NREL team uncovers new metabolic pathway for cellulosic sugars to biofuel, chemical precursors

January 3, 2016 |

In Colorado, NREL researchers have discovered that a metabolic pathway previously only suggested to be functional in photosynthetic organisms is actually a major pathway and can enable efficient conversion of carbon dioxide to organic compounds.

NREL scientists engineered a cyanobacterium, Synechocystis, that is unable to store carbon as glycogen into a strain that could metabolize xylose (a main sugar component of cellulosic biomass), thus turning xylose and carbon dioxide into pyruvate and 2-oxoglutarate, organic chemicals that can be used to produce a variety of bio-based chemicals and biofuels. While testing this mutant strain under multiple growth conditions, the scientists discovered, unexpectedly, that it excreted large amounts of acetic acid.

Starting from a previously studied phosphoketolase, the researchers were able to identify the gene slr0453 as the likely source of the phosphoketolase in Synechocystis. The researchers were zeroing in on their quarry.

The next step in the detective work was to disable the gene and see what happened. Disabling it in both the wild and mutant strains of Synechocystis slowed the growth in sunlight-that is, conditions dependent only on CO2 assimilation by photosynthesis-demonstrating that the gene played a role in photosynthetic carbon metabolism.

The strains with the disabled gene did not excrete acetic acid in the light in the presence of xylose. Synechocystis was able to produce acetic acid in the dark when fed with sugars, but the strains with the disabled gene could not. The researchers found that the phosphoketolase pathway was solely responsible for the production of acetic acid in the dark and also contributed significantly to carbon metabolism in the light when xylose was supplied.

The discovery was led by NREL senior scientist Jianping Yu and Wei Xiong, an NREL Director’s Postdoc Fellow. The findings were published in the online edition of Nature Plants.

“There are two aspects that are important in this discovery,” Yu said. “One is that it is an important native metabolic pathway in the cyanobacterium whose role was not studied previously. Second is that this pathway is more efficient than the traditional pathways, so it can be exploited to increase photosynthetic productivity.”

Category: Research

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