Hydrogenases from the hyperthermophilic bacterium Aquifex aeolicus: electrocatalysis of the hydrogen production/consumption reactions at carbon electrodes

Hydrogenase-based electrode for hydrogen sensing in a fermentation bioreactor

The hydrogen-based economy will require not only sustainable hydrogen production but also sensitive and cheap hydrogen sensors. Commercially available H₂ sensors are limited by either use of noble metals or elevated temperatures. In nature, hydrogenase enzymes present high affinity and selectivity for hydrogen, while being able to operate in mild conditions. This study aims at evaluating the performance of an electrochemical sensor based on carbon nanomaterials with immobilised hydrogenase from the hyperthermophilic bacterium Aquifex aeolicus for H₂ detection. The effect of various parameters, including the surface chemistry, dispersion degree and amount of deposited carbon nanotubes, enzyme concentration, temperature and pH on the H₂ oxidation are investigated. Although the highest catalytic response is obtained at a temperature around 60 °C, a noticeable current can be obtained at room temperature with a low amount of protein less than 1 μM. An original pulse-strategy to ensure H₂ diffusion to the bioelectrode allows to reach H₂ sensitivity of 4 μA cm^-2 per % H₂ and a linear range between 1 and 20%. Sustainable hydrogen was then produced through dark fermentation performed by a synthetic bacterial consortium in an up-flow anaerobic packed-bed bioreactor. Thanks to the outstanding properties of the A. aeolicus hydrogenase, the biosensor was demonstrated to be quite insensitive to CO₂ and H₂S produced as the main co-products of the bioreactor. Finally, the bioelectrode was used for the in situ measurement of H₂ produced in the bioreactor in steady-state.

Enhanced oxygen-tolerance of the full heterotrimeric membrane-bound [NiFe]-hydrogenase of Ralstonia eutropha

Hydrogenases are oxygen-sensitive enzymes that catalyze the conversion between protons and hydrogen. Water-soluble subcomplexes of membrane-bound [NiFe]-hydrogenases (MBH) have been extensively studied for applications in hydrogen-oxygen fuel cells as they are relatively tolerant to oxygen, although even these catalysts are still inactivated in oxidative conditions. Here, the full heterotrimeric MBH of Ralstonia eutropha, including the membrane-integral cytochrome b subunit, was investigated electrochemically using electrodes modified with planar tethered bilayer lipid membranes (tBLM). Cyclic voltammetry and chronoamperometry experiments show that MBH, in equilibrium with the quinone pool in the tBLM, does not anaerobically inactivate under oxidative redox conditions. In aerobic environments, the MBH is reversibly inactivated by O2, but reactivation was found to be fast even under oxidative redox conditions. This enhanced resistance to inactivation is ascribed to the oligomeric state of MBH in the lipid membrane.

The hyperthermophilic bacterium Aquifex aeolicus: from respiratory pathways to extremely resistant enzymes and biotechnological applications

Aquifex aeolicus isolated from a shallow submarine hydrothermal system belongs to the order Aquificales which constitute an important component of the microbial communities at elevated temperatures. This hyperthermophilic chemolithoautotrophic bacterium, which utilizes molecular hydrogen, molecular oxygen, and inorganic sulfur compounds to flourish, uses the reductive TCA cycle for CO(2) fixation. In this review, the intricate energy metabolism of A. aeolicus is described. As the chemistry of sulfur is complex and multiple sulfur species can be generated, A. aeolicus possesses a multitude of different enzymes related to the energy sulfur metabolism. It contains also membrane-embedded [NiFe] hydrogenases as well as oxidases enzymes involved in hydrogen and oxygen utilization. We have focused on some of these proteins that have been extensively studied and characterized as super-resistant enzymes with outstanding properties. We discuss the potential use of hydrogenases in an attractive H(2)/O(2) biofuel cell in replacement of chemical catalysts. Using complete genomic sequence and biochemical data, we present here a global view of the energy-generating mechanisms of A. aeolicus including sulfur compounds reduction and oxidation pathways as well as hydrogen and oxygen utilization.

Membrane-bound hydrogenase I from the hyperthermophilic bacterium Aquifex aeolicus: enzyme activation, redox intermediates and oxygen tolerance

The membrane-bound hydrogenase (Hase I) of the hyperthermophilic bacterium Aquifex aeolicus belongs to an intriguing class of redox enzymes that show enhanced thermostability and oxygen tolerance. Protein film electrochemistry is employed here to portray the interaction of Hase I with molecular oxygen and obtain an overall picture of the catalytic activity. Fourier transform infrared (FTIR) spectroscopy integrated with in situ electrochemistry is used to identify structural details of the [NiFe] site and the intermediate states involved in its redox chemistry. We found that the active site coordination is similar to that of standard hydrogenases, with a conserved Fe(CN)(2)CO moiety. However, only four catalytic intermediates could be detected; these correspond structurally to the Ni-B, Ni-SI(a), Ni-C, and Ni-R states of standard hydrogenases. The Ni-SI/Ni-C and Ni-C/Ni-R midpoint potentials are approximately 100 mV more positive than those observed in mesophilic hydrogenases, which may be the reason that A. aeolicus Hase I is more suitable as a catalyst for H(2) oxidation than production. Protein film electrochemistry shows that oxygen inhibits the enzyme by reacting at the active site to form a single species (Ni-B); the same inactive state is obtained under oxidizing, anaerobic conditions. The mechanism of anaerobic inactivation and reactivation in A. aeolicus Hase I is similar to that in standard hydrogenases. However, the reactivation of the former is more than 2 orders of magnitude faster despite the fact that reduction of Ni-B is not thermodynamically more favorable. A scheme for the enzymatic mechanism of A. aeolicus Hase I is presented, and the results are discussed in relation to the proposed models of oxygen tolerance.

Hydrogenase-coated carbon nanotubes for efficient H2 oxidation

Multiwalled carbon nanotubes grown on gold electrodes manufactured by microtechnology techniques have been used as a platform for oriented and stable immobilization of a Ni-Fe hydrogenase. Microscopic and electrochemical characterization of the system are presented. High-density currents due to H2 oxidation electrocatalysis, stable for over a month under continuous operational conditions, were measured. The functional properties of this nanostructured hydrogenase electrode are suitable for hydrogen biosensing and biofuel applications.

Oriented Immobilization of Desulfovibrio gigas Hydrogenase onto Carbon Electrodes by Covalent Bonds for Nonmediated Oxidation of H2

The orientation of hydrogenase bound covalently to a pyrolytic graphite edge electrode modified with a 4-aminophenyl monolayer can be modulated via electrostatic interactions during the immobilization step. At low ionic strength and when the amino groups of the electrode surface are mostly protonated, the hydrogenase is immobilized with the negatively charged region that surrounds its 4Fe4S cluster nearer to the protein surface facing the electrode. This allows direct electron transfer between the immobilized hydrogenase and the electrode, which is observed by the strong catalytic currents measured in the presence of the H2 substrate. Therefore, a very stable enzymatic electrode is produced that catalyzes nonmediated H2 oxidation.