Proteins fibrous conformations

Most fibrous proteins have regular extended structures representing a structural complexity intermediate between pure secondary and tertiary structures of globular proteins. This conformational regularity is derived from regularities in their amino acid sequences. Table 5.9 lists some of their structural elements. 

Globular proteins are compact, are roughly spherical or ovoid in shape, and have axial ratios (the ratio of their shortest to longest dimensions) of not over 3. Most enzymes are globular proteins, whose large internal volume provides ample space in which to construct cavities of the specific shape, charge, and hy-drophobicity or hydrophilicity required to bind substrates and promote catalysis. By contrast, many structural proteins adopt highly extended conformations. These fibrous proteins possess axial ratios of 10 or more. 

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

Soluble proteins have a more complex structure than the fibrous, completely insoluble structural proteins. The shape of soluble proteins is more or less spherical (globular). In their biologically active form, globular proteins have a defined spatial structure (the native conformation). If this structure is destroyed (denaturation see p. 74), not only does the biological effect disappear, but the protein also usually precipitates in insoluble form. This happens, for example, when eggs are boiled the proteins dissolved in the egg white are denatured by the heat and produce the solid egg white. 

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

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. 

One of the key arguments for neutral site binding is the presence of (3-turns and associated conformations. This puts certain restrains on the structure of the fibrous protein. For elastin, conformations with bound calcium are likely to be inside-out with respect to hydrophobicity. Such structures are acceptable only for molecules functioning in a non-polar environment (cell-membranes) but not for a hydrated elastin fibre. Binding of calcium would stabilize a rigid inside-out conformation437. ... 

Type C repeats are very common in proteins. They are quantal in length, but the repeats themselves do not contain residues that are conserved absolutely in any position. However, several positions within the repeats are strongly conserved in character. A classic example of a Type C repeat is that given by the heptad substructure in a-fibrous proteins. This has the form (a—b-c—d—e—f—g)n with the a and d positions generally occupied by apolar residues, and the e and g positions by charged or hydrophilic residues. The heptad is characteristic of an Q-helical conformation (Cohen and Parry, 1986, 1990 Lupas, 1996), but comparison of any two sequences with a heptad substructure generally reveals only about 15—20% identity. The motif also implies that several Q-helices will aggregate to form a multistranded left-handed coiled-coil rope to shield the apolar stripes on the surface of the Q-helices from the aqueous environment. 

Fraser,R.D.B MacRae,T.P. "Conformation in Fibrous Proteins", Academic Press, New York, 1973. 

Proteins are often referred to as globular and fibrous proteins according to their conformation. Globular proteins are usually soluble in water, whilst fibrous proteins are usually insoluble. The complex nature of their structures has resulted in the use of a sub-classification, sometimes referred to as the order of protein structures. This classification divides the structure into into primary, secondary, tertiary and quaternary orders of structures. 

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