolysis of carboxyamides of asparagine and glutamine residues from collagen in carboxylic groups [41]. 

Krimm, 1968a,b Mattice and Mandelkern, 1971 Krimm and Tiffany, 1974). This conformation is similar to that of a single strand from collagen, with average backbone dihedrals of (0,0) = ( 75°, +145°). These dihedrals lead to an extended left-handed helical conformation with precisely three residues per turn and 9 A between residues i and i + 3 (measured Cft to C/3). A cartoon of a seven-residue alanine peptide in this conformation is shown in Figure 1. Notably, backbone carbonyl and amide groups point perpendicularly out from the helical axis into the solvent and are well-exposed. 

Figure 19.12 Water helps to stabilize collagen by forming inter- and intra-hydrogen bonds with hydrophilic residues. (From Nyman et al., 2005. Copyright 2003, with permission from Elsevier.)...

Fig. 2. Kinetics of cross-linking of chondroitin 6-sulfate, a glycosaminoglycan (GAG), to collagen following exposure to 105 °C under 6.7 Pa (50 mtorr). The mechanism of cross-linking is most probably interchain amide condensation involving e-amino groups of lysyl residues on collagen chains with carboxylic groups on glucuronic acid residues in neighboring GAG chains (From [30] with permission).

The absence of half-cystine residues in collagens with chain compositions [ai(I)]2a2 and [ 2(11)13 exclude cystine disulfide bridges from participation in cross-linking in these collagens. However, half-cystine residues have been identified in [ai(III)]3 collagen and disulfide bridges may serve as cross-links in this type of collagen191). 

The active form of vitamin C is ascorbate acid (Figure 28.8). The main function of ascorbate is as a reducing agent in several different reac lions. Vitamin C has a well-documented role as a coenzyme in hydroxy lation reactions, for example, hydroxylation of prolyl- and lysyl-residues of collagen (see p. 47). Vitamin C is, therefore, required for the mainte nance of normal connective tissue, as well as for wound healing. Vitamin C also facilitates the absorption of dietary jron from the intestine. 

Hydroxyproline and hydroxylysine result from the hydroxylation by specific hydroxylases of proline and lysine residues after their incorporation into a-chains. The enzymes require ascorbic acid as a cofactor. [Note An ascorbic acid deficiency results in scurvy.] The hydroxyl group of the hydroxylysine residues of collagen may be enzymatically glycosy lated (most commonly, glucose and galactose are added sequentially to the triple helix). 

Collagen-like triple helices also occur within other proteins. One of these is protein Clq, a component of the complement system of blood (Chapter 31). This protein interacts with antibodies to trigger a major aspect of the immune response. Clq has six subunits, each made up of three different polypeptide chains of about 200 residues apiece. Beginning a few residues from the N termini, there are over 80 residues in each chain with collagen-like sequences. The three chains apparently form a triple helix within each subunit. However, the C-terminal portions are globular in nature.200 Collagen-like tails also are present on some forms of the enzyme acetylcholinesterase (see Chapter 12C,10). Tire extensins of plant cell walls contain 4-hydroxyproline and evidently have a structure... 

Collagen fibers are collected, as described by Bomstein et al. (1958). Fibers of each tail are incubated in 50 ml 0.1 % (v/v) acetic acid at 4 °C for 24 h. Centrifugation for 2 h at 5000 x g separates the fiber residues from solubilized collagen which is aspirated. Total protein content of the collagen solution is determined and adjusted to 0.1 mg/ml. Transwell or Transwell-Clear filters are coated by adding 100 pi of rat-tail collagen solution per... 

B. Residues from Enzymatic Digests of Denatured Collagen. ... 

Hermann (1957), on the basis of the accumulation of hexose in the residues from trypsin or elastase digestion of denatured collagen, has suggested that the hexose occurs in certain favored regions of the collagen molecule. This is also indicated by electron microscope studies of collagen stained by the periodic acid-Schiff method. 

Fig. 4.3 Collagen helix. This secondary structure is created by peptide bond conformation around the proline and hydroxyproline residues (From Fig. 2-39 in Biochemistry. L. Stryer, 4th Ed. 1995. W.H. Freeman Co., New York)...

G4. Goll, D. E., Bray, B. W., and Hoekstra, W. G., Age-associated changes in muscle composition. The isolation and properties of a collagenous residue from bovine muscle. J. Food Sci. 28, 503-509 (1963). 

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