Alpha-Keratin protein

Moreover, we reached the conclusion, as did Crick, that in the alpha-keratin proteins the alpha helices are twisted together into ropes or cables. This idea essentially completed our understanding of the alpha-keratin diffraction patterns. 

Alpha farnesene, structure of, 207 Alpha helix (protein), 1038 Alpha-keratin, molecular model of, 1039... 

Fibrous proteins also gain strength due to interactions between the side chains of the residues. The alpha-keratin polypeptides, for example, have a large number of cystine residues, which can form disulfide bonds. 

Coils such as those found in alpha-keratin are not the only structural motifs present in fibrous proteins. Silk, for example, is largely composed of fibrous proteins whose structures resemble interleaved sheets, see also Quaternary Structure Secondary Structure Tertiary Structure. 

The wide angle x-ray diffraction pattern of undeformed corneum exhibits diffuse halos at 4.6 A and 9.8 A common to proteins (Figure 4). The lack of the 5.1-A reflection characteristic of alpha-keratin structures in undeformed comeum suggests that the protein is considerably less oriented and perhaps of a lower alpha content than wool. This is supported by the fact that the 5.1-A reflection begins to appear in samples of comeum which were hydrated and stretched to 100% or more (Figure 6) and allowed to dry in the extended state. The increased orientation of the lipid reflections in the stretched sample demonstrates further their association with the orienting protein fibrils. 

The morphological location of the fibrous protein principally responsible for the deformation and viscoelastic behavior is uncertain. Both the cell membrane and intracellular regions are composed of fibrous proteins which differ considerably in amino acid composition. Since the alpha-keratin within the cells shows few orientation properties until high elongations, it has been suggested that the membrane proteins determine the viscoelastic behavior at low deformations (84). 

Human hair is made of protofibrils, which are twisted bundles of the coiled protein alpha-keratin. Alpha-keratin is so strong and flexible that a human hair can be tied in a knot without breaking. 

In alpha-keratin, shown in Figure 5, the entire length of the protein has an a-helix structure. However, other proteins will have only sections that are a-helixes. Different sections of the same protein may have a pleated sheet secondary structure. These different sections of a protein can fold in different directions. These factors, combined with the inter-molecular forces acting between side chains give each protein a distinct three-dimensional shape. This shape is the tertiary structure of the protein. 

Protein Alpha-Keratin X-ray repeat distances = 5.4 A 1.5 A 5.4 A = Pitch of the helix... 

Alpha keratin fibers occur in hairs, wool, quills, and together with fibroin fibers such as silks and spiders webs are all highly extensible fibrous protein while collagen is a relatively inextensible fibrous protein. Because of their commercial applications and the relatively complex structure at the molecular and near molecular level, the interpretation of the physical properties of a-keratin fibers has been object of recent studies [188]. 

Alpha helices are sufficiently versatile to produce many very different classes of structures. In membrane-bound proteins, the regions inside the membranes are frequently a helices whose surfaces are covered by hydrophobic side chains suitable for the hydrophobic environment inside the membranes. Membrane-bound proteins are described in Chapter 12. Alpha helices are also frequently used to produce structural and motile proteins with various different properties and functions. These can be typical fibrous proteins such as keratin, which is present in skin, hair, and feathers, or parts of the cellular machinery such as fibrinogen or the muscle proteins myosin and dystrophin. These a-helical proteins will be discussed in Chapter 14. 

Keratin Structure and Orientation. Acute flattening of the flbrous protein-fllled cells in the flnal stages of keratinization establishes a biaxial orientation. As would be expected, no birefrigence is observed normal to the plane of the comeum surface, but significant birefrigence is observed parallel to the plane of the corneum surface (I, 42). The x-ray diflFraction pattern of this isolated epidermal protein, when highly drawn, exhibits the classical alpha pattern (7, 43). 

Great advances have been made in protein chemistry since the time of Astbury s pioneering work. Polyalanine, like keratin, exists in an alpha form which may be transformed into the beta conhguration by stretching. It is much easier to interpret X-ray diffraction diagrams of substances of known constitution, such as polyalanine, than those obtained from the infinitely more complex keratin molecule. 

KER, keratin, detected by a mixture of GAMS.2, MAK-6, and AEl /AE3 EMA, epithelial membrane antigen VIM, vimentin DES, desmin MSA, muscle-specific actin SMA, smooth muscle (alpha isoform) actin GALD, h-caldesmon S-IOOP, S-100 protein OCN, osteocalcin LM, laminin UL, Ulex europaeus I lectin binding FS, fibrosarcoma SGRMS, spindle cell rhabdomyosarcoma LMS, leiomyosarcoma MPNST, malignant peripheral nerve sheath tumor MSS, monophasic spindle cell synovial sarcoma SCAS, spindle cell angiosarcoma KS, Kaposi s sarcoma FOS, fibroblastic osteosarcoma. 

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