Differential substrate behaviour of phenol and aniline derivatives during conversion by horseradish peroxidase

For the first time saturating overall kcat values for horseradish peroxidase (HRP) catalysed conversion of phenols and anilines are described. These kcat values correlate quantitatively with calculated ionisation potentials of the substrates. The correlations for the phenols are shifted to higher kcat values at similar ionisation potentials as compared to those for anilines. 1H-NMR T1 relaxation studies, using 3-methylphenol and 3-methylaniline as the model substrates, revealed smaller average distances of the phenol than of the aniline protons to the paramagnetic Fe3 centre in HRP. This observation, together with a possibly higher extent of deprotonation of the phenols than of the anilines upon binding to the active site of HRP, may contribute to the relatively higher HRP catalysed conversion rates of phenols than of anilines.