The Structure of Keratin

FIGURE 9-25 Schematic diagram of tropocollagen which is composed of three helices. 

FIGURE 9-27 Schematic diagram of the double a-hfilices of a-keratin. 

As we have mentioned, there are various types of keratins, and this brings us to one final aspect of sulfur cross-link bridges. The a-keratins can be classified as soft or hard by their sulfur content The low sulfur content keratins of wool and hair are much more flexible than the hard, high sulfur 

FIGURE 9-28 Schematic diagram of a protofibril (P in the figure). Eight protofibrils form a structure, called a microfibril, which is the basis of the structure of wool and hair. 

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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...

Steinert, P. M., Marekov, L. N., and Parry, D. A. D. (1993b). Conservation of the structure of keratin intermediate filaments Molecular mechanism by which different keratin molecules integrate into pre-existing keratin intermediate filaments during differentiation. Biochemistry 32, 10046-10056. 

Figure 2.25. Structure of keratin protofibrils. The diagram illustrates the structure of keratin in intermediate filaments containing a helical sequences. Two coils are wound around each other and then packed into protofilaments. Eight protofilaments are packed into a filament.

After the structure of keratin in the wool is loosened by breaking the disulfide bonds, extracellular proteolytic enzymes hydrolyze the peptide bonds as shown in Fig. 1.4. The hydrolysis releases soluble peptides that are further hydrolyzed to amino acids. 

The hair fibers derived from furry mammals are mainly made up (over 80%) of the structural protein keratin. The distinction between wool and hair is not compositional, but related to size wool fibers are generally fine and short, whereas those of hair are usually thicker and longer. The molecule of keratin consists essentially of a combination of amino acids about 18 amino acids make up the keratin molecule (see Textbox 67). The nature of the amino acids, their relative amounts, and their sequence and arrangement within the molecule of keratin vary from one animal species to another but are characteristic of any variety of wool or hair (Asquith 1977) (see Table 89). 

Cysteine disulfide formation is one of the most important posttranslational modifications involved in protein structure. Disulfides play a crucial role in maintaining the structure of many proteins including insulin, keratin, and many other structurally important proteins. While the cytoplasm and nucleus are reducing microenvironments, the Golgi and other organelles can have oxidizing environments and process proteins to contain disulfide bonds (Scheme 5). 

Astbury, W. T. Street, A., X-ray Studies of the Structure of Hair, Wool and Related Fibres. I. General. Trans. R. Soc. London 1931, A230,75 Astbury, W. T. Woods, H. J., II. The Molecular Structure and Elastic Properties of Hair Keratin. ibid. 1934, A232, 333 Astbury, W. T. Sisson, W. A., III. The Configuration of the Keratin Molecule and its Orientation in the Biological Cell, Proc. R. Soc. London 1935, A150, 533. 

We have noted previously (see Section 2.1) the role played in biochemistry by the thiol disulfide interconversion, and the disulfide unit is a fundamental feature of the structure of peptides and proteins. The sulfur-sulfur bond between two cysteins link remote parts of a peptide chain or cross-link two such chains. Cleavage of these bonds in the hair protein keratin, followed by reoxidation, gives hair its desired shape. 

The Structure of the a-Keratins Was Determined with the Help of Molecular Models The fi-Keratins Form Sheetlike Structures with Extended Polypeptide Chains Collagen Forms a Unique Triple-Stranded Structure Globular Protein Structures Are Extremely Varied and Require a More Sophisticated Form of Analysis Folding of Globular Proteins Reveals a Hierarchy of Structural Organization... 

The Structure of the a-Keratins Was Determined with the Help of Molecular Models... 

FIGURE 24.7 Then-helical secondary structure of keratin. The amino acid backbone winds in a spiral, much like that of a telephone cord. 

Powell, B. C., and Rogers, G. E. (1997). The role of keratin proteins and their genes in the growth, structure and properties of hair. In Formation and structure of human hair (P. Jolles, H. Zahn, and H. Hocker, Eds.), pp. 59-148. Basel Birk-hauser Verlag, Boston. 

The hair matrix possesses a large monitoring window (months/years) that allows a retrospective analysis and the study of past history. Hair is also characterized by a high stability and a minimum possibility of adulteration [41, 42], The structure of the hair can be considered as a repeated network of keratin fibers. There are also melanins, lipids, and all the compounds of cells that led to the formation of the stem. The growth of hair is in the range of 0.6-1.4 cm/month [43,44],... 

The optimization of the extraction phase from hair with dichloromethane and methanol as modifier has recently been published by Fernandez et al. [140], In this case MAE has been used, obtaining a very rapid extraction procedure and allowing the chlorinated solvent to permeate into the structure of the hair keratin. 

These processes provide a complete destruction of the structure of the hair keratin by the complete hydrolysis of proteins that compose it. Hydrolysis can occur enzymatically or chemically. 

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