For many parts of the country, spring can bring heavy snowstorms that cause trees to sag and snap. While most bounce back with the warmer weather, the exact process by which plants build the pectin components of their cell walls has long puzzled scientists. Pectin, which interconnects with other important components like lignin, cellulose, and hemicellulose, is crucial for providing strength and flexibility to plants.
Now, a team of researchers from the National Renewable Energy Laboratory, the University of Georgia, and Lawrence Berkeley National Laboratory has made a significant discovery about the biological mechanism responsible for producing one specific component of pectin. In an article published in Nature Plants, the team describes the enzyme Galactan Synthase 1 (GalS1), which is responsible for turning the sugar galactose into galactan, a polymeric form that is essential to pectin.
While pectin is often overlooked by those outside of the plant science community, it is an essential component of any plant. The team's work sheds light on the processes involved in plant biopolymer synthesis and provides insight into how plant enzymes work together to build complex polymers. This knowledge could one day be used to more easily extract useful cell wall components from biomass or manufacture sustainable bioproducts.
GalS1 decorates pectin with sidechains of arabinose and galactose sugars
Plant enzymes work together to transform sugars into polymers, creating the flexible limbs, deep roots, and sturdy trunks that plants need. One such enzyme, GalS1, plays a critical role in linking galactose sugars to a pectin chain, much like tree branches are fastened to a central trunk.
However, understanding the complex workings of plant enzymes has been a challenge for researchers. To address this, NREL computational scientist Vivek Bharadwaj set out to investigate how GalS1 operates at the atomic level, including how substrates bind to the active site and how sugars are attached to increase galactan chain length.
These processes have traditionally been difficult to study experimentally, making NREL's computational approach a valuable tool for gaining insight into the mechanisms behind plant enzyme activity.
Bharadwaj and Bomble utilized computational tools to gain an unprecedented level of insight into the structure and biochemistry of GalS1. Through their efforts, they were able to uncover details about the enzyme's substrate binding process and the biological machinery that GalS1 employs to carry out its specialized biochemical functions.
Their findings revealed that GalS1 is distinct from other enzymes due to the presence of a unique module that allows it to bind to a pectin backbone created by other enzymes. Once attached, GalS1 positions its catalytic domain to start linking sugar molecules together, ultimately creating branches composed of galactan chains that terminate with arabinose. These branches contribute to the distinct structure and function of pectin, with large amounts of galactose observed in tension wood, a type of wood that is particularly resilient against environmental factors.
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Understanding the function of enzymes like GalS1 can have a significant impact on controlling the ratio of different sugars in plant cell walls, specifically in pectin and hemicelluloses. This information could enable researchers to modify or overexpress enzymes like GalS1, influencing the chemical composition of pectin and other key polymers in cell walls. By adjusting the ratio of sugars, plant materials can be tailored to exhibit specific properties required for various applications.
For example, GalS1 may be one of the enzymes that give extra flexibility to certain areas of plants, such as tree limbs susceptible to spring snowstorms. Moreover, modifying GalS1 to increase galactose or glucose concentrations could be useful in the production of biofuels, where higher concentrations of C6 sugars are more easily convertible by microbes than C5 sugars.
However, the process of modifying enzymes requires further research and peer review to better understand the impact of changing the sugar ratios on plant cell wall strength. Despite this, the potential for developing more efficient and sustainable routes for producing biofuels and bioproducts through enzyme modification is significant.
Source: National Renewable Energy Laboratory