MIT engineers yeast to improve efficiency of biofuels
Scientists at the Massachusetts Institute of Technology (MIT) have engineered strains of the oleaginous yeast Yarrowia lipolytica to yield 25% more lipids for use in biofuels than in other engineered strains. The experiments also managed to convert surplus NADH to NADPH, which can be used to synthesise lipids, an important feedstock for biofuels. Although the process would be "economically feasible at current prices for cornstarch", the researchers said they were looking at ways to make the process even more efficient.
A paper on their work is published in the journal Nature Biotechnology ; the MIT team, led by Gregory Stephanopoulos, the Willard Henry Dow Professor of Chemical Engineering and Biotechnology at MIT, is now working on additional improvements to the lipids yield.

MIT engineers have genetically engineered strains of the oleaginous yeast Yarrowia lipolytica to boost the production of lipids by about 25% compared to previously engineered yeast strains. Their approach could enable commercialization of microbial carbohydrate-based lipid production, supporting the renewable production of high-energy fuels such as diesel.

Lipids, specifically fatty-acid-derived lipids, are important feedstocks for the fuel and oleo-chemical industries. Owing to the depletion of fossil fuels, plant-oil- and animal-fat-derived lipids are being developed as renewable feedstocks for biodiesel production. However, availability of these edible oils is insufficient for production of biofuels at scale, and their use for fuel production conflicts with food supply. Carbohydrates are the most abundant renewable feedstocks, and advances in lignocellulosic biomass hydrolysis technologies mean that microbial conversion of non-food carbohydrates to lipids is a promising option for sustainable lipid production.

Oleaginous yeasts are characterized by complex internal membranes that enable high storage capacity of neutral lipids (mainly triacylglycerides). These yeasts grow fast and produce lipids at high rates. The model oleaginous yeast Yarrowia lipolytica has been engineered to improve lipid production. Engineering strategies include increasing the availability of biosynthetic precursors and lipogenic pathway flux, shutting down degradation pathways including lipolysis and β-oxidations, and removing inhibitory intermediates. Despite much progress, commercialization of microbial oils is limited to the production of high-value commodity chemicals. For low-priced biofuels, commercialization depends on lowering the feedstock cost, which is the single most expensive component of total production cost. This requires improving the overall yield of carbohydrate to lipid conversion and productivity. —Qiao et al.

The researchers engineered 13 strains of Yarrowia lipolytica with synthetic pathways converting glycolytic NADH into the lipid biosynthetic precursors NADPH or acetyl-CoA. The best engineered strain achieved a productivity of 1.2 g/L/h and a process yield of 0.27 g–fatty acid methyl esters/g-glucose—a 25% improvement over previously engineered yeast strains.

Further, oxygen requirements of the highest producer were reduced owing to decreased NADH oxidization by aerobic respiration.

What we’ve done is reach about 75 percent of this yeast’s potential, and there is an additional 25 percent that will be subject of follow-up work. —Gregory Stephanopoulos

Stephanopoulos and his colleagues focused on fully utilizing the electrons generated from the breakdown of glucose. To achieve this, they transformed Yarrowia with synthetic pathways that convert surplus NADH, a product of glucose breakdown, to NADPH, which can be used to synthesize lipids. They ended up testing more than a dozen modified synthetic pathways.

It turned out that the combination of two of these pathways gave us the best results that we report in the paper. The actual mechanism of why a couple of these pathways work much better than the others is not well-understood. —Gregory Stephanopoulos

Using this improved pathway, the yeast cells require only two-thirds of the amount of glucose needed by unmodified yeast cells to produce the same amount of oil.

While this new glucose-to-lipid conversion process could be economically feasible at current prices for cornstarch, the researchers are hoping to make the process even more efficient, Stephanopoulos said. The researchers are also exploring using cheaper sources of plant material, such as grass and agricultural waste, which would require converting the cellulose that makes up those plant materials into glucose.

The research was funded by the US Department of Energy.

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