| Popis: |
The mammalian hard α-keratins constitute a homologous group of epidermal appendages comprising wool, hair, hoof, the horns of cattle, goats, sheep, and rhinoceros, claw, baleen, and the quills of porcupine, echidna, and hedgehog. Although of common embryological origin and sharing a common structure, they cover a very large range in amino acid compositions. Some examples are given in Table I where it can be seen that there is a large variation in the proportion of nearly every amino acid. This precludes a simple compositional definition for hard α-keratins and instead requires a detailed list of chemical and structural characteristics, which can be summarized as follows: 1. Epidermal appendages, usually cystine-rich, which are insoluble in usual protein solvents except at extremes of pH, due primarily to an extensive network of disulfide bonding. 2. After solubilization following fission of disulfide bonds, hard α-keratins yield two or three characteristic and unique families of constituent proteins, named because of peculiarities in composition, low-sulfur, high-sulfur, and high-tyrosine proteins. 3. X-ray diffraction analysis and electron microscopy show that these tissues have a unique arrangement of the constituent proteins, comprising intermediate filaments (IFs) traditionally termed microfibrils, usually aligned in the growth direction, surrounded by a nonfilamentous matrix of IF-associated protein (IFAP). |
