Electrocatalytic cycling of nicotinamide cofactors by Ralstonia eutropha soluble hydrogenase
Keywords: Ralstonia eutropha
nicotinamide
protein film electrochemistry
soluble hydrogenase

Nicotinamide cofactors in their reduced and oxidised forms are important redox agents in biology. Of about 3000 dehydrogenases available to date, many require these cofactors for their activity. Dehydrogenases are of interest to chemists as they offer asymmetric catalysis to yield chiral products. The requirement of dehydrogenases for nicotinamide cofactors necessitates research into finding the best way of recycling the oxidised or reduced forms of these cofactors. Electrocatalytic NAD(P)H oxidation and NAD(P)⁺ reduction on standard electrodes is problematic due to unwanted side reactions and high overpotential requirements, but in Nature efficient enzyme catalysts are available to facilitate these reactions.
The focus of this Thesis, the Soluble Hydrogenase of R. eutropha (SH) is a multimeric bidirectional hydrogenase that couples H2 oxidation to the reduction of NAD⁺ to NADH. Protein Film Electrochemistry (PFE) has been employed to study NAD⁺-reducing catalytic moieties of the SH for the first time. It is shown that SH subunits on an electrode are able to catalyse NADH oxidation and NAD⁺ reduction efficiently with minimal overpotential, which is significant because in vivo, NAD(H) cycling is coupled to 2H⁺/H₂ cycling and these reactions are closely spaced in potential. Substrate affinities and inhibition constants for the SH, determined using PFE are discussed in the context of the SH function and the related catalytic domains of respiratory Complex I. A range of molecules that are known to inhibit the related Complex I have been investigated for their ability to inhibit the SH moieties: the similarity between inhibition constants is consistent with structural and functional similarity between the SH and Complex I. The ability of the SH moieties to sustain NAD(H) catalysis in the presence of O₂ is also demonstrated and is consistent with the requirement for the SH to function under aerobic conditions and to reactivate the inactivated hydrogenase moiety by supplying low potential electrons from NADH.
Engineered variants of the SH, designed to enhance the affinity towards NADP⁺, were investigated for the first time, using PFE. Electrochemical characterisation of the variants is presented and results are discussed alongside findings on the wild type SH. The variants are shown to exhibit NADP⁺ reduction, and to have higher affinity towards NADP⁺ than the wild type SH. The first efficient NADP⁺ reduction and NADPH oxidation is observed for one of the variants on a graphite electrode and the best variant showed a KM of 1.7 mM for NADP⁺.
This Thesis also provides evidence for the ability of moieties of the SH to be used in cofactor regeneration systems. Two novel systems are demonstrated. The first involves H₂ driven NADH recycling based on the NAD⁺-reducing moiety of the SH immobilised on graphite particles together with a hydrogenase or platinum, with electrons from H₂ passed from the hydrogenase through the graphite to the NAD⁺-reducing moiety. The second involves an electrode modified with the NAD⁺-reducing moiety of the SH, and is demonstrated as an electrochemical NADH recycling system coupled with NADH-dependent pyruvate reduction to lactate by lactate dehydrogenase. The ability of variants of the SH to catalyse NADP⁺ reduction suggests that it may also be possible to use these systems for recycling NADPH for catalysis of important biotransformation reactions by NADPH-dependent dehydrogenases.