1. Based on pKa values, which amino acid side chains are easiest to protonate an
ID: 185475 • Letter: 1
Question
1. Based on pKa values, which amino acid side chains are easiest to protonate and deprotonate at cellular pH (7.4)? (Choose all that apply)
Aspartic Acid
Histidine
Lysine
Serine
Arginine
Glutamic Acid
Cysteine
Tyrosine
2. At pH 7.4, which of the amino acid sidechains or R-groups below will be mostly deprotonated? (Chose all that apply)
Glutamatic Acid
Aspartic Acid
Arginine
Tyrosine
Serine
Lysine
Histidine
Cysteine
3. Acetyltransferase enzymes modify a lysine side chain with an acetyl group. Titration of the pKa of the lysine substrate reveals the pKa value to be 8.1. What is the likely environment surrounding the lysine sidechain before it is acetylated? (Choose all that apply)
Cannot hypothesize based on the given information and prior knowledge
Water accessible active site in the enzyme
Water inaccessible highly polar active site
Largely non-polar and water inaccessible active site
Solvent exposed and near the enzyme surface
Explanation / Answer
Aspartic acids, Glutamic acid have very low pKa value so they lose a proton, and becomes negatively charged at pH 7.4. So they are prone to deprotonation.
Aminoacids like Arginine, Lysine, Histidine have a larger Pka that they tend to gain a proton and becomes positively charged. So they are protonated at pH 7.4.
Serine, Tyrosine, and cysteine will also get deprotonated but at very slow rate as they have very less difference in the PKa value.
Ans. The pI value of side chains of aspartic acid and glutamic acid is lesser than the Pka value at pH 7.4, so they are more prone to deprotonation.
Ans. Water inaccessible highly polar active site is the condition when there will be a negative charge on the lysine that can facilitate the addition of acetyl group to the lysine side chain.
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