Added sugar takes on a whole new meaning in the National Renewable Energy Laboratory’s (NREL’s) work to uncover the secrets behind one of nature’s most powerful tools—cellulose-degrading enzymes. 

Researchers at NREL are seeking to unlock the bioenergy stored in plant cell walls—with a little sugar on top. Funded by the U.S. Department of Energy’s Bioenergy Technologies Office (BETO) and recently published in the Proceedings of the National Academy of Sciences, NREL’s research focuses on understanding the mechanisms that cellulose-degrading enzymes (i.e., cellulase) use to break down cellulose and identifying the means to improve their performance. This insight could enable more efficient and cost-effective biofuel and bioproducts production. 

And as you have probably guessed by now, it involves sugar.

Cellulose in plant cell walls is the largest reservoir of renewable carbon on Earth. However, accessing that cellulose has long proved challenging, as it is packed densely within a heterogeneous, tight structure. In the natural world, plant biomass is degraded by microbes like fungi and bacteria, which secrete a cocktail of enzymes that work in concert to break down cellulose into sugars. Just as microbes use these sugars as a source of energy, we too can convert them into fuels and products. The application of these enzymes in the biofuel and bioproducts industry, however has met with several obstacles, given the sheer complexity and scale associated with large-scale production. . 

One strategy to overcome these obstacles is to engineer cellulose-degrading enzymes for improved performance—a task that requires detailed knowledge of enzyme structure and function. One of the more interesting yet least studied features of these enzymes are the glycans—or small sugar molecules—that decorate their surface. These sugars are added to enzymes through a process known as “glycosylation,” and it is known to have a substantial impact on enzyme function. However, the specific ways that different glycans affect enzyme functions have been elusive.

NREL set out to understand how these glycans affect cellulase activity—by engineering mutants. With uncertainty about how different kinds of glycans relate to different enzyme functions, NREL created a line of cellulase mutants that lacked various combinations of glycosylation sites. Then, a team of researchers from NREL, the University of Georgia, and the University of Colorado Boulder compared the function of mutant enzymes to those of the native ones. By doing so, the researchers were able to gather critical data about the relationships between each specific glycan, its function, and its location.

NREL’s findings have revealed that, depending on the type of glycan and where it is attached, glycosylation is important to achieve the following advantages: