Shown below are free energy diagrams representing the unfolding of 3 proteins (N = native; U = unfolded).

The following graph shows three protein unfolding curves.

Which of the following options is most likely to indicate the correct relationship between the three free energy diagrams and the three unfolding curves?

Shown below is a curve representing the unfolding of a protein with increasing temperature.

At what temperature are 90% of protein molecules folded?

Enter the temperature in degrees Celsius (deg C) but do NOT type the units. To be marked correct you must type only numbers.

One way to destabilise a folded protein structure is to change the solution conditions so that the increase in the entropy of the polypeptide chain (when the protein unfolds) makes a greater contribution to the free energy of unfolding than it would under physiological buffer conditions.

Which of the following perturbations is most likely to destabilise a protein in this way?

Which of the following is the major factor (component of free energy) favouring the formation of hydrogen bonds between two groups in a protein?

To test the importance of certain amino acid residues in stabilising a protein structure, a research student performs equilibrium unfolding experiments with three forms of the protein:

(A) the wild type protein;

(B) a mutant in which a slightly stabilising interaction is introduced; and

(C) a mutant in which two slightly stabilising interactions are introduced.

They obtain the three unfolding curves shown below.

Which curves correspond to which forms of the protein?

Shown below are free energy diagrams representing the unfolding of 3 proteins (N = native; U = unfolded).

The following graph shows three protein unfolding curves.

Which of the following options is most likely to indicate the correct relationship between the three free energy diagrams and the three unfolding curves?

An enzyme is known to contain four Cys residues that form two disulfide bonds (and no other Cys residues).

[Note that four Cys residues could randomly form three possible disulfide bond arrangements]

A researcher performs the following sequence of treatments on a sample of this enzyme:

1. Treatment with high concentrations of mercaptoethanol and urea.
2. Removal of the mercaptoethanol while keeping the urea concentration high (and avoiding formation of any intermolecular disulfide bonds).
3. Removal of the urea (returning the sample to the original buffer).

They find that the sample obtained from the final treatment has one third of the activity of the original sample of the enzyme.

Which of the following is the best conclusion from this experiment?

Consider the following two ionisable groups positioned close to each other within a protein structure:

• The carboxy terminus (pKa = 2.3)
• An arginine side chain (pKa = 12.5)

Based on the pKa values of these functional groups and their charge states when they are ionised, select the correct statement below.

[Note that these statements are specifically referring to the attraction or repulsion of ionic interactions between these groups]

Shown below is an unfolding curve for a protein that contains two domains of approximately equal size.

Which point on the curve represents the form of this protein in which one domain is fully unfolded and the other is 50% unfolded?

The protein vMIP-II from Kaposi's sarcoma-associated herpesvirus (which subverts the host immune system by blocking the migration of white blood cells) consists of a 3-stranded β-sheet packed against an α-helix.

The side chain of residue Lys-61 is located on the exposed surface of the α-helix.

Examine the structure of vMIP-II (in the vicinity of Lys-61) in the PDB 3D View or by downloading the PDB file 1HFG into PyMOL.

Which of the following mutations is most likely to result in a new ionic interaction with the side chain of Lys-61, causing the folded structure of vMIP-II to be more stable at neutral pH (but not at pH 3 or pH 11)?