Collagen polypeptide backbones

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.]... 

Figure 1.8 Amino add residue side-chain interactions further restrict free rotation in peptide or polypeptide backbone. Rotational possibilities are defined by allowed values of dihedral angle 4> subtended about N—Co, bond and V subtended about Co,—C(0) bond (left). Theoretically allowed angles are shown in Ramachandran plot (right) together with positions of actual angles found in real protein secondary structures a right-handed a-helix ai. left-handed a-helix parallel/S-sheet f anti-parallel 8-sheet C collagen, Pn helix (see later). (Ramachandran plot from Voet, Voet Pratt, 1999 [Wiley], Fig. 6-6).

All of us are held together by fibrous proteins. A quarter of our body protein is collagen, possibly the most abundant protein on earth. There are actually at least twenty collagens, all very similar in structure, but differently distributed in the animal body. Like silk, collagen is rich in glycine, which occupies every third position in the sequence, and the other abundant amino acid is proline. This amino acid is unique in that its side chain makes a loop, attached at its other end to the nitrogen atom of the polypeptide backbone thus ... 

The liquid phase synthesis on PEG has also been used for the conformational analysis of collagen-like sequences by CD studies 237). The attachment to PEG has also permitted the CD spectral delineation of the specific interactions between the polypeptide chains and sidechain groups 238,239). Thus, Anzinger et al. observed that onset of local ordered structures in the mesogenic side chains of polylysine blocks attached to PEG leads to significant, specific alteration in the backbone conformations of the peptide chain 239). 

It is this secondary amino function which gives rigidity, and a change in direction, to the peptide backbone of which proteins are built. Thus, the direction of the collagen helix (collagen is a triple helix with three separate polypeptide chains wrapped around one another) is continually changing as a result of its proline and hydroxyproline content. Collagen is the only protein in which hydroxyproline is found. 

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