Can a pectin-producing enzyme aid trees in surviving storms and enable the development of eco-friendly bioproducts?

Spring means blizzards across much of the country, which can bring inches of thick, wet snow. Tree branches dangle, a few start – but most quickly bounce back as the spring sun warms.

Plant cells contain high amounts of lignin, cellulose, and hemicellulose, which are the building blocks of plant stems and stems. However, not much is known about a fourth important polymer: pectin, which is thought to provide strength and elasticity by bonding with the other three ingredients.

What has particularly puzzled scientists is exactly how plants build the pectin components of their cell wall.

Now, researchers from the National Renewable Energy Laboratory (NREL), the University of Georgia, and Lawrence Berkeley National Laboratory have discovered the biological mechanism involved in making one specific component of pectin. Posted in Nature plantsThe article details the structure and biochemical activity of Galactan Synthase 1 (GalS1), an enzyme involved in converting the sugar galactose into its polymeric form, called galactan—an important component of pectin.

“If you ask most people in the biofuel industry what the main components of plant cell walls are, they will tell you cellulose, hemicellulose, and lignin,” said Yannick Pompel, an NREL biophysicist. “Pectin is often overlooked unless you talk to a botanist, but it is a very essential component of any plant.”

The study complements the team’s recent work detailing different processes in plant biopolymer synthesis. Together, they provide a picture of how plant enzymes work together to build complex polymers–polymers that scientists may one day modify to more easily extract beneficial cell wall components from biomass or manufacture sustainable bioproducts.

GalS1 decorates pectin with side chains of the sugars arabinose and galactose

To create flexible limbs, deep roots, and strong trunks, plant enzymes work in tandem to reconstitute raw resources like sugars into polymers they can later use as building materials for their cell walls. Among the many enzymes involved in these processes, GalS1 performs a specific role: It binds galactose sugar molecules together into long chain pectin at specific locations, almost like attaching tree branches to a central trunk.

The result is a structurally complex molecular component of pectin. According to Vivek Bharadwaj, an NREL computing scientist, researchers have struggled to understand which plant enzymes, such as GalS1, are involved in making polymers like pectin.

“We wanted to understand, at the atomic level, how the substrates are connected at the active site and the mechanisms by which the sugar is bound to increase the length of the galactan branch chain,” he said. “This was really challenging experimentally, and that’s where NREL came in.”

Using computational tools, Bharadwaj and Bomble provided an unprecedentedly detailed view of both the enzyme’s structure and its biochemistry. This effort has revealed details about how and where the substrates bind, as well as the biological mechanism used by GalS1 to complete such specific biochemical tasks.

They found that GalS1 is unique, and contains a special module that allows it to bind to the pectin backbone laid down by other enzymes. Once attached, GalS1 positions its catalytic domain to initiate the stacking of polysaccharides, one on top of the other. The resulting branches—formed of galactan chains ending in rabinose—provide unique structure and function to the resulting pectin. Significant amounts of galactose have been observed in tension wood, for example—a form of wood that is particularly adept at weathering the elements.

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Why is it so difficult to understand a single enzyme with a very specific role? According to Bomble, the long-term goal is to be able to modulate the concentrations of sugars in plant cell walls in general and in pectin and hemicellulose more precisely.

“We want to be able to control the ratio of all the different ingredients to give the lignocellulosic materials the properties that we want,” Bumble explained.

Overexpression or modification of enzymes such as GalS1 can affect the properties of cell walls, altering the chemical structure of pectin and other key polymers. For example, GalS1 may be one of the enzymes involved in conferring more flexibility in key regions of the plant, such as the limbs of trees exposed to a spring blizzard.

In the future, Bumble said, scientists may be able to modify GalS1 to add more galactose, or even glucose, which is favored in biofuel manufacturing processes.

“Microbes usually have an easier time converting C6 sugars, such as galactose, than C5 sugars, such as arabinose,” he said. “It would be better to have higher concentrations of C6 polysaccharides in the plant cell wall to promote them into useful products.”

Of course, Bomble and Bharadwaj caution that effectively modifying enzymes will require many focused, peer-reviewed scientific studies. As Bharadwaj puts it, “What happens to the strength of these plant cell walls when you shift the ratio of C5 to C6? We don’t know much about it.”

For now, the next time you see branches swaying in the wind or flexing under the snow, think about GalS1 and the many other enzymes needed to accomplish this feat. These same enzymes could hold the keys to more efficient and cost-effective methods for biofuels and sustainable bioproducts.