This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Metabolism of carbohydrate, Subject name: Biochemistry
Keratin of skin and nail differ because 
Select and explain. 
A. Vander waal bond, B. Lipolysis, C. Disulphide bond, D. Covalent bond
C: Disulphide bond
Answer: C: Disulphide bond 
Explanation: Keratins - Keratins are proteins present in hair, skin and nails, horn, hoof, etc. The fibers present are called alpha keratins and matrix as keratohyalin. They mainly have the alpha helical structure. Each fibril has 3 polypeptide chains and each bundle has about 10-12 fibrils. The keratohyalin matrix has cysteinerich polypeptide chains which are held together by disulfide bonds. The more the number of disulfide bonds, the harder the keratin is. REF: DM VASUDEVAN TEXTBOOK OF BIOCHEMISTRY,EIGHTH EDITION,PG.NO.,674.