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

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.