In reference to pages 29 & 185 of Cambridge checkpoints 2013:
Q: In fibrous proteins, the polypeptide chains are arranged in parallel to form long fibres or sheets. In globular proteins, the polypeptide chains are folded into compact spherical or globular shapes. Describe a distinctive property of a fibrous protein and explain how this property is due to the arrangement of its polypeptides?
The answer says that one of the properties is its elasticity. But I'm confused because silk is an example of a fibrous protein and I know that silk does NOT stretch due its tightly constructed beta pleats secondary structure.
Could someone please explain this to me??
Thanks!
Notice that the actual answer addresses two points, which I have summarized below:
1) The parallel arrangement of polypeptides means that they form long fibers or sheets. This refers to the beta pleated sheets as a secondary structure. Silk is well known for this property.
2) Some, but not all, fibrous proteins can be stretched and then return to their original shape. This refers to a protein's alpha helices.
You've only quoted one of them in your question.
You may have reached the correct conclusion if you payed attention to when the question asked "explain how this property as due to the
arrangement of polypeptides". Fibrous proteins can have either an alpha helix or a beta pleated sheet. Recall that alpha helices can be twisted or stretched and return to their original shape. Beta pleated sheets, on the other hand, do the opposite. They are rigid and cannot be stretched. I would negate the comparison to a globular protein. That's not exactly what the question asks for. Anyway, hopefully I answered your question ( :