Microfibrils, keratin

Figure 7-31 A model for the structure of keratin microfibrils of intermediate filaments. (A) A coiled-coil dimer, 45-nm in length. The helical segments of the rod domains are interrupted by three linker regions. The conformations of the head and tail domains are unknown but are thought to be flexible. (B) Probable organization of a protofilament, involving staggered antiparallel rows of dimers. From Jeffrey A. Cohlberg297...

Our own skin is made up of specialized cells which become filled with microfibrils of keratin as they move outward and become the relatively dry nonliving external surface (Box 8-F). Internal epithelial cells secrete protein and carbohydrate materials that form a thin basement membrane around the exposed parts of the cells. The connective tissue that lies between organs and which also includes tendons, cartilage, and bone consists of a relatively small number of cells surrounded by a "matrix" consisting of the protein fibers collagen and elastin in a "ground substance" rich in proteoglycans (Chapter 4).616 618 in bone, the calcium phosphate is deposited within this matrix. 

Figure 25-17 Representation of the quaternary structure of a-keratin showing (a) three a-helical polypeptide strands coiled into a rope and (b) eleven units of the three-stranded rope arranged to form one microfibril...

The assembly of hair a-keratin from one a helix to a protofibril, to a microfibril, and finally, to a single hair. (Illustration copyright by Irving Geis. Reprinted by permission.)... 

An example of a longer exact repeat is found in chick scale keratin. The sequence contains a fourfold tandem repeat, each IS residues long (G—Y-G—G-S-S-L-G—Y-G—G—L-Y Fig. 1). Interestingly, feather and scale keratin share a common microfibril structure with a 3.4 nm diameter. 

The most important discovery in recent years has been the clear evidence obtained by Filshie and Rogers (1961) of an organized protofibrillar substructure within the microfibril. This substructure is illustrated in Fig. 13 together with an example of the less definite evidence for longitudinal substructure. As far as is known the size and substructure of the microfibril seem to be a constant feature of a-keratins from a variety of animals. Thus the microfibril may be regarded as the fibrillar unit of structure, although it is to be anticipated that some species-to-species variation in detail will occur in view of the known variability in amino acid composition (Section III,R,1). 

Keratin has a crystallinity of about 30 Z. Since 1950,X-ray diffraction and electron microscopy have led to a model in which the structural elements are (13,14) the a-helix with a diameter of 10, the elementary fibril containing two to three helicoidal chains of 20 diameter, the microfibril or association of ten elementary fibrils through the amorphous regions (of 80 A diameter) and finally the fibrils or structures of several microfibrils within an amorphous matrix. 

In keratin fibers like human hair and wool fiber, the helical proteins of the intermediate filaments (microfibrils) are oriented parallel to the axis of... 

In modem usage, the observed filaments in fiber sections are strictly called hard keratin IFs. Traditionally, these IFs— known as microfibrils—have probably received more intensive study over the years than other fiber stmctural components. The range of the various... 

Small-angle x-ray diffraction patterns of porcupine quill fiber saturated with water show that the microfibrils and matrix have increased in volume 11 and 53%, respectively, compared with those of the dry quill [135,293]. Wide-angle patterns of keratin fibers show [135,293] that between 0 and 80% RH there is a 4-5% increase in the distance between the a-helices, compared with a 9% increase in the diameter of the fiber. When the RH is increased to 100%, there is no further increase in the distance between the a-helices, but the increase in the diameter of the fiber is 16%, compared with that of the dry fiber. The a-helices are present in the microfibrils, and they represent about half the total mass of the microfibrils. The wide-angle x-ray diffraction patterns may therefore imply that the extra water absorbed between 80 and 100% RH is absorbed by the matrix. 

Wool, hair and other animal fibres have a hierarchical microstruclure and no reliable model has been developed for prediction of the failure stress of lhe.se fibres which encompasses all the relevant length scales. Three models are available to explain the tensile properties of a-keratin fibres all deal with a system of parallel microfibrils embedded in a proteinaceous matrix at a scale of 10 nm. In 1959, Feughelman laid the foundations of stmctural interpretation of the stress-strain curve with his two-phase model of microfibrils imbedded in a matrix, a model that was improved in 1994 (Feughelman, 1994). In the same year, Wortmann and Zahn (1994) proposed another version of the microfibril model. The third model, the Chapman and Hearle model (Hearle, 1967 Chapman, 1969) is based on the mechanics of stress transfer in a composite system consisting of microfibrils, which undergo an a p transition, in parallel with an elastomeric amorphous matrix. The Wortmann and Zahn model does not explicitly mention breakage of fibres, but it is implicit that this must be triggered... 

Wool and hair have the most complex structures of any textile fibres. In the paper by Viney, fig. 1 shows how keratin proteins, of which there are more than one type, all having a complicated sequence of amino acids, assemble into intermediate filaments (IFs or microfibrils). But, as shown in Fig. 5a, this is only one part of the story. The microfibrils are embedded in a matrix, as shown in Fig. 5b. The keratin-associated proteins of the matrix contain substantial amounts of cy.stine, which cross-links molecules by -CH2-S-S-CH2- groups. Furthermore, terminal domains (tails) of the IFs, which also contain cystine, project into the matrix and join the cross-linked network. At a coarser scale, as indicated in Fig. 5c, wool is composed of cells, which are bonded together by the cell membrane complex (CMC), which is rich in lipids. As a whole, wool has a multi-component form, which consists of para-cortex, ortho-cortex, meso-cortex (not shown in Fig. 5a), and a multi-layer cuticle. In the para-and meso-cortex the fibril-matrix is a parallel assembly and the macrofibrils, if they are present, run into one another, but in the ortho-cortex the fibrils are assembled as helically twisted macrofibrils, which are clearly apparent in cross-section.s. 

The majority part of the interior of the fiber mass is the cortex, which, from the point of view of mechanical properties, is also the most important component. The cortex consists of elongated, spindle-shaped cells aligned in the direction of the fiber axis. Within these cells resides the major part of the keratinized protein in the form of macrofibrils, which in turn are formed by lower levels of organization, i.e., microfibrils and finally protofibrils. The latter two are low-sulfur proteins and more or less crystalline in nature with their a-helical parts as crystalline lattice components. They are embedded in a noncrystalline, nonfibrillar matrix of disulfide cross-linked, globular proteins. 

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