Histidine can contribute to buffering because its side chain can gain or lose a proton.

### Why this is right Histidine’s imidazole side chain can accept or donate a proton, allowing it to participate in acid-base reactions. This makes histidine useful in protein active sites and buffering roles. ### The concept A buffer limits changes in pH by reversibly binding or releasing hydrogen ions. A group is most effective as a buffer when its protonation state can change in the pH range of interest. ### Terms - **Imidazole**: a nitrogen-containing ring in the histidine side chain. - **Buffer**: a system that resists a change in pH when acid or base is added. ### Remember Histidine’s side-chain chemistry makes it a common participant in enzyme acid-base catalysis.