25. What will happen to protein with an aspartate residue and a glutamate residue near each other if the pH is increased from 1.0 to 7.4? a. The aspartate and glutamate carboxyl side chains will be protonated and gain a positive charge, causing repulsion. b. The aspartate and glutamate carboxyl side chains will be protonated and form a hydrogen bond with each other. c. The aspartate and glutamate carboxyl side chains will gain a negative charge and repel each other. d. The aspartate and glutamate carboxyl side chains will form disulfide bonds.
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21. Which of the following statements is TRUE when aspartic acid in a protein is positioned in proximity with glutamic acid? a. The pKa of aspartic acid is unaffected b. The protein dissociation from aspartic acid side chain is favored c. The pKa of aspartic acid is increased d. The pKa of aspartic acid is decreased
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Isaac H.
Consider a protein in which a negatively charged glutamic acid side chain (pKa = 4.2) makes a salt bridge (ion-ion interaction) with a positively charged histidine side chain (pKa = 6.5). Part A: Do you predict that this salt bridge will become stronger, become weaker, or be unaffected as pH increases from pH = 7.2 to pH = 7.8? The salt bridge will become stronger. The salt bridge will become weaker. The salt bridge will be unaffected. Correct At pH = 7.2 the glutamic acid (Glu) side chain will carry a charge of ~ -1 (at 3 pH units above the pKa for Glu, the side chain will be almost fully ionized); whereas the histidine (His) side chain will carry a charge of < +0.5 (at pH = pKa the charge on His would be +0.5; since pH = 7.2 is above its pKa, it will carry less (+) charge as it becomes more deprotonated). As the pH increase to 7.8, the charge on Glu will remain ~ -1 and the charge on His will decrease; thus, this salt bridge is predicted to become weaker as pH is raised from 7.2 to 7.8. Part B: Justify your answer with calculations of partial charges on these amino acid side chains at pH = 7.2. (Hint: Consider lessons from Coulomb's law, and the Henderson-Hasselbalch equation.) Express your answers using two significant figures separated by a comma. -0.99, 0.33
Adi S.
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