Chapter 3, Problem 3.5PS

Hair Versus Silk. The a-keratin of human hair is a good example of a fibrous protein with extensive a-helical structure. Silk fibroin is also a fibrous protein, but it consists primarily of b-sheet structure. Fibroin is essentially a polymer of alternating glycines and alanines, whereas a-keratin contains most of the common amino acids and has many disulfide bonds.

(a) If you were able to grab onto both ends of an α-keratin polypeptide and pull, you would find it to be both extensible (it can be stretched to about twice its length in moist heat) and elastic (when you let go, it will return to its normal length). In contrast, a fibroin polypeptide has essentially no extensibility, and it has great resistance to breaking. Explain these differences.

(b) Can you suggest why fibroin assumes a pleated sheet structure, whereas α-keratin exists as an α helix and even reverts spontaneously to a helical shape when it has been stretched artificially?