The Structures of the Horseradish Peroxidase C-Ferulic Acid Complex and the Ternary Complex with Cyanide Suggest How Peroxidases Oxidize Small Phenolic Substrates

We have solved the x-ray structures of the binary horseradish peroxidase C-ferulic acid complex and the ternary horseradish peroxidase C-cyanide-ferulic acid complex to 2.0 and 1.45 Ã , respectively. Ferulic acid is a naturally occurring phenolic compound found in the plant cell wall and is an in vivo substrate for plant peroxidases. The x-ray structures demonstrate the flexibility and dynamic character of the aromatic donor binding site in horseradish peroxidase and emphasize the role of the distal arginine (Arg 38 ) in both substrate oxidation and ligand binding. Arg 38 hydrogen bonds to bound cyanide, thereby contributing to the stabilization of the horseradish peroxidase-cyanide complex and suggesting that the distal arginine will be able to contribute with a similar interaction during stabilization of a bound peroxy transition state and subsequent O-O bond cleavage. The catalytic arginine is additionally engaged in an extensive hydrogen bonding network, which also includes the catalytic distal histidine, a water molecule and Pro 139 , a proline residue conserved within the plant peroxidase superfamily. Based on the observed hydrogen bonding network and previous spectroscopic and kinetic work, a general mechanism of peroxidase substrate oxidation is proposed.