Native lignin peroxidase (LIP) can adopt either a stable penta- or hexa-coordinate state. We have examined catalysis with hexa-coordinate ferric LIP as the starting material, using rapid scanning spectrophotometry. Initial two-electron oxidation of hexa-coordinate native LIP by H2O2 (Compound I formation) was accompanied by a shifting isosbestic point (419 --> 416 nm), consistent with displacement of a resident water molecule, prior to the reaction of the ferric iron with H2O2. The Compound I species derived from a hexa-coordinate ferric state shows an unusual peak at 520 nm, which may be due to water retention in the vicinity of the heme active site. Compound I reduction by veratryl alcohol showed saturation kinetics, which contrasts with the situation observed when Compound I is derived from a pent a-coo rdinate ferric state. The data inferred that water can interfere with heme access by electron donors, altering the mechanism of Compound I reduction. (C) 2002 Elsevier Science (USA). All rights reserved.