University of Queensland researchers, working in collaboration with the Technical University of Munich (TUM), have found a way to convert sugarcane into isobutanol— a building block of aviation fuel and other products—more efficiently. An open-access paper on the work is published in Chemistry - a European Journal.

Enzymatic cascade to produce isobutanol from D-glucose. Representatives from the ilvD/EDD superfamily (DHAD and DHT) catalyse multiple reactions in the cascade (highlighted in red). Bayaraa et al.

Enzyme-catalyzed reaction cascades play an increasingly important role for the sustainable manufacture of diverse chemicals from renewable feedstocks. For instance, dehydratases from the ilvD/EDD superfamily have been embedded into a cascade to convert glucose via pyruvate to isobutanol, a platform chemical for the production of aviation fuels and other valuable materials. These dehydratases depend on the presence of both a Fe−S cluster and a divalent metal ion for their function. However, they also represent the rate-limiting step in the cascade.

Here, catalytic parameters and the crystal structure of the dehydratase from Paralcaligenes ureilyticus (PuDHT, both in presence of Mg²⁺ and Mn²⁺) were investigated. Rate measurements demonstrate that the presence of stoichiometric concentrations Mn²⁺ promotes higher activity than Mg²⁺, but at high concentrations the former inhibits the activity of PuDHT. Molecular dynamics simulations identify the position of a second binding site for the divalent metal ion. Only binding of Mn²⁺ (not Mg²⁺) to this site affects the ligand environment of the catalytically essential divalent metal binding site, thus providing insight into an inhibitory mechanism of Mn²⁺ at higher concentrations.

Furthermore, in silico docking identified residues that play a role in determining substrate binding and selectivity. The combined data inform engineering approaches to design an optimal dehydratase for the cascade.

—Bayaraa et al.

By zeroing in on a specific enzyme, the team sped up the slowest step in processing sugar into isobutanol.

Our research into this particular enzyme means we can accelerate the production rate and yield of isobutanol from sugarcane, ultimately enabling biomanufacturers to make diverse products at scale sustainably and efficiently.

Usually during a biomanufacturing process, cells such as yeasts are used as a production platform, but in our research only a small number of a sugar acid-specific dehydratase enzyme was used. Having sugar-converting enzymes operate outside a cellular environment meant we could bypass many of the pitfalls of the more traditional cell-based biomanufacturing methods.

This has led to much higher yields of isobutanol with fewer unwanted side products.

—Professor Gary Schenk from UQ’s School of Chemistry and Molecular Biosciences

Cell-based production of isobutanol from sugar creates about 25 grams per liter of liquid cell culture but in the study, the cell-free method produced at least 10 times that amount.

Resources

• Bayaraa, T., Lonhienne, T., Sutiono, S., Melse, O., Brück, T. B., Marcellin, E., Bernhardt, P. V., Boden, M., Harmer, J. R., Sieber, V., Guddat, L. W., Schenk, G. (2022), “Structural and Functional Insight into the Mechanism of the Fe−S Cluster-Dependent Dehydratase from Paralcaligenes ureilyticus” Chem. Eur. J. doi: 10.1002/chem.202203140