A cysteine found in the interior of a protein can act as a base in a...
A cysteine found in the interior of a protein can act as a base in a chemical reaction. Since the pH for living organisms is held constant near 7.3, what would have to happen to the pKa of the side chain (as a result of microenvironmental effects) to optimize cysteine’s ability to act as a base. Explain your answer.
Homework Answers
Cysteine contains a side chain with an -SH group. This acts as a nucleophile due to the lone pair electrons on the sulfur group, which is why it acts as a base. In order to optimize cysteine's ability to act as a base, its side chain should be deprotonated and the sulfur group should exist as -S-, and this happens when the pKa is lower than the pH.
However, this is not what happens at physiological pH. The pKa of cysteine side chain is 8.33 and at physiological pH, only a fraction of cysteine residues are deprotonated.
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