Plant peroxidases: substrate complexes with mechanistic implications.

Plant peroxidases are capable of binding phenolic substrates, and it has been possible to crystallize complexes between horseradish peroxidase C (HRP C) and benzhydroxamic acid. The X-ray structures of the binary HRP C:ferulic acid complex and the ternary HRP C:CN(-):ferulic acid complex to 2.0 and 1.45 A resolution, respectively, have also been solved recently. Ferulic 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 structures demonstrate the flexibility of the aromatic-donor-binding site in plant peroxidases and highlight the role of the distal arginine in substrate oxidation and ligand binding. A general mechanism of peroxidase substrate oxidation (compound I-->compound II and compound II-->resting state) can be proposed on the basis of the complexes and a large body of biochemical evidence.