Collagen chain

Collagen chains are synthesized as longer precursors, called procollagens, with globular extensions—propeptides of about 200 residues—at both ends. These procollagen polypeptide chains are transported into the lumen of the rough endoplasmic reticulum where they undergo hydroxylation and other chemical modifications before they are assembled into triple chain molecules. The terminal propeptides are essential for proper formation of triple... 

The part of non-imino acid-containing tripeptides amounts to nearly 50%. It is astonishing that at some places in the collagen chain, containing 1044 amino acid units, such sequences are accumulated, forming imino add-free ranges of remarkable length. 

Gel filtration on Sephadex G 25, G 50 or G 75 beads was just available. After standardization with peptides and proteins of known length, the molecular weight of the respective fraction could be determined. Also cyanogen bromide peptides of collagen chains were available in the later sixties, thus leading to a more consistent standardization15"17. ... 

Collagen Chain Selection, Trimerization, and Triple Helix Formation 506... 

The three collagen chains are designated A, B, and C, where chain A has a one-residue stagger... 

This enzyme [EC 3.4.24.14], also known as procollagen A-proteinase, catalyzes the hydrolysis of the A-propep-tide of the collagen chain a-l(l) at Pro—Gin and of a-2(11) chain at Ala—Gin. As a result, A-terminal propeptides of type I and II collagens are released prior to fibril assembly. However, it does not act on type III procollagen. 

Every third amino acid in most collagen chains is glycine, in triplet repeats of the sequence Gly-Pro-X and Gly-X-hydroxyproline, where X = any amino acid. 

The high frequency of glycine, with its small side chain, allows the three collagen chains to pack very tightly together for strength. 

Covalent cross-linking of collagen chains adds markedly to the strength of the triple helix as well as to the larger structures formed by these connections. 

Many types of EDS are inherited in an autosomal dominant manner because the mutant collagen chains interfere with function of the normal proteins with which they Interact. 

Brittle bone disease, or osteogenesis imperfecta (01), is caused by mutations or absence of one of the genes encoding type I collagen chains, which interferes with assembly and function of the triple helix. 

Proline and lysine residues of collagen chains may be modified by hydrojgrla-tion (see Chapter 2). 

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 limited solubility of membrane proteins and related polypeptides in aqueous mobile phases can also cause problems. These could be solved, e.g., by adding guanidine hydrochloride (6 M) or urea (8 M) to the portion of initial eluent used for sample preparation 69). The urea was always eluted in the breakthrough volume of the column. Thus, the retained hydrophobic polypeptides might have been temporarily precipitated upon the column. Collagen chains, dissolved in 0.5 M acetic acid, were successfully separated by RP-HPLC through gradients of 0.1 M TFA/acetonitrile 70> or (0.05 M ammonium bicarbonate + TFA)/ tetrahydrofuran 57>. 

Finally, note that, as in all other supercoiled molecules (e.g., the o-helical coiled coil), the sense of the twist changes from the Type VI molecular supercoil (left-handed) to the supercoiling of the individual collagen chains in the 10/3 helix (right-handed) to the twist within each chain (left-handed). This is characteristic of regular lay rope structures and produces strength and rigidity in the assembly. 

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