Question

What are the two types of beta sheets?

Answer 1

1. Proteins can either be fibrous or globular based on their shape.
2. The structure of globular proteins is in 4 levels: primary, secondary, tertiary and quarternary.
3. The secondary structure of proteins refers to the shape in which a long polypeptide chain can exist. The shapes are of two types: $\mathrm{\alpha }$-helix and $\mathrm{\beta }$-pleated sheets.
4. $\mathrm{\beta }$-pleated sheets consist of $\mathrm{\beta }$-strands which are polypeptide chains (3 to 10 amino acids long) stretched out to almost maximum extension. They are laid side-by-side and bonded by hydrogen bonds existing between the $\mathrm{C}=\mathrm{O}$ and $-\mathrm{NH}-$ groups of adjacent polypeptide chains. Adjacent $\mathrm{\beta }$-strands can form hydrogen bonds in either parallel, antiparallel or mixed arrangements.
5. In the antiparallel arrangement, the $\mathrm{\beta }$-strands are aligned such that the $\mathrm{C}=\mathrm{O}$ groups of one strand are adjacent to those of the next strand and $-\mathrm{NH}-$ groups of one strand are adjacent to those of the next strand. The inter-strand hydrogen bonds are planar in this arrangement which makes this arrangement more stable. The antiparallel arrangement is shown below.

6. In the parallel arrangement, the $-\mathrm{NH}-$ groups of adjacent strands are aligned and the inter-strand hydrogen bonding is non-planar. This makes the parallel arrangement less stable. The parallel arrangement is shown below.

7. Thus, $\mathrm{\beta }$-sheets can have hydrogen bonding between the adjacent strands in a parallel or antiparallel arrangement.