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.
Highlights
Hydrogenases catalyze the reversible cleavage of H2 into protons and electrons, and play an important role in the energy metabolism of a broad range of microorganisms [1]
Whereas the majority of well characterized [NiFe]hydrogenases are inactivated by very low levels of molecular O2 [4], in certain species there is a requirement for H2 oxidation to occur in microaerobic and even fully aerobic environments
In this article we describe the characterization of E. coli hydrogenases 1 and 2 and report striking differences in the catalytic properties of the two enzymes, most notably their contrasting activities under the different redox conditions and oxidizing environments likely to be encountered by the organism
Summary
Hydrogenases catalyze the reversible cleavage of H2 into protons and electrons, and play an important role in the energy metabolism of a broad range of microorganisms [1]. Using the Hyd-2 voltammogram as a reference for the reversible cell potential, a close examination of Fig. 1 shows that the overpotential required for H2 oxidation by Hyd-1 decreases as the partial pressure of H2 is lowered.
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