Which amino acid is found in the greatest abundance in the active site of laccase?

• A. Phe
• B. Tyr
• C. His
• D. Cys

Answer: (C)

The active site of laccase, an enzyme that catalyzes the oxidation of phenolic compounds, prominently features the amino acid histidine (His). This is due to histidine's unique properties that make it an excellent candidate for binding metal ions, particularly copper, which is critical for the enzymatic activity of laccase. Histidine contains an imidazole side chain that can easily coordinate with metal ions and can also function as both an acid and a base, facilitating the enzyme's catalytic mechanism.

In the context of laccase, copper ions are essential for the oxidation-reduction reactions that the enzyme performs, and histidine plays a crucial role in stabilizing these metal ions at the active site. Its ability to exist in protonated and deprotonated forms at physiological pH enhances the enzyme's flexibility in catalyzing reactions. Therefore, histidine is the most abundant amino acid present in the active site of laccase, reflecting its functional significance in the enzyme's mechanism.