Constructing and testing the thermodynamic limits of synthetic NAD(P)H:H2 pathways.

NAD(P)H:H(2) pathways are theoretically predicted to reach equilibrium at very low partial headspace H(2) pressure. An evaluation of the directionality of such near-equilibrium pathways in vivo, using a defined experimental system, is therefore important in order to determine its potential for application. Many anaerobic microorganisms have evolved NAD(P)H:H(2) pathways; however, they are either not genetically tractable, and/or contain multiple H(2) synthesis/consumption pathways linked with other more thermodynamically favourable substrates, such as pyruvate. We therefore constructed a synthetic ferredoxin-dependent NAD(P)H:H(2) pathway model system in Escherichia coli BL21(DE3) and experimentally evaluated the thermodynamic limitations of nucleotide pyridine-dependent H(2) synthesis under closed batch conditions. NADPH-dependent H(2) accumulation was observed with a maximum partial H(2) pressure equivalent to a biochemically effective intracellular NADPH/NADP(+) ratio of 13:1. The molar yield of the NADPH:H(2) pathway was restricted by thermodynamic limitations as it was strongly dependent on the headspace:liquid ratio of the culture vessels. When the substrate specificity was extended to NADH, only the reverse pathway directionality, H(2) consumption, was observed above a partial H(2) pressure of 40 Pa. Substitution of NADH with NADPH or other intermediates, as the main electron acceptor/donor of glucose catabolism and precursor of H(2), is more likely to be applicable for H(2) production.