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

• A lysine side chain (pKa = 10.5)
• An aspartic acid side chain (pKa = 3.7)

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

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?

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

[Note that six Cys residues could randomly form fifteen 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. Simultaneous removal of the mercaptoethanol and the urea (while avoiding formation of any intermolecular disulfide bonds).

They find that the sample obtained from the final treatment has 100% of the activity of the original sample of the enzyme.

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

Which of the following is the major factor (component of free energy) favouring the formation of ionic interactions between oppositely charged side chain groups in a 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?

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?