Direct heterogeneous electron transfer (ET) of sulfite oxidase (SOx), a heme- and
molybdopterin cofactor-containing intermembrane enzyme, was studied on alkanethiol-modified
Au electrodes both with SOx entrapped between the modified Au electrode and a permselective
membrane and with SOx adsorbed at the electrode surface, in the absence of any membrane.
SOx in direct electronic communication with the electrode surface gave a quasi-reversible
electrochemical signal with a midpoint potential of--120 mV vs Ag/AgCl corresponding
to the redox transformations of the heme domain of SOx and with a heterogeneous ET
constant in the order of 15 s(-1). The efficiency of the bioelectrocatalytic 2e- oxidation
of sulfite catalyzed by SOx in direct ET exchange with the electrode was shown to
depend essentially on the nature of the alkanethiol layer. Adsorption and orientation
of SOx on an 11-mercapto-1-undecanol (MuD-OH) self-assembled monolayer, i.e., terminally
functionalized with OH groups, provided efficient catalytic oxidation of sulfite,
contrary to nonfunctionalized alkanethiols, e.g., 1-decanethiol, or alkanethiol layers
terminally functionalized with NH2 groups. Comparative studies with short-chain alkanethiols,
e.g., cysteamine and 2-mercaptoethanol, revealed an evidently different mode of adsorption
of SOx on these layers, onto which SOx was not catalytically active. Coadsorption
of MuD-OH and 11-mercapto-1-undecanamine improved the surface properties of the SAM,
resulting in a higher surface coverage with bioelectrocatalytically active SOx but
not in an increased apparent catalytic rate constant, kcat, ranging in the order of
18-24 s(-1) at pH 7.4. The achieved efficiency of SOx bioelectrocatalysis in direct
ET reaction between the modified electrode and the enzyme approached the rates characteristic
for the catalysis mediated by cytochrome c, the natural redox partner of SOx, thus
implying the retention of the biological function of SOx under the heterogeneous electrode
reaction conditions. Results obtained enable the development of a third-generation
biosensor for sulfite monitoring.