NAD+-Dependent Enzyme Electrodes: Electrical Contact of Cofactor-Dependent Enzymes and Electrodes

Abstract

NAD+-dependent lactate dehydrogenase (LDH) is assembled onto a pyrroloquinoline quinone-NAD+ monolayer. The redox active monolayer is assembled via covalent attachment of pyrroloquinoline quinone (PQQ) to a cystamine monolayer associated with a Au electrode, followed by covalent linkage of N6-(2-aminoethyl)-NAD+ to the monolayer. The surface coverage of PQQ and NAD+ units is ca. 1.2 × 10-10 mol cm-2. The surface coverage of LDH bound to the redox active monolayer is ca. 3.5 × 10-12 mol cm-2. The assembled LDH monolayer is active in the bioelectrocatalytic oxidation of lactate. The bioelectrocatalyzed process involves the PQQ-mediated oxidation of the immobilized NADH in the presence of Ca2+ ions. The LDH associated with the PQQ-NAD+ monolayer assembled on the electrode surface exhibits moderate stability, and the biocatalyst dissociates to the electrolyte solution. Dissociation of LDH is enhanced in the presence of solubilized NAD+. Cross-linking of the monolayer-bound LDH with glutaric dialdehyde yields an integrated stable enzyme electrode for the bioelectrocatalyzed oxidation of lactate. The electrode acts as an amperometric biosensor for lactate. Affinity binding of NAD+-dependent alcohol dehydrogenase to the PQQ-NAD+-monolayer-modified Au electrode, followed by cross-linking of the enzyme, yields an enzyme electrode for the bioelectrochemical detection of ethanol.