Protein Chemistry - Part 2 - Selected Topics in Special Protein Chemistry - Ashmarin I. P. 1968

Scleroproteins
Keratins

Keratins form The basis of a wide range of integumentary structures, including Hair, wool, feathers, hooves, horns, and fish scales. They are also found in eggshells and Nervous Tissue. In the Structure/133.html">Discussion that follows, we will focus primarily on wool and hair keratins, as they are the most thoroughly studied.

Amino Acid Composition in relation to the Characteristic Properties of keratin. Unlike Fibroin, keratin is characterized by a full Complement of Amino Acids, featuring a significant proportion of amino acids with polar side chains. The features of keratin's Primary Structure are determined by a high content of cystine and Cysteine (10–14%), as well as Proline and hydroxyproline (9%) (see Table 1).

The difficulties encountered in dissolving keratin, its high stability, and its inertness to various agents (including several Proteolytic Enzymes) cannot be explained solely by a high content of amino acids with nonpolar radicals, as is the case with fibroin. In keratin, these properties are governed by a relatively high cysteine content. The SH groups of cysteine form numerous transverse disulfide cross-links that bind The polypeptide chains together. Consequently, Specific Methods are required for keratin dissolution. Even the mechanical grinding of wool breaks a portion of these Disulfide Bonds. A number of chemical treatments facilitate this process even more completely. Commonly used agents include reducing agents such as metal sulfides, thioglycol, and mercaptoethanol; oxidizing agents such as performic and peracetic acids; and hydrolytic agents, such as sodium carbonate solutions. In Practical Applications, these treatments are used to temporarily or permanently alter the mechanical properties of hair (particularly in hair waving) and to remove hair and wool during leather Processing.

Dissolution of keratin. Subunits. When the disulfide cross-links are sufficiently disrupted through chemical and mechanical treatments, keratin becomes soluble and susceptible to the action of proteolytic enzymes.

The components of keratin that pass into solution are compositionally heterogeneous, yet they share a roughly uniform Molecular Weight of 55,000–75,000. These components are subunits that retain their original structure. Following the oxidative Cleavage of disulfide Bonds, particles are obtained that consist of chains with an a-conformation, which apparently indicates that the subunits do not undergo Denaturation during this preparation method. Subunits can also be obtained through the action of urea. The existence of large structural units in this group of Proteins has been confirmed by Electron Cell/15.html">Microscopy and X-Ray Diffraction Analysis. Their minimum number per original keratin molecule can be determined by analyzing The ratio of fragments that differ in their N-terminal amino acids. For instance, there is 1 fragment with a terminal aspartic acid residue for every 36 fragments with other N-terminal amino acids. Consequently, the minimum molecular weight of keratin can be estimated as 37 × 55,000 = 2 · 10⁶ (assuming, of course, that all these fragments originate from the same protein).

Secondary and tertiary structures in relation to mechanical properties. The Introduction/11.html">Secondary structure of keratin polypeptide chains can only be properly examined in the context of its various states, which are governed by mechanical stress, humidity, and Temperature. Hair and wool fibers are highly elastic, with their extensibility increasing alongside humidity and temperature. As is well known, this principle forms the basis of hair hygrometers. At low and room temperatures, dry wool fibers stretch by 20%, whereas under high humidity and elevated temperatures, a wool fiber can be stretched up to 100% or even more. Upon release of the load, the wet fiber recovers its initial length. As the magnitude and duration of stretching increase, along with humidity or temperature, a phenomenon known as "set" (or Setting) occurs, which is characterized by a reduced rate of retraction upon load removal. Finally, if a stretched fiber is exposed to steam for a short time at room temperature and then released, it undergoes supercontraction, shrinking to ²/₃ of its original length. This latter phenomenon—the precise nature of which remains poorly understood—is utilized, for example, in certain hair-waving techniques and in crease-setting during the ironing of woolen garments.

As it turns out, the three characteristic states of a wool fiber—natural, stretched, and supercontracted—correspond to specific configurations of the keratin polypeptide chains.

The normal Native State is characterized by an a-conformation (see Chapter V, Part I). This is evidenced by X-ray diffraction data showing identity periods along the chain axis of 5.1 Å and a repeat periodicity of 1.5 Å. These values, along with an equatorial reflection of 9.8 Å, closely match the theoretical values predicted for an a-helix.

The stretched state is characterized by an axial identity period of 6.7 Å. This differs slightly from the theoretical value of 7.27 Å predicted for an ideal ß-Structure; however, the discrepancy is not large enough to suggest any alternative structure. It is possible that these deviations stem from a greater distorting influence of side chains on the ideal ß-structure than is observed in fibroin. It has also been suggested that these data align even more closely with a hypothetical layered structure composed not of parallel, but of antiparallel ß-chains. In this scenario, a different spatial arrangement of the CO and NH groups from adjacent chains results in a slightly different identity period—around 6.5 Å. The strongest equatorial reflection characterizing the stretched state is 4.65 Å, which is in satisfactory agreement with theory.

These concepts regarding the secondary structure of keratin are further supported by Infrared Spectra, which show the orientation of CO and most NH groups parallel to the axis in unstretched keratin, and perpendicular to the axis in stretched keratin. Furthermore, infrared spectra indicate that NH groups participate in Hydrogen bond formation.

The X-ray diffraction results for supercontracted keratin are highly distinctive: a strong meridional reflection appears at 4.65 Å, resembling the equatorial reflection of stretched keratin. This apparently indicates either the folding of polypeptide chains transverse to the fiber axis or The formation of very gentle, large-diameter helices. At present, however, views on the secondary structure of supercontracted keratin remain a subject of debate among researchers.

The three characteristic states of the keratin fiber—reversibly stretched, set, and supercontracted—have also been analyzed in terms of the role played by cross-links formed by disulfide bridges (—S—S—) between polypeptide chains. The reversibility of the standard stretching-contraction cycle in keratin indicates that the covalent bonds remain intact, and that the transition from the a-conformation to the ß-conformation can occur without breaking the —S—S— bridges. At the same time, The Setting phenomenon (resulting from prolonged and intensive stretching or extended exposure to steam) can be explained by the disruption of preexisting —S—S— bonds and the formation of new ones. From this perspective, supercontraction can be interpreted as an intermediate state in which old —S—S— bonds have been broken, new ones have not yet formed, and the load has already been removed. Nevertheless, it would be imprudent to attribute these processes exclusively to —S—S— or other covalent bonds, as Hydrogen Bonds also play a part. This is supported by experiments involving urea and Other Reagents that affect hydrogen bonds, which render the set state partially reversible. It has also been observed that prior deamination or the blocking of amino groups (using dinitrobenzene) reduces the capacity for setting. Based on this, it has been hypothesized that the Hydrolysis of —S—S— groups generates new bonds between the NH₂ groups of Lysine or Arginine and the aldehyde groups formed following The breakdown of sulfonic acid via the reaction:

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