The unfolding reaction for many small proteins can be represented by the equilibrium: Folded Unfolde — Analytical Chemistry Chemistry Question
Protein Folding
The unfolding reaction for many small proteins can be represented by the equilibrium:
Folded Unfolded
You may assume that the protein folding reaction takes place in a single step. The position of this equilibrium changes with temperature; the melting temperature Tm is defined as the temperature at which half of the molecules are unfolded and half are
Assuming that the fluorescence intensity from each species is directly proportional to its concentration, calculate the fraction, x, of unfolded molecules present at each temperature.
Model Answer
Temp / °C 58 60 62 64 66
x 0.27 0.41 0.59 0.73 0.86
Give an expression for the equilibrium constant, K, in terms of x, and hence calculate the value of K at each temperature.
Model Answer
Temp / °C 58 60 62 64 66
K 0.38 0.69 1.4 2.7 6.3
Estimate the value of Tm for this protein (to the nearest 1 °C).
Model Answer
Answer: Tm = 61 °C
Assuming that the values of ∆Ho and ∆So for the protein unfolding reaction are constant with temperature then:
o∆ ln
H K C
RT = − +
where C is a constant.
3.4 Plot a suitable graph and hence determine the values of ∆Ho and ∆So for the protein unfolding reaction.
[If you have been unable to calculate values for ∆Ho and ∆So, you should use the following incorrect values for the subsequent parts of the problem:
∆Ho = 130 kJ mol–1; ∆So = 250 J K–1 mol–1.]
Model Answer
Answers: ∆Ho = 330 kJ mol–1 ; ∆So = 980 J mol–1 K–1
Calculate the equilibrium constant for the unfolding reaction at 25 °C.
[If you have been unable to calculate a value for K, you should use the following
Model Answer
∆Ho = 330000 J mol–1 and ∆So= 980 J mol–1 K–1 then ∆Go = 35000 J mol–1 at 25 oC, hence K = 6.9 · 10–7.
protein folding reaction, kf, at 25 °C.
[If you have been unable to calculate the value for kf, you should use the following incorrect value for the subsequent parts of the question: kf = 60 s–1.]
Model Answer
Answer: Rate constant for the protein folding reaction, kf = 50 s–1.
Determine the value of the rate constant for the protein unfolding reaction, ku, at 25 °C.
Model Answer
Answer: Rate constant for the protein unfolding reaction, ku = 3.5 · 10–5 s–1.
At 20 °C the rate constant for the protein folding reaction is 33 s–1. Calculate the activation energy for the protein folding reaction.
Model Answer
Answer: Activation energy = 61 kJ mol–1.