Plant peroxidases: substrate complexes with mechanistic implications

Plant peroxidases are capable of binding phenolic substrates, and it has be en possible to crystallize complexes between horseradish peroxidase C (HRP C) and benzhydroxamic acid. The X-ray structures of the binary HRP C:feruli c acid complex and the ternary HRP C:CN-:ferulic acid complex to 2.0 and 1. 45 Angstrom resolution, respectively, have also been solved recently. Ferul ic acid is a naturally occurring phenolic compound found in the plant cell wall and it is an in vivo substrate for plant peroxidases. The X-ray struct ures demonstrate the flexibility of the aromatic-donor-binding site in plan t peroxidases and highlight the role of the distal arginine in substrate ox idation and ligand binding. A general mechanism of peroxidase substrate oxi dation (compound I --> compound II and compound II --> resting state) can b e proposed on the basis of the complexes and a large body of biochemical ev idence.