Use of cytoplasmic hydrogenase from alcaligenes eutrophus for NADH regeneration

Bicyclo[3.2.0]carbocyclic Molecules and Redox Biotransformations: The Evolution of Closed-Loop Artificial Linear Biocatalytic Cascades and Related Redox-Neutral Systems

The role of cofactor recycling in determining the efficiency of artificial biocatalytic cascades has become paramount in recent years. Closed-loop cofactor recycling, which initially emerged in the 1990s, has made a valuable contribution to the development of this aspect of biotechnology. However, the evolution of redox-neutral closed-loop cofactor recycling has a longer history that has been integrally linked to the enzymology of oxy-functionalised bicyclo[3.2.0]carbocyclic molecule metabolism throughout. This review traces that relevant history from the mid-1960s to current times.

Enzymes as modular catalysts for redox half-reactions in H2-powered chemical synthesis: from biology to technology

The present study considers the ways in which redox enzyme modules are coupled in living cells for linking reductive and oxidative half-reactions, and then reviews examples in which this concept can be exploited technologically in applications of coupled enzyme pairs. We discuss many examples in which enzymes are interfaced with electronically conductive particles to build up heterogeneous catalytic systems in an approach which could be termed synthetic biochemistry We focus on reactions involving the H+/H2 redox couple catalysed by NiFe hydrogenase moieties in conjunction with other biocatalysed reactions to assemble systems directed towards synthesis of specialised chemicals, chemical building blocks or bio-derived fuel molecules. We review our work in which this approach is applied in designing enzyme-modified particles for H2-driven recycling of the nicotinamide cofactor NADH to provide a clean cofactor source for applications of NADH-dependent enzymes in chemical synthesis, presenting a combination of published and new work on these systems. We also consider related photobiocatalytic approaches for light-driven production of chemicals or H2 as a fuel. We emphasise the techniques available for understanding detailed catalytic properties of the enzymes responsible for individual redox half-reactions, and the importance of a fundamental understanding of the enzyme characteristics in enabling effective applications of redox biocatalysis.

Reversible active site sulfoxygenation can explain the oxygen tolerance of a NAD+-reducing [NiFe] hydrogenase and its unusual infrared spectroscopic properties

Oxygen-tolerant [NiFe] hydrogenases are metalloenzymes that represent valuable model systems for sustainable H2 oxidation and production. The soluble NAD(+)-reducing [NiFe] hydrogenase (SH) from Ralstonia eutropha couples the reversible cleavage of H2 with the reduction of NAD(+) and displays a unique O2 tolerance. Here we performed IR spectroscopic investigations on purified SH in various redox states in combination with density functional theory to provide structural insights into the catalytic [NiFe] center. These studies revealed a standard-like coordination of the active site with diatomic CO and cyanide ligands. The long-lasting discrepancy between spectroscopic data obtained in vitro and in vivo could be solved on the basis of reversible cysteine oxygenation in the fully oxidized state of the [NiFe] site. The data are consistent with a model in which the SH detoxifies O2 catalytically by means of an NADH-dependent (per)oxidase reaction involving the intermediary formation of stable cysteine sulfenates. The occurrence of two catalytic activities, hydrogen conversion and oxygen reduction, at the same cofactor may inspire the design of novel biomimetic catalysts performing H2-conversion even in the presence of O2.

Concepts in bio-molecular spectroscopy: vibrational case studies on metalloenzymes

Challenges and chances in bio-molecular spectroscopy are exemplified by vibrational case studies on metalloenzymes.

H₂-driven cofactor regeneration with NAD(P)⁺-reducing hydrogenases

A large number of industrially relevant enzymes depend upon nicotinamide cofactors, which are too expensive to be added in stoichiometric amounts. Existing NAD(P)H-recycling systems suffer from low activity, or the generation of side products. H₂-driven cofactor regeneration has the advantage of 100% atom efficiency and the use of H₂ as a cheap reducing agent, in a world where sustainable energy carriers are increasingly attractive. The state of development of H₂-driven cofactor-recycling systems and examples of their integration with enzyme reactions are summarized in this article. The O₂-tolerant NAD⁺-reducing hydrogenase from Ralstonia eutropha is a particularly attractive candidate for this approach, and we therefore discuss its catalytic properties that are relevant for technical applications.

NAD(H)-coupled hydrogen cycling - structure-function relationships of bidirectional [NiFe] hydrogenases

Hydrogenases catalyze the activation or production of molecular hydrogen. Due to their potential importance for future biotechnological applications, these enzymes have been in the focus of intense research for the past decades. Bidirectional [NiFe] hydrogenases are of particular interest as they couple the reversible cleavage of hydrogen to the redox conversion of NAD(H). In this account, we review the current state of knowledge about mechanistic aspects and structural determinants of these complex multi-cofactor enzymes. Special emphasis is laid on the oxygen-tolerant NAD(H)-linked bidirectional [NiFe] hydrogenase from Ralstonia eutropha.

A modular system for regeneration of NAD cofactors using graphite particles modified with hydrogenase and diaphorase moieties

Pyrolytic graphite particles modified with hydrogenase and an NAD(+)/NADH cycling enzyme provide a modular heterogeneous catalyst system for regeneration of oxidised or reduced nicotinamide cofactors using H(2) and H(+) as electron source or sink. Particles can be tuned for cofactor supply under different conditions by appropriate choice of hydrogenase.

Redox-dependent inactivation of the NAD-dependent hydrogenase from Alcaligenes eutrophus Z1

A novel inactivation mechanism of the NAD-dependent hydrogenase from Alcaligenes eutrophus Z1 comprising redox-dependent steps is described. The model of the hydrogenase inactivation process is proposed which implies that the enzyme may exist in several forms which differ in their stability and spectral properties. One of these forms, existing within a limited (approximately -200 +/- 30 mV) potential range, undergoes a rapid and irreversible inactivation. The dissociation of the FMN prosthetic group from the apohydrogenase appears to be the main reason for the enzyme inactivation. The rationale for the enzyme stabilization under real operational conditions based on the chemical modification of the hydrogenase molecule is suggested.

Regeneration of nicotinamide cofactors for use in organic synthesis

The high cost of nicotinamide cofactors requires that they be regenerated in situ when used in preparative enzymatic synthesis. Numerous strategies have been tested for in situ regeneration of reduced and oxidized cofactors. Regeneration of reduced cofactors is relatively straightforward; regeneration of oxidized cofactors is more difficult. This review summarizes methods for preparation of the cofactors, factors influencing their stability and lifetime in solution, methods for their in situ regeneration, and process considerations relevant to their use in synthesis.