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-ferul ic acid complex to 2.0 and 1.45 Angstrom, respectively. Ferulic acid is a n aturally 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 horsera dish peroxidase-cyanide complex and suggesting that the distal arginine wil l be able to contribute with a similar interaction during stabilization of a bound peroxy transition state and subsequent O-O bond cleavage. The catal ytic arginine is additionally engaged in an extensive hydrogen bonding netw ork, which also includes the catalytic distal histidine, a water molecule a nd Pro(139), a proline residue conserved within the plant peroxidase superf amily. Based on the observed hydrogen bonding network and previous spectros copic and kinetic work, a general mechanism of peroxidase substrate oxidati on is proposed.