MCB124_ANSWERS_SET3B

MCB124_ANSWERS_SET3B - MCB 124 Enoch Baldwin Electrostatics...

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MCB 124 Protein Stability Enoch Baldwin ANSWERS SET IIIB page 1 Electrostatics Question: An enzyme binds peptide substrates. This protein has a carboxylate residue, Glu238. The pKa of this residue in the absence of substrate by NMR is 6.5. The pKa in the unfolded form is 4.8. a) How does the ionization of this residue contribute to protein stability? ______favorably OR __X__ unfavorably. The pKa is shifted away from ionization. This means the environment of the carboxylate is less favorable in the folded protein, therefore it is increasing the free energy of the F state. b) If the pH dependence of folding was only determined by this residue, what would happen to the protein Tm at pH 7 compared to pH 4.0 ? The Tm would be reduced because Glu238 is half-ionized at pH 7 but mostly protonated at pH 4. (although in reality, the Tm would be different because of other titratable residues) c) Why? The ionized form is not accommodated by the protein, hence the increased pKa. d). If this residue were mutated to a Gln residue, what would you estimate the change in protein
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This note was uploaded on 04/06/2011 for the course MCB 124 taught by Professor Baldwin during the Summer '09 term at UC Davis.

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MCB124_ANSWERS_SET3B - MCB 124 Enoch Baldwin Electrostatics...

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