The effect of pH on the kinetics of the bioelectrocatalytic reduction of H(2)O(2) catalysed by horseradish peroxidase (HRP) has been studied at -50 mV vs. Agmid R:AgCl on HRP-modified Au electrodes placed in a wall-jet flow-through electrochemical cell. Native HRP (nHRP) and a nonglycosylated recombinant form containing a six-histidine tag at the C-terminus, C(His)rHRP, produced by genetic engineering of nonglycosylated recombinant HRP using an E. coli expression system, have been used for adsorptive modification of Au electrodes. A favourable adsorption of C(His)rHRP on pre-oxidized Au from a protein solution at pH 6.0 provided a high and stable current response to H(2)O(2) due to its bioelectrocatalytic reduction based on direct (mediator-less) electron transfer (ET) between Au and the active site of HRP. The heterogeneous ET rate constant, k(s), calculated from experimental data on direct ET, on mediated ET in the presence of catechol as well as from microbalance data, increased more than 30 times when changing from nHRP to C(His)rHRP. For both forms of HRP, the increasing efficiency of bioelectrocatalysis with increasing [H(3)O(+)] was observed. The values of the apparent k(s) between C(His)rHRP and Au changed from a value of 12+/-2 s(-1) in PBS at pH 8.0 to a value of 434+/-62 s(-1) at pH 6.0; a similar k(s)-pH dependence was also observed for nHRP, providing the possibility to consider the reaction mechanism involving the participation of a proton in the rate-determining step of the charge transfer.