The labels 1 2 3 in the following image of the extracellular

The labels (1, 2, 3) in the following image of the extracellular domain of the nicotinic acetylcholine receptor protein refer to which aspects of secondary structure?

Select one:

a. 1 - alpha turn, 2 - beta sheet, 3 - loop

b. 1 - loop, 2 - beta-alpha-beta barrel, 3 - Greek key

c. 1 - alpha helix, 2 - parallel beta sheet, 3 - anti-parallel beta sheet

d. 1 - alpha helix, 2 & 3 - beta-barrel

e. 1 - alpha helix, 2 - anti-parallel beta sheet, 3 - parallel beta sheet

Solution

Answer:

a. 1 - alpha turn, 2 - beta sheet, 3 - loop

1. Secondary structure of protein mainly made up of alfa helix and beta sheets. In the structure pink color shows the alfa helix which contain turns in the coil called alfa turn. A turn is a element of a secondary structure of the proteins where polypeptide chain reverse its overall direction. Examples: Alfa turn, beta turn etc. In all turns most common turn is beta turn.

End residue in an alfa turn are separated by four peptide bonds whereas in beta turn end residue are separated by three peptide bonds.

2. Beta Sheets:In the structure yellow color shows the beta sheets.Singal chain of beta sheet forms H-bond with its adjacent chain. H-bond form between donor amide and the acceptor carbonyl group. Beta sheets can be parallel or antiparallel.

3. Loops: White color in the structure shows the loops. It is a connecting part of the alfa helix and the beta sheets. Loops are hydrophilic residue found on the surface of the proteins that contain 4-5 amino acid residue.