Protein Structure and Function Questions and Answers

Answer: D

The angle that does not rotate is \(\omega\) (omega).

Omega is the torsion angle about the peptide bond (C-N). Because of resonance, this bond has partial double-bond character, so the peptide group is planar and omega is fixed near 180° (trans), or rarely 0° (cis, mostly before proline).

Phi (about N-Cα) and psi (about Cα-C) are the freely rotating backbone angles plotted in a Ramachandran diagram. Chi angles describe rotation in the side chains, not the backbone.

Answer: D

The residue required at every third position is glycine.

Collagen's triple helix is built from repeating triplets, usually written Gly-X-Y, in which this question's 'X' is the glycine position. Every third residue lies at the centre of the tightly wound triple helix, where there is room only for glycine, whose side chain is a single hydrogen atom.

Proline and hydroxyproline are common at the other two positions, where they rigidify the chains, and lysine (with hydroxylysine) is used for cross-linking. None of them fits at the centre of the helix, where a glycine substitution causes diseases such as osteogenesis imperfecta.

Answer: A

The component is ubiquitin.

Ubiquitin is a small 76-amino-acid protein. A cascade of E1, E2 and E3 enzymes attaches it to lysine residues of the target, and a chain of ubiquitins marks the protein for destruction by the 26S proteasome.

Methylation, acetylation and phosphorylation are chemical modifications, not components that bind proteins. They regulate activity, chromatin structure and signalling; they can influence stability indirectly, but none is the tag that guides a protein to degradation.