Helix collagen

Another type of helix occurs in the collagens, which are important constituents of the con-nectivetissue matrix (see pp. 70, 344). The collagen helix is left-handed, and with a pitch of 0.96 nm and 3.3 residues per turn, it is steeper than the a-helix. In contrast to the a-helix, H bonds are not possible within the collagen helix. However, the conformation is stabilized by the association of three helices to form a righthanded collagen triple helix (see p. 70). 

Secondary structure refers to regularities or repeating features in the conformation of the protein chain s backbone. Four major types of secondary structure in proteins are (1) the alpha (a) helix, formed from a single strand of amino acids (2) the beta (P) sheet, formed from two or more amino acid strands (from either the same chain or from different chains) (3) the beta (P) bend or reverse turn, in a single strand and (4) the collagen helix, composed of three strands of amino acids. 

Collagen Like the a-keratins, collagen has evolved to provide strength. It is found in connective tissue such as tendons, cartilage, the organic matrix of bone, and the cornea of the eye. The collagen helix is a unique... 

The polypeptide backbone does not assume a random three-dimensional structure, but instead generally forms regular arrangements of amino acids that are located near to each other in the linear sequence. These arrangements are termed the secondary structure of the polypeptide. The a-helix, 3-sheet, and 3-bend are examples of secondary structures frequently encountered in proteins. [Note The collagen helix, another example of secondary structure, is discussed on p. 43.]... 

Fig. 3. Ramachandran plot showing the allowed angles for poly-L-alanine (grey regions), a, (p-y/values that produce the right-handed a-helix ji, the antiparallel fi-pleated sheet /3, the parallel (5-pleated sheet C, the collagen helix.

A third type of ordered structure giving x-ray patterns is the collagen helix. The basic collagen unit consists of three intertwined chains, in which one twist accounts for a distance of 9 A, with three amino acid residues present in each twist. 

In scurvy, vitamin C deficiency disease, hydroxylation of proline is defective, and the smaller hydroxyproline content of the collagen helix results in weaker hydrogen bonding. 

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)...

Among helical structures yet to be analyzed are the w and tt helices. Preliminary analyses have already been done on the polyproline I and polyproline 11 helices (Johnston, 1975), and these studies can provide the basis for a detailed analysis of the triple-stranded collagen helix. Since helices in proteins are found in short segments and often with distorted axes, it will be important to study the influence of such defects on the vibrational spectrum. 

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