Rewiring Algae's Catalytic Circuits
Opening a promising avenue to increased biohydrogen production.
In nature, photosynthesis uses the energy in sunlight to split water into carbon dioxide and hydrogen. A typical plant cell relies on a series of electron carriers, which create a photosynthetic circuit that allows plants to capture the carbon dioxide they need, and then convert it into the biomass that fuels cell growth. At the same time, plants produce hydrogen, a molecule that can be used in a variety of renewable and sustainable fuel technologies, but that is also expensive to produce in large quantities and currently involves non-renewable natural gas reformation.
A photosynthetic organism such as green algae tends to use solar energy to generate either fixed carbon or hydrogen—while this is fine for growth, it is not particularly efficient for making greater quantities of hydrogen. Facing this challenge, NREL researchers wondered if they could find ways to boost the hydrogen-making capacity of photosynthesis. They posed a key question: What controls the partitioning of electrons between these two competing metabolic pathways?
Photosynthetic electron transport pathways that support carbon dioxide fixation and hydrogen production. Light-activated PSII extracts electrons from water and transfers them, while parallel circuits couple Fd to either FNR for carbon dioxide fixation or hydrogenase production.
Image provided by Paul King, NREL
Engineering of the hydrogen-producing enzyme to create an Fd-H2ase fusion changes the composition of the hydrogen production circuit to include both direct (box 1) and indirect (box 2) H2 production modes. The CO2 fixation circuit (box 3) remains open, but operates at a reduced level.
Credit: Image provided by Paul King, NREL
A team from NREL, along with colleagues from the Massachusetts Institute of Technology and Tel Aviv University, set out to answer this question. They hypothesized that they could engineer the process by "rewiring" algae's catalytic circuits, or pathways. To do so, they would replace the normal hydrogen-producing enzyme, hydrogenase (H2ase), with a ferredoxin and hydrogenase fusion protein. They speculated that inserting this kind of a fusion protein into this reaction path could divert more electrons into hydrogen production and push the algae into making more hydrogen and fixing less carbon dioxide. If successful, this engineered photosynthetic circuit could potentially increase efficiencies and thus bring down the price of hydrogen. In its more than 30-year history of innovation, NREL has been a leader in working with green algae for hydrogen and biofuel production, as well as with finding ways to speed renewable fuels to market to help meet the nation's clean energy goals. It is this expertise that encouraged MIT's Iftach Yacoby to partner with NREL, which enabled the researchers to collaborate on technical innovations such as the CdTe-H2ase.
During NREL's work with green algae, the lab's own Senior Scientist Paul King and other researchers worked with hydrogenase enzymes as a key component of the photosynthetic hydrogen production equation. These biological catalysts can convert electrons and protons into hydrogen gas, or convert hydrogen into electrons and protons. For this work, the team chose to use in vitro tests under anaerobic conditions. They were able to demonstrate how the hydrogenase and other enzymes compete to regulate whether algae uses the solar energy it captures through photosynthesis to produce carbon compounds or hydrogen. As they studied these interactions, they were able to devise a procedure to engineer the proteins that compose electron transfer circuits.
The first element of their strategy was based on their hypothesis that they could have more of the electrons go to hydrogen if they altered the composition to replace hydrogenase with a ferredoxin-hydrogenase fusion. In the anaerobic test tubes, the team confirmed that the photosynthetic circuit can switch from capturing carbon dioxide to producing hydrogen by substituting the fusion. The hydrogen production was carried out in the presence of the CO2 fixation enzyme ferredoxin:NADP-oxidoreductase (FNR). This process is a biological model for using solar power to convert water into hydrogen. The basis for this switch was modeled as two new Fd-hydrogenase circuits (boxes 1 and 2, Figure 2), and a reduced level of FNR activity modeled as a third circuit (box 3, Figure 2).
King considered these results promising, because they suggest that fusion is an engineering strategy to improve hydrogen production efficiencies, and might be useful in resolving the biochemical mechanisms that control photosynthetic electron transport circuits and product levels from competing pathways. The next phase, already underway, is to introduce the fusion protein into green algae Chlamydomonas and determine if rewiring can take place to improve hydrogen-production efficiencies. Even though this is only one of a number of variables to consider, this strategy has already signaled an avenue to pursue in the drive to reduce the cost of hydrogen fuel and make it cost-competitive for industry.