This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Miscellaneous (Structure & Function of Proteins), Subject name: Biochemistry
Keratin of skin and nail differ because of 
Select and explain. 
A. Disulphide bond, B. Covalent bond, C. Vander Waal bond, D. Hydrogen bond
A: Disulphide bond
Answer: A: Disulphide bond 
Explanation: Ans. is 'a' i.e Disulphide bond " the keratins are a family of closely related fibrous proteins, whose structure is nearly entirely alpha- helical. They are a major component of tissues such as hairs and skin; their rigidity is determined by the number of disulphide bonds between the constituent polypeptide chains " -Lippincott's BiochemThus the difference in rigidity between the keratins of hairs and nail in because of the number of disulphide bonds.Keratins are rich in cysteine*( a sulphur containing amino acid ) which provides covalent disulphide cross-link between adjacent polypeptide chains.