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

For the first time saturating overall k(cat) values for horseradish peroxid ase (HRP) catalysed conversion of phenols and anilines are described. These k(cat) values correlate quantitatively with calculated ionisation potentia ls of the substrates. The correlations for the phenols are shifted to highe r k(cat) values at similar ionisation potentials as compared to those for a nilines. H-1-NMR T-1 relaxation studies, using 3-methylphenol and 3-methyla niline as the modal substrates, revealed smaller average distances of the p henol than of the aniline protons to the paramagnetic Fe3+ centre in HRP. T his observation, together with a possibly higher extent of deprotonation of the phenols than of the anilines upon binding to the active site of HRP, m ay contribute to the relatively higher HRP catalysed conversion rates of ph enols than of anilines. (C) 1999 Elsevier Science B.V. All rights reserved.