Oriented Attachment of Cytochrome P450 2C9 to a Self-Assembled Monolayer on a Gold Electrode as a Biosensor Design
Cytochrome P450s (CYPs) are a family of enzymes implicated in the metabolism of drugs in the body. Consequently, P450 reactions are of high interest to the pharmaceutical industry, where lead compounds in drug development are screened as potential substrates of CYPs. The P450 reaction involves electron transfer to an iron heme via NADPH and the electron transfer partner enzyme P450 reductase (CPR). By immobilizing CYPs on an electrode however, NADPH and CPR are potentially no longer needed and the immobilized CYP can act as a biosensor by accepting electrons directly from the electrode. Such a biosensor could be used as an initial screening tool for CYP reactivity of pharmaceuticals in development. In this study, the drug-metabolizing enzyme CYP 2C9 was immobilized to a self-assembled monolayer (SAM) on a gold electrode in three different orientations to investigate the effect that orientation has on the direct electrochemistry of CYP and to evaluate oriented attachment of CYP to an electrode as a biosensor design. Three attachment methods were investigated: random attachment via amine coupling to a carboxy-terminated SAM, oriented attachment via C-terminal His-tag coupling to a Ni-NTA-functionalized SAM, and oriented attachment via maleimide/thiol coupling to a maleimide-functionalized SAM. Three 2C9 mutants (R125C, R132C, and K432C) were developed with a single cysteine mutation at the binding site for CPR on the side of the enzyme closest to the heme; attachment of these mutants to a gold electrode via maleimide/thiol coupling would orient the enzyme such that electron transfer occurs on the electrode in the same orientation that it does in vivo with CPR. Therefore, we expected oriented attachment via maleimide/thiol coupling to produce the most electroactive CYP biosensor. Electrochemical analysis and surface characterization of the SAMs on gold electrodes confirmed that electron transfer occurs through the SAMs, and activity assays of the 2C9 electrodes confirmed that wild-type 2C9 and the single Cys mutants R125C, R132C, and K432C were immobilized to the gold electrode via all three attachment methods. Cyclic voltammetry of the 2C9 electrodes revealed however, that direct electron transfer to 2C9 was not possible for all three attachment methods. Similar redox processes were observed for both the 2C9 electrodes and no-enzyme electrodes modified only with SAMs, suggesting that the redox process observed on the 2C9 electrodes is related to the underlying SAM. Thus, we were unable to make any conclusions regarding the effectiveness of oriented attachment in creating a 2C9 biosensor. However, to our knowledge, there are no examples in the literature of the oriented attachment of 2C9 to an electrode via coupling of an engineered cysteine to a maleimide-functionalized SAM on gold and therefore this study represents the first attempt towards such an electrode system.