Unformatted text preview: of the histidine in the mutant protein? Note that under chemical standard conditions the
standard states for all reactants and products, including H+, is assumed to be 1 M.
(iii) (3 points) The protein undergoes a mutation so that an aspartic acid sidechain is placed next Q2 A (10 points)
Consider a histidine sidechain on the surface of a protein. The pKA value of the histidine is 6.5,
and the temperature is 300 K.
(i) (3 points) What fraction of the protein molecules in a solution would have this histidine
residue protonated at pH 7.0? (ii) (4 points) The acid/base reaction governing histidine protonation is: His + His + H +
Assume that the pH is held fixed at 7.0 ([H+]=10-7) due the presence of a buffer. Under these
conditions, the equilibrium constant KA (acid dissociation constant) is given by: KA = [ His ] ⎡ H + ⎤
⎡ His + ⎤
⎦ . What is the value of the standard free energy change for the reaction (chemical standard states) ( ΔG 0 )? (iii) (3 points) The protein undergoes a mutation so that an aspartic acid sidechain is placed next
to the histidine. The aspartic acid stabilizes the protonated form of the histidine, so that the
value of ΔG 0 for histidine deprotonation is +60 kJ.mol-1 for the mutant protein. What is the pKA
value of the histidine in the mutant protein? Note that under chemical standard conditions the
standard states for all reactants and products, including H+, is assumed to be 1 M. Final Exam MCB C100A / Chem C130 Fall 2012
Question 8. (15 points total)
A student in a biochem lab made a buffer solution using imidazole for studies of a protein. She looked up the
thermodynamics for the ionization reaction: ImH+ → Im + H+ and found ΔG° = +34 kJ mol-1 (chemist’s
standard state), and ΔH° = + 30 kJ mol-1, both at 25° C.
Q8. A. (8 points) She dissolved 0.001 moles of ImH+ and 0.001 moles of Im in 100 ml of water. What will the
pH of the buffer solution be?
Q8. B. (7 points)
Before she put her protein into the buffer she realized she needed to add some salt. She found a concentrated
stock solution of sodium chloride that she could add, but it was at a pH of 3 (but no buffer in it). If sh...
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- Fall '09