Question 21.6: Amino Acid Structure Why is histidine often found in the act...

What makes histidine unique is that it has a basic side chain (one based on a nitrogen that can associate with a hydrogen ion), but that group has an acidic \text{p}K_{a} (6.0). This makes histidine a unique amino acid because its side chain can be found protonated or deprotonated under physiological conditions. This is one reason why histidine is found in the active sites of so many enzymes due to its ability to change forms.

■ Quick Check 21.5
Give two reasons why the aromatic amino acids are of considerable importance.