A-Keratin structure

TABLE lO-III Principal Developments in a-Keratin Structural Models... 

The jS form of keratin requires still additional models. And, continuing the order of decreasing certainty, these models are again less well corroborated by experimental observations than are the a-helix or a-keratin structures. Pauling and Corey (1598) presented the pleated sheets to explain /3-keratins. These sheets are made up of extended peptide chains H bonded essentially side by side. Two are shown in Fig. 10-7. 

Why balance pH in skin products The outer layer of skin has a keratin structure just as hair does. Products aimed at making the skin look brighter and clearer have a higher pH. Their purpose is to... 

The a-Keratin Structure.—It seems not unlikely that the polypeptide chains in unstretched hair, contracted muscle, horn, nail, quill, and other proteins that give the a-keratin x-ray pattern have the 3.7-residue helical configuration (which for convenience we shall call the a helix). 

There are several facts that favor the view that the 3.7-residue helix is represented in hemoglobin. First, there is the similarity to a keratin, pointed out by Perutz, and the evidence supporting the 3.7-residue helical configuration for the fibrous proteins with the a-keratin structure. Closely related is the fact that from the density and the average residue weight for... 

Two structures, the a-helix and the pleated sheet, that have been derived on the assumption that the amide group in polypeptides is planar and has interatomic distances and bond angles as found in simpler substances, and that hydrogen bonds are formed between the imino groups and the carbonyl oxygen atoms, have been found to be present in polypeptides and proteins with the a-keratin structure and the / -keratin structure respectively. The dimensions found in polypeptides and proteins substantiate the assumed values of the structural parameters in particular the N— H>"0 hydrogen-bond distance between amide groups in proteins may be assumed to be close to 2-75A. 

Fig. 3. A cross-section of the a-keratin structure, sliowing tlie a-cables AB, and the interstitial compound helixes C. The orientation of the cross-section of the cable changes with coordinate along-the fibre axis. The central cable is shown in tlie most unfavourable orientation for the interstitial a-hclixes. The protein chains are not so nearly circular in cross-section as indicated in the drawing, and space is filled more effectively th.an is indicated...

Flagellin the main protein component of bacterial flagella. The filaments of F. have an a-keratin structure, which is reversibly converted to the -keratin structure on stretching of the aggregate. The F. monomer (M, 33,000 to 40,000) is dissociated by mild acid treatment (pH 3-4). It has 304 amino acid residues, but no cysteine or tryptophan and only traces of proline and histidine. 

The homy layer consists of about 10% extracellular components such as lipids, proteins, and mucopolysaccharides. Around 5% of the protein and lipids form the cell wall. The majority of the remainder is present in the highly organized cell contents, predominantly as keratin fibers, which are generally assigned an a-helical structure. They are embedded in a sulphur-rich amorphous matrix, enclosed by lipids that probably he perpendicular to the protein axis. Since the stratum comeum is able to take up considerably more water than the amount that corresponds to its volume, it is assumed that this absorbed fluid volume is mainly located in the region of these keratin structures. 

Under normal conditions, the transcellular route is not considered as the preferred way of dermal invasion, the reason being the very low permeability through the corneocytes and the obligation to partition several times from the more hydrophilic corneocytes into the lipid intercellular layers in the stratum corneum and vice versa. The transcellular pathway can gain in importance when a penetration enhancer is used, for example, urea, which increases the permeability of the corneocytes by altering the keratin structure. 

The secondary structure is responsible for some of the physical properties of proteins. For example, structural proteins such as a-keratins in skin and hair are fibrous in nature, and have good elastic... 

An individual polypeptide in the a-keratin coiled coil has a relatively simple tertiary structure, dominated by an a-helical secondary structure with its helical axis twisted in a left-handed superhelix. The intertwining of the two a-helical polypeptides is an example of quaternary structure. Coiled coils of this type are common structural elements in filamentous proteins and in the muscle protein myosin (see Fig. 5-29). The quaternary structure of a-keratin can be quite complex. Many coiled coils can be assembled into large supramolecular complexes, such as the arrangement of a-keratin to form the intermediate filament of hair (Fig. 4-1 lb). 

FIGURE 4-11 Structure of hair, (a) Hair a-keratin is an elongated a helix with somewhat thicker elements near the amino and carboxyl termini. Pairs of these helices are interwound in a left-handed sense to form two-chain coiled coils. These then combine in higher-order structures called protofilaments and protofibrils. About four protofibrils—32 strands of a-keratin altogether—combine to form an intermediate filament. The individual two-chain coiled coils in the various substructures also appear to be interwound, but the handedness of the interwinding and other structural details are unknown, (b) A hair is an array of many a-keratin filaments, made up of the substructures shown in (a). 

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