Despite the assumptions of shampoo ads you might not want to

Despite the assumptions of shampoo ads. you might not want to have \"hair like silk.\" Both hair and silk are composed of fibrous protein, but hair is somewhat elastic and silk is not. Based on what you know of protein secondary structural elements, propose an explanation for this difference.

Solution

Both hair and silk are made up of fibrous proteins. Silk has fibroin protein and hair has keratin in it. Both fibroin and keratin are structural proteins. The keratin is alpha-helix and fibroin is a beta-pleated sheet. The alpha keratins can also found in feathers and nails along with hair. The difference between the hair (alpha helix) and silk(beta-pleated sheet) is that the type of amino acids within them. Silk has glycine, alanine and serine and these amino acids do not have side chains. So they cannot form helices, but they get arranged one amino acid on top of another to give a sheet like structure. In silk, thus beta-sheets formed lie on one another giving a pleated structure. Thus the silk shows less elasticity compared to hair due to lesser interaction between sheets and due to loose hydrogen bonds of the sheets which lie on one another. The elasticity of hair is due to its hydrogen bonding between C=O of one amino acid residue with NH at 5th position of another amino acid residue. These alpha helices due to their crossing linking with disulfide bonds (cysteine residues) give curly hair.