Tropocollagen triple helix

E. TRImming the ends of the procollagen TRIple helix to form a TROpocollagen triple helix. 

Fig. 4.5 Glycine residues make up the interior of a tropocollagen triple helix. The same three-stranded collagen super-helix is depicted as in Fig. 4.4, but looking down the center of a ball-and-stick representation. Glycine residues ( 11) are shown in red. Because of its small size, glycine is required where the three chains contact The balls in this illustration do not represent the van der Waals radii of the individual atoms (Figure 4-12d in Lehninger Principles of Biochemistry. D.L. Nelson and M.M. Cox, 4th Ed. 2005. W.H. Freeman Co., New York)...

Fig. 6.9 Hyaluronan and other molecules. The volume of a single, hydrated hyaluronan molecule is compared with those of monomeric tropocollagen (triple helix), glycogen and albumin (Adapted from Fig. 19-37 in The Molecular Biology of the Cell. B. Alberts et al., 4th Ed. 2002, Garland Science, Taylor Francis Group, New York)...

Cleavage of N-terminal and C-terminal regions, yielding the tropocollagen triple helix ... 

The usual alkali employed is lime. The raw material for gelatine is tropocollagen, which is present in the original hides or bones. This protein consists of three polypeptide chains arranged in a triple helix. In contrast, gelatine consists of several free or interassociated chains, ranging in molecular weight from around ten thousand to several hundred thousand. On extraction, monomers (a-chains MW 100 000), dimers (P-chains) and trimers ( -chains) and some lower order peptides are released. 

Gly-Pro-Y > Gly-X-Pro > Gly-X-Y. In a given polypeptide chain of native tropocollagen, about one third of the molecule contains the Gly-Pro-Hyp sequence and two thirds involve Gly-X-Y, which decreases the stability of the triple helix. Amino acid residues other than proline and hydroxyproline that occupy the X- and Y-positions decrease helix stability but are essential for the next level of organization of collagen—the formation of microfibrils. 

Hydroxylysine glycosides occur at the Y-position and may play a role in determining fibril diameter. The side chains of these amino acids project outward from the center of the triple helix, permitting hydrophobic and ionic interactions between tropocollagen molecules. These interactions determine the manner in which individual tropocollagen molecules aggregate to form microfibrils initially, then larger fibrils, and eventually fibers. 

The molecule of collagen has the form of a rigid rod ( ) with a length of 2900 A and a diameter of 12.5 A with a molecular weight of SOOjOOO. This rod consists of three helical polypeptide chains with three parallel axes separated by 4.5 X (triple helix or tropocollagen). 

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