A new model for the biochemistry of pectin synthesis: GAUTs synthesize diverse HG glycans in structurally and functionally distinct plant cell wall polymers

Abstract

Each year more than 100 billion tons of carbon dioxide are fixed via photosynthesis into plant biomass, the base of our food chain and a renewable resource for production of chemicals, fuels, materials and bio‐based products. The bulk of plant biomass is plant cell walls, which have evolved during hundreds of millions of years of biotic and abiotic challenge into very complex polymers that support plant longevity up to thousands of years and provide mechanical strength needed for trees to reach heights of 90 meters. Such striking mechanical and physical properties arise from the fine structure of individual wall polymers, the covalent and non‐covalent interactions between them, and the architectural arrangement between the polymers. Among the three major types of cell wall polysaccharides, cellulose, hemicellulose and pectin, pectin is the most complex. Pectin is comprised of the three pectic polysaccharides, homogalacturonan (HG), rhamnogalacturonan I (RG‐I) and rhamnogalacturonan II (RG‐II). Our research focuses on the synthesis of the most abundant pectic polysaccharide, HG, which is a partially methyl‐esterified and acetylated homoglycan of α‐1,4‐linked D‐galacturonic acid. HG is synthesized by the GAUT gene family of 15 proven and putative HG:α‐1,4‐galacturonosyltransferases (HG:GalATs) in Arabidopsis. This family has expanded in grasses and in trees. Our long term goal is to understand which HG glycans are synthesized by the diverse GAUTs, and how the polymers containing these HG glycans contribute to cell wall structure, integrity, plant growth, cell adhesion and plant development. The current biochemical, transgenic and mutant studies showing that at least 6 of the 15 Arabidopsis GAUTs encode HG:GalATs will be summarized and data in support of the hypothesis that different GAUTs synthesize HG glycan regions in unique polymers with different functions in the wall will be presented. Enzymatic properties of a heterologously expressed GAUT1:GAUT7 HG:GalAT complex will be described and a two‐phase model for the non‐processive biosynthesis of homogalacturonan polysaccharides by the GAUT1:GAUT7 complex will be presented. We show that the GAUT1:GAUT7 complex is a distributive glycosyltransferase that catalyzes polymerization of high‐molecular‐weight polysaccharides and has full activity only with acceptors longer than a critical chain length.Support or Funding InformationResearch was supported by USDA AFRI 2010‐65115‐20396 and the BioEnergy Science Center and the Center for Bioenergy Innovation which are US Department of Energy Bioenergy Research Centers supported by the Office of Biological and Environmental Research in the Department of Energy's Office of Science. Also partially funded by Department of Energy Center Grant DESC0015662.This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.