Probing the Aromatic‐Donor‐Binding Site of Horseradish Peroxidase Using Site‐Directed Mutagenesis and the Suicide Substrate Phenylhydrazine

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Abstract
The haem groups from two classes of site‐directed mutants of horseradish peroxidase isoenzyme C (HRP‐C) (distal haem pocket mutants, [H42L]HRP‐C* and [R38K]‐HRP‐C* and peripheral‐haem‐access‐channel mutants, [F142A]HRP‐C* and [F143A]HRP‐C*) were extracted and analysed by reverse‐phase HPLC after phenylhydrazine‐induced suicide inactivation. The relative abundance of the two covalently modified haems, C20‐phenyl (δ‐meso phenyl) and C18‐hydroxymethyl haem, provided a sensitive topological probe for changes induced in the protein architecture in the vicinity of the haem active site and substrate‐access channel. Although differing considerably in their efficiency as peroxidases ([H42L]HRP‐C* exhibited only approximately 0.03 % of the peroxidase activity of wild type), the variants studied gave rise to a modification pattern typical of an exposed haem edge thereby strengthening the argument that it is the overall protein topology rather than the intrinsic catalytic activity of the active site that determines the sites of covalent haem modification. Mutants which showed impaired ability to bind the aromatic donor benzhydroxamic acid were less readily modified by the phenyl radical at the haem C18‐methyl position although the level of arylation at the haem C20 position remained remarkably constant. Our findings suggest that the overall efficacy of haem modification catalysed by HRP‐C during turnover with phenylhydrazine and its vulnerability towards inactivation are related to its general ability to bind aromatic donor molecules. Results from phenylhydrazine treatment of HRP‐C wild‐type and mutant variants were compared with those obtained for Coprinus cinereus peroxidase, an enzyme which from its structure is known to have a remarkably open access channel to the haem edge. We show evidence that C. cinereus peroxidase is able to bind benzhydroxamic acid, albeit with a relatively high Kd (Kd 3.7 mM), a probe for aromatic‐donor binding. We suggest reasons why phenylhydrazine‐treated C. cinereus peroxidase was more resistant to haem modification and phenyl‐radical‐based inactivation than HRP‐C.

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