Question

Describe the hydropathic index plot for a fibrous protein such as collagen or keratin.

Short answer

The hydropathic index plot of collagen will mostly shift toward the negative region of the plot. For keratin, every third or fourth residue will show a highly positive shift in the plot.

Step-by-step solution

Hydropathic index plot

The hydropathic index plot measures the degree of hydrophobicity and hydrophilicity in a protein.

The graph shows a hydropathic index plot of nine consecutive residues.

A large positive hydropathic index in the plot represents a hydrophobic region in the protein. On the other hand, a hydrophilic region is represented by a large negative hydropathic index in the plot.

Hydropathic index of collagen

Collagen consists of repetitive triplet amino acids, Gly-X-Y, whereX is often Pro, and Y is often a derivative of Pro residue. The hydropathy of Gly and Pro are both less than 0 according to the hydropathy scale for amino acid side chains.

Hence, the hydropathic index plot of collagen is expected to be mostly in the negative region.

Hydropathic index of keratin

Keratin consists of repetitive heptad amino acids, where every third of the fourth residue is hydrophobic. Due to these hydrophobic residues, every third or fourth amino acid will show a positive shift in the hydropathic index plot.