Virginia Tech team engineers optimized synthetic enzymatic pathway for high-yield production of H2 directly from biomass
A team of Virginia Tech researchers and colleagues has demonstrated the complete conversion of glucose and xylose from pretreated plant biomass to H2 and CO2 based on an in vitro synthetic enzymatic pathway crafted from more than 10 purified enzymes. Glucose and xylose were simultaneously converted to H2 with a yield of two H2 per carbon, the maximum possible yield.
The researchers used a nonlinear kinetic model fitted with experimental data to identify the enzymes that had the greatest impact on reaction rate and yield. After optimizing enzyme loadings using this model, volumetric H2 productivity was increased 3-fold to 32 mmol H2⋅L−1⋅h−1. The productivity was further enhanced to 54 mmol H2⋅L−1⋅h−1 by increasing reaction temperature, substrate, and enzyme concentrations—an increase of 67-fold compared with the initial studies using this method.
The gaseous hydrogen can be separated from aqueous substrates easily, greatly decreasing product separation costs, and avoid reconcentrating sugar solutions. This greatly reduces processing costs from biomass hydrolyzate sub- strates and avoids inhibition from fermentation products and/or potentially toxic compounds from biomass hydrolyzate.
In an open access paper describing the technology published in Proceedings of the National Academy of Sciences (PNAS), the team suggests that distributed hydrogen production based on evenly distributed less-costly biomass could accelerate the implementation of a hydrogen economy.
In this pathway cellulose and hemicellulose were first completely converted to glucose and xylose, which in turn served as substrates for phosphorylation and hydrogen generation, using an enzyme mixture. In preliminary experiments we attempted simultaneous saccharification and hydrogen production but this required compromising the pH optimum of either the hydrogen-producing enzyme mixture (pH optimum of 7.5) or the cellulase–hemicellulase mixture (pH optimum of 4.8). Separating the two processes allowed for more careful control of each system. However, combining these process steps is a logical step forward to minimize production costs.
A more advanced and likely more efficient integrated approach could make use of cello-oligomer phosphorylases. For example, a cellulase phosphorylase could use the energy of glycosidic bond hydrolysis to catalyze phosphorylation of long chain cellulose molecules, although so far such an enzyme has yet to be discovered or engineered.—Rollin et al.
The team already has received significant funding for the next step of the project, which is to scale up production to a demonstration size.
Lonnie O. Ingram, director of the Florida Center for Renewable Chemicals and Fuels at the University of Florida, who is familiar with the work but not associated with the team, commented that the work represents a revolutionary approach that offers many new advantages.
These researchers have certainly broadened the scope of our thinking about metabolism and how it plays into the future of alternative energy production.—Lonnie Ingram
Joe Rollin, a former doctoral student of Zhang’s at Virginia Tech and co-founder with Zhang of the start-up company Cell-free Bioinnovations, is the lead author on the paper.
This work builds upon previous studies Zhang’s team has done with xylose, the most abundant simple plant pentose sugar, to produce hydrogen yields that previously were attainable only in theory.
We believe this exciting technology has the potential to enable the widespread use of hydrogen fuel cell vehicles around the world and displace fossil fuels.—Joe Rollin
The project was funded in part by the Shell GameChanger initiative and the National Science Foundation’s Small Business Technology Transfer program.
Joseph A. Rollin, Julia Martin del Campo, Suwan Myung, Fangfang Sun, Chun You, Allison Bakovic, Roberto Castro, Sanjeev K. Chandrayan, Chang-Hao Wu, Michael W. W. Adams, Ryan S. Senger, and Y.-H. Percival Zhang (2015) “High-yield hydrogen production from biomass by in vitro metabolic engineering: Mixed sugars coutilization and kinetic modeling” PNAS doi: 10.1073/pnas.1417719112