Cyanobacterial hydrogenases and hydrogen metabolism revisited: recent progress and future prospects

Abstract

Cyanobacteria have garnered interest as potential cell factories for hydrogen production. In conjunction with photosynthesis, these organisms can utilize inexpensive inorganic substrates and solar energy for simultaneous biosynthesis and hydrogen evolution. However, the hydrogen yield associated with these organisms remains far too low to compete with the existing chemical processes. Our limited understanding of the cellular hydrogen production pathway is a primary setback in the potential scale-up of this process. In this regard, the present review discusses the recent insight around ferredoxin/flavodoxin as the likely electron donor to the bidirectional Hox hydrogenase instead of the generally accepted NAD(P)H. This may have far reaching implications in powering solar driven hydrogen production. However, it is evident that a successful hydrogen-producing candidate would likely integrate enzymatic traits from different species. Engineering the [NiFe] hydrogenases for optimal catalytic efficiency or expression of a high turnover [FeFe] hydrogenase in these photo-autotrophs may facilitate the development of strains to reach target levels of biohydrogen production in cyanobacteria. The fundamental advancements achieved in these fields are also summarized in this review.

Keywords: [FeFe] hydrogenase; [NiFe] hydrogenase; bidirectional Hox hydrogenase; biohydrogen; cyanobacteria; ferredoxin.

Figure 1

Biochemistry of hydrogen production in Synechocystis PCC 6803 by direct biophotolysis. In this pathway the reducing equivalents are obtained directly from the splitting of water at PSII. The electrons are transferred into the photosynthetic electron transport chain through a series of transport molecules including plastoquinone (PQ), cytochromeb₆F (Cyt b₆f) and plastocyanin (PC), and move through photosystem I (PSI) to reduce ferredoxin (Fdx) which goes on to reduce NADP⁺ to NADPH via the enzyme ferredoxin NADP⁺ reductase (FNR). At the same, time the reduced ferredoxin also has the capacity to directly donate the electrons to the Hox hydrogenase indicated by the highlighted arrow. Under the present conditions hydrogenase can compete with the Calvin cycle for hydrogen production till it is inactivated due to the evolution of oxygen at PSII. The dotted arrow speculates NADPH as another possible electron donor to the hydrogenase. The dotted double-headed arrow under TH suggests a hypothetical conversion of NADPH into NADH and vice versa. Abbreviations: OPP: Oxidative Pentose Phosphate pathway; TH: transhydrogenase; ATP: adenosine triphosphate; ADP: adenosine diphosphate.

Figure 2

Biochemistry of hydrogen production in Synechocystis PCC 6803 by indirect biophotolysis. In darkness, under anaerobiosis, when the PSII is inactivated, glycogen is catabolized to pyruvate by glycolysis. Pyruvate is further oxidized to acetyl CoA during which ferredoxin is reduced by pyruvate formate oxidoreductase (PFOR). Reduced ferredoxin has the capacity to directly donate electrons to the Hox hydrogenase.