Question

Explain why tyrosine has a lower pKa than threonine.

Answer 1

Lower pKa means higher acidity. The acidity of organic compounds depends on the nature of electron donating and electron withdrawing groups. The electron withdrawing groups withdraws negative charge from the carboxylate anion and thus increases its acidity. The electron donating groups on the other hand increases charge density of the carboxylate anion and therefore decreases acidy.

Threonine contain hydroxyl group, while tyrosine contains most acidic phenolic hydroxyl group. The phenolic hydroxyl group of tyrosine has a lower pKa (~10) than serine and threonine pKa (~13) and is considered more reactive. As with all ionizable groups, the precise pK_a will depend to a major degree upon the environment within the protein. Tyrosines that are on the surface of a protein will generally have a lower pK_a than those that are buried within a protein; ionization yielding the phenolate anion would be exceedingly unstable in the hydrophobic interior of a protein.

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