Polypeptides fibrous proteins

We provide here a brief summary of the computational methodology and some examples of calculations on model systems. We also show the results of computations on the conformations of natural polypeptides, fibrous proteins, globular proteins, and binary complexes of enzymes and substrates. 

Fibrous protein (Section 26.9) A protein that consists of polypeptide chains arranged side by side in long threads. Such proteins are tough, insoluble in water, and used in nature for structural materials such as hair, hooves, and fingernails. 

Collagen, the principal fibrous protein in mammalian tissue, has a tertiary structure made up of twisted a-helices. Three polypeptide chains, each of which is a left-handed helix, are twisted into a right-handed super helix to form an extremely strong tertiary structure. It has remarkable tensile strength, which makes it important in the structure of bones, tendons, teeth, and cartilage. 

Fraser, R. D. B., and MacRae, T. P. (1973). Conformation in Fibrous Proteins and Related Synthetic Polypeptides. Academic Press, London, New York. 

Figure 11.4 Pleated sheets of fibrous proteins. Parallel pleated sheets are composed of polypeptide chains which all have their N-terminal amino acid at the same end whereas anti-parallel pleated sheets involve polypeptide chains which are alternately reversed in direction. Both forms of sheet show a high degree of hydrogen bonding between the chains.

The nature of the amino acid residues is of prime importance in the development and maintenance of protein structure. Polypeptide chains composed of simple aliphatic amino acids tend to form helices more readily than do those involving many different amino acids. Sections of a polypeptide chain which are mainly non-polar and hydrophobic tend to be buried in the interior of the molecule away from the interface with water, whereas the polar amino acid residues usually lie on the exterior of a globular protein. The folded polypeptide chain is further stabilized by the presence of disulphide bonds, which are produced by the oxidation of two cysteine residues. Such covalent bonds are extremely important in maintaining protein structure, both internally in the globular proteins and externally in the bonding between adjacent chains in the fibrous proteins. 

Proteins may be structural, functional, or catalytic. Structural proteins are frequently fibrous proteins (insoluble polypeptide chains arranged side by side in long filaments)... 

In addition to the 20 common amino acids, proteins may contain residues created by modification of common residues already incorporated into a polypeptide (Fig. 3-8a). Among these uncommon amino acids are 4-hydroxyproline, a derivative of proline, and 5-hydroxylysine, derived from lysine. The former is found in plant cell wall proteins, and both are found in collagen, a fibrous protein of connective tissues. 6-N-Methyllysine is a constituent of myosin, a contractile protein of muscle. Another important uncommon amino acid is y-carboxyglutamate, found in the bloodclotting protein prothrombin and in certain other proteins that bind Ca2+ as part of their biological function. More complex is desmosine, a derivative of four Lys residues, which is found in the fibrous protein elastin. 

The strength of fibrous proteins is enhanced by covalent cross-links between polypeptide chains within the multihelical ropes and between adjacent chains in a supramolecular assembly. In a-keratins, the cross-links stabilizing quaternary structure are disulfide bonds (Box 4-2). In the hardest and toughest a-keratins, such as those of rhinoceros horn, up to 18% of the residues are cysteines involved in disulfide bonds. 

Conformation in Fibrous Proteins and Related Synthetic Polypeptides, Academic Press, New 53. 

Although living organisms contain additional types of fibrous proteins, as well as polysaccharide-based structural motifs, we focused here on the three arrangements that are the most widely distributed. Two of these, the a-keratins and the /3-keratins incorporate polypeptide secondary structures that also commonly occur in globular proteins. Colla-... 

The structures of fibrous proteins are determined by the amino acid sequence, by the principle of forming the maximum number of hydrogen bonds, and by the steric limitations of the polypeptide chain, in which the peptide grouping is in a planar conformation. 

X-ray diffraction played a major role in the discovery of the structure of fibrous proteins. In most cases the fibers under study are oriented in two dimensions by stretching. In this analysis we illustrate how the technique is used to study the a form of the synthetic polypeptide poly-L-alanine. 

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