<prompt> Keratin is present in both skin and nail. However, skin is much more flexible than the nail. This is because keratin of skin has?Less number of disulfide bondsLesser number of salt bridgesHigher Na contentDifferent affinities of water  <response>: Keratins are fibrous proteins present in hair, skin and nails, horn, hoof, etc. They mainly have the alpha helical structure. Each fibril has 3 polypeptide chains and each bundle has about 10-12 bundles. The matrix has cysteine-rich polypeptide chains which are held together by disulfide bonds. The more the number of disulfide bonds, the harder the keratin is. This is the reason behind skin is more flexible than nail. Ref: Essentials of Medical Biochemistry: With Clinical Cases By N. V. Bhagavan, Chung-Eun Ha, 2011, Page 33 