Aromatic amino acid methyl ester analogs form quinonoidal species with Dopa decarboxylase.

This study reports for the first time that binding of aromatic methyl ester analogs to Dopa decarboxylase in the native and inactive nicked forms causes the appearance of a dead-end quinonoidal species absorbing at 500 nm, in addition to an external aldimine absorbing at 398 nm. The equilibrium mixture of these species varies depending on both the analog structure and the enzyme form. The above mentioned intermediates are also characterized with respect to their CD properties and the equilibria for their formation are determined as a function of pH. The results have provided evidence that the establishment of proper contacts between the active site and hydroxyl groups of the ligand are indispensable in order to limit unwanted side reactions.