Collagen, protein structure

The basement membrane is an acellular meshwork consisting of collagen and glycoproteins. The collagen provides structural support and the negatively charged glycoproteins prevent the filtration of plasma proteins into Bowman s capsule. 

Sasisekharan, V. 1962. Stereochemical Criteria for Polypeptide and Protein Structures, in Collagen, Ramanathan, N., ed. 39-78. Madras, India, Wiley. 

A few other helical conformations occur occasionally in globular protein structures. The polyproline helix, of the same sort as one strand out of a collagen structure, has been found in pancreatic trypsin inhibitor (Huber et al., 1971) and in cytochrome c551 (Almassy and Dickerson, 1978). An extended e helix has been described as occurring in chymotrypsin (Srinivasan et al., 1976). In view of the usual variability and irregularity seen in local protein conformation it is unclear that either of these last two helix types is reliably distinguishable from simply an isolated extended strand however, the presence of prolines can justify the designation of polyproline helix. 

The Pro side chain (R group), is unique in that it incorporates the terminal amino group in the side chain. The incorporation of this amine group into a five-membered pyrrolidine ring, constrains the rotational freedom around the N—Ca bond in Pro to a specific rotational angle. This has important consequences for the protein structures in which Pro participates, for example, collagen. ... 

The Structure of the a-Keratins Was Determined with the Help of Molecular Models The fi-Keratins Form Sheetlike Structures with Extended Polypeptide Chains Collagen Forms a Unique Triple-Stranded Structure Globular Protein Structures Are Extremely Varied and Require a More Sophisticated Form of Analysis Folding of Globular Proteins Reveals a Hierarchy of Structural Organization... 

Rasmussen, M., Jacobsson, M., and Bjorck, L. (2003). Genome-based identification and analysis of collagen-related structural motifs in bacterial and viral proteins. J. Biol. Chem. 278, 32313-32316. 

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