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

Fully integrated biocatalytic electrodes based on bioaffinity interactions

Integrated bioelectrocatalytically active electrodes are assembled by the deposition of enzymes onto respective electrically contacted affinity matrices and further cross-linking of the enzyme monolayers. A catalyst-NAD(+)-dyad for the binding of the NAD(+)-dependent enzymes and cytochrome-like molecules for the binding of the heme-protein-dependent enzymes are used to construct integrated electrically contacted biocatalytic systems. NAD(+)-dependent lactate dehydrogenase (LDH) is assembled onto a pyrroloquinoline quinone-NAD+ monolayer. The redox-active monolayer is organized 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 interface modified with the PQQ-NAD(+)-dyad provides temporary affinity binding for LDH and allows cross-linking of the enzyme monolayer. The cross-linked LDH is bioelectrocatalytically active towards oxidation of lactate. The bioelectrocatalyzed process involves the PQQ-mediated oxidation of the immobilized NADH. Integrated, electrically contacted bioelectrodes are produced by the affinity binding and further cross-linking of nitrate reductase (NR) (cytochrome-dependent, E.C. 1.9.6.1 from E. coli) or CoII-protoporphyrin IX reconstituted myoglobin (CoII-Mb) atop the microperoxidase-11 (MP-11) monolayer associated with a Au-electrode. The MP-11 monolayer provides an affinity interface for the temporary binding of the enzymes, that allows the cross-linkage of the enzyme molecules. The MP-11 assembly acts as electron transfer mediator for the reduction of the secondary enzyme layer. The integrated bioelectrodes consisting of NR and CoII-Mb show catalytic activities for NO3- reduction and acetylene-dicarboxylic acid hydrogenation, respectively. Two FeIII-protoporphyrin IX units are reconstituted into a four alpha-helix bundle de novo protein assembled as a monolayer on a Au-electrode. Vectorial electron transfer proceeds in the synthetic heme-protein monolayer. Cross-linking of an affinity complex generated between the FeIII-protoporphyrin IX reconstituted de novo protein monolayer and NR yields an integrated, electrically contacted enzyme electrode that stimulates the bioelectrocatalyzed reduction of nitrate.

Newly discovered redox cofactors: possible nutritional, medical, and pharmacological relevance to higher animals

Research spurred by the discovery of pyrroloquinoline quinone (PPQ) in 1979 led to the discovery of four additional oxidation-reduction (redox) cofactors, all of which result from transmogrification of amino acyl side chains in respective enzymes. These cofactors are (a) topa quinone in copper-containing amine oxidases, enzymes found in nearly all forms of life, including human; (b) lysyl topa quinone of the copper protein lysyl oxidase, an enzyme required for proper cross-linking of collagen and elastin; (c) tryptophan tryptophylquinone of alkylamine dehydrogenases from gram-negative soil bacteria; and (d) the copper-complexed cysteinyltyrosyl radical of fungal galactose oxidase. Originally, PQQ was thought to be a covalently bound cofactor in numerous enzymes from eukaryotes and prokaryotes. Today, PQQ is only found as a noncovalent cofactor in bacterial enzymes. The ubiquity of PQQ in the environment and its steady accessibility in the human diet has raised questions concerning its role as a vitamin, or an essential or helpful nutrient. The relevance to nutrition, medicine, and pharmacology of PQQ, topa quinone, lysyl topa quinone, tryptophan trytophylquinone, the galactose oxidase cofactor, and the enzymes harboring these cofactors are discussed in this review.

Model studies on calcium-containing quinoprotein alcohol dehydrogenases. Catalytic role of Ca2+ for the oxidation of alcohols by coenzyme PQQ (4,5-dihydro-4,5-dioxo-1H-pyrrolo[2,3-f]quinoline-2, 7,9-tricarboxylic acid)

Mechanistic studies on the action of calcium-containing quinoprotein alcohol dehydrogenases have been performed by using a series of PQQ model compounds in anhydrous organic media. The PQQ model compounds are shown to form 1:1 complexes with a series of alkaline earth metal ions by spectroscopic methods and theoretical calculations. The site of coordination of the PQQ molecule to the metal ions in solution is indicated to be the same as in the case of enzymatic systems. It has also been found that Ca2+ binds to the quinone most strongly among the alkaline earth metal ions. Formation of the C-5 hemiacetal derivatives with methanol, ethanol, and 2-propanol is also investigated spectrophotometrically to show that the alcohol addition to the quinone is enhanced in the presence of the metal ions. In this case as well, Ca2+ shows the highest efficiency for the enhancement of the C-5 hemiacetal formation. Addition of a strong base such as DBU into an MeCN solution containing the Ca2+ complex of the PQQ model compounds and the alcohols leads to the redox reactions to afford reduced PQQ derivatives and the corresponding aldehydes. On the basis of detailed kinetic studies on the redox reactions, including structural effects of PQQ analogues and metal ion effects, we propose the addition-oxidative elimination mechanism through the C-5 hemiacetal intermediate.