Keratins supercoil

The two-stranded a-helical coiled coil is now recognized as one of natures favorite ways of creating a dimerization motif and has been predicted to occur in a diverse group of over 200 proteins.111 This structure consists of two amphipathic, right-handed a-helices that adopt a left-handed supercoil, analogous to a two-stranded rope where the nonpolar face of each a-helix is continually adjacent to that of the other helix. 2 This structure was first postulated by Crick to explain the X-ray diffraction pattern of a-keratin in the absence of sequence information.Pl The coiled-coil dimerization motif is natures way of creating a rod-like molecule that perhaps plays only a structural role in many fibrous proteins, such as the kmef (keratin, myosin, epidermis, fibrinogen) class 3,4 and the intermediate filament proteins)5 6 ... 

The a-helical rodlike domains of two keratin polypeptides form a coiled coil. Two staggered antiparallel rows of these dimers form a supercoiled protofilament. Hundreds of filaments, each containing four protofilaments, form a macrofibril. 

FIGURE 10.16 Structures of fibrous proteins, (a) Three-stranded supercoil of collagen, (b) three-strand supercoil of oc-keratin (wool and hair), (c) two strand supercoil of myosin (muscle). Each strand of a-keratin and myosin is a right handed helix. The two three stranded supercoils have different amino acid contents and periodicities, and arrange themselves into different fibrous sub-structures which are not shown. 

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