How Escherichia coli Is Equipped to Oxidize Hydrogen under Different Redox Conditions

Abstract

The enterobacterium Escherichia coli synthesizes two H 2 uptake enzymes, Hyd-1 and Hyd-2. We show using precise electrochemical kinetic measurements that the properties of Hyd-1 and Hyd-2 contrast strikingly, and may be individually optimized to function under distinct environmental conditions. Hyd-2 is well suited for fast and efficient catalysis in more reducing environments, to the extent that in vitro it behaves as a bidirectional hydrogenase. In contrast, Hyd-1 is active for H 2 oxidation under more oxidizing conditions and cannot function in reverse. Importantly, Hyd-1 is O 2 tolerant and can oxidize H 2 in the presence of air, whereas Hyd-2 is ineffective for H 2 oxidation under aerobic conditions. The results have direct relevance for physiological roles of Hyd-1 and Hyd-2, which are expressed in different phases of growth. The properties that we report suggest distinct technological applications of these contrasting enzymes.Hydrogenases catalyze the reversible cleavage of H 2 into protons and electrons, and play an important role in the energy metabolism of a broad range of microorganisms (1). Hydrogenases are classified according to their active site metal ion content, and three phylogenetically distinct classes have so far been identified: di-iron [FeFe]-, nickel-iron [NiFe]-, and mono-iron [Fe]-hydrogenases (1). Nickel-iron hydrogenases are the most abundant of the three types (1), and many members of this class are membrane bound, with the membrane-extrinsic domain consisting of a large subunit containing the active site, and a small subunit accommodating one to three electron-transferring iron-sulfur clusters. The active sites of [NiFe]-hydrogenases contain a nickel atom coordinated by four cysteine-S ligands, two of which bridge to an iron atom that is further coordinated by three unusual diatomic ligands, two cyanides and one carbonyl (2).Hydrogenases are inactivated by O 2

Physiology and Bioenergetics of [NiFe]-Hydrogenase 2-Catalyzed H2-Consuming and H2-Producing Reactions in Escherichia coli

“…Recent studies have also revealed that during glycerol fermentation in the presence of Casamino Acids, Hyd-2 can evolve hydrogen (22), suggesting that under certain conditions the enzyme can function bidirectionally. This is in agreement with the electrochemical analysis of purified Hyd-2 (23). Therefore, in this study we wished to determine the requirements of Hyd-2 to function in H 2 evolution during fermentative growth with glycerol and in H 2 oxidation during respiratory growth on glycerol and fumarate.…”

“…The E. coli Hyd enzymes are active across a wide range of redox potentials and in the presence of different electron acceptors (23,32). As well as fumarate, electron transport chains coupling H 2 oxidation to dimethyl sulfoxide (DMSO) and TMAO have also been described (14).…”

H2 Conversion in the Presence of O2 as Performed by the Membrane‐Bound [NiFe]‐Hydrogenase of Ralstonia eutropha

“…[51] The ability to be catalytically active in a wide potential window at aerobic conditions and the capability for fast reactivation at high potentials are vital features of hydrogenases that are active in the presence of O 2 and has so far been observed for MBH proteins from R. eutropha and R. metallidurans, as well as for the hydrogenase I proteins from Aquifex aeolicus and Escherichia coli. [17,51,53,54] Chronoamperometric experiments conducted with both the anaerobic "standard" hydrogenase of A. vinosum and the aerobic MBH led to a conclusive definition of O 2 -tolerant H 2 catalysis ( Figure 3). [17,55] In the case of MBH, the H 2 -oxidation current decreases stepwise upon addition of increasing amounts of O 2 .…”

Biohydrogen for a New Generation of H2/O2 Biofuel Cells: A Sustainable Energy Perspective

“…During the last ten years, several O 2 -tolerant Hases have been purified and characterized especially by spectroscopy (electron paramagnetic resonance, electron nuclear double resonance, Mçssbauer, FTIR), electrochemistry essentially at graphite electrodes, and more recently by molecular dynamics. [100] This includes the membrane-bound Hases from Ralstonia eutropha (Re MbH), [76] from Aa, [95,101] from Hydrogenovibrio marinus [102] and from the bacteria Escherichia coli (E. coli) [103] and Salmonella. [104] The O 2 tolerance means that the enzyme can retain at least 40 % of its activity at ambient levels of O 2 (200 mbar).…”