Abstract
Nucleotide sugars are the key precursors for all glycosylation reactions and are required both for oligo- and polysaccharides synthesis and protein and lipid glycosylation. Among all nucleotide sugars, UDP-sugars are the most important precursors for biomass production in nature (e.g., synthesis of cellulose, hemicellulose, and pectins for cell wall production). Several recent studies have already suggested a potential role for UDP-Glc in plant growth and development, and UDP-Glc has also been suggested as a signaling molecule, in addition to its precursor function. In this review, we will cover primary mechanisms of formation of UDP-sugars, by focusing on UDP-sugar metabolizing pyrophosphorylases. The pyrophosphorylases can be divided into three families: UDP-Glc pyrophosphorylase (UGPase), UDP-sugar pyrophosphorylase (USPase), and UDP-N-acetyl glucosamine pyrophosphorylase (UAGPase), which can be distinguished both by their amino acid sequences and by differences in substrate specificity. Substrate specificities of these enzymes are discussed, along with structure-function relationships, based on their crystal structures and homology modeling. Earlier studies with transgenic plants have revealed that each of the pyrophosphorylases is essential for plant survival, and their loss or a decrease in activity results in reproductive impairment. This constitutes a problem when studying exact in vivo roles of the enzymes using classical reverse genetics approaches. Thus, strategies involving the use of specific inhibitors (reverse chemical genetics) are also discussed. Further characterization of the properties/roles of pyrophosphorylases should address fundamental questions dealing with mechanisms and control of carbohydrate synthesis and may allow to identify targets for manipulation of biomass production in plants.
Highlights
Many processes during a plant’s life are dependent on targetedglycosylation of different compounds
One group stands out—the UDP-sugars. They are vital to a plant because of the importance of the processes they are involved in (Figure 1) (Bar-Peled and O’Neill, 2011; Kleczkowski and Decker, 2015) and because of their abundance (UDP-sugars may comprise up to 55% of the total nucleotide pools) (Wagner and Backer, 1992) and the profuse number of reactions where they serve as substrates
UDP-Glc pyrophosphorylase (UGPase)-B is expressed in Arabidopsis leaf, stems, and flowers and is co-expressed with sulfoquinovosyl diacylglycerol synthesis genes that are involved in sulfolipid biosynthesis, where UDP-Glc is a key precursor (Okazaki et al, 2009; Shimojima, 2011)
Summary
Many processes during a plant’s life are dependent on targetedglycosylation of different compounds (carbohydrates, polysaccharides, lipids, proteins, hormones, etc.). They are vital to a plant because of the importance of the processes they are involved in (e.g., sucrose and cell wall formation, etc.) (Figure 1) (Bar-Peled and O’Neill, 2011; Kleczkowski and Decker, 2015) and because of their abundance (UDP-sugars may comprise up to 55% of the total nucleotide pools) (Wagner and Backer, 1992) and the profuse number of reactions where they serve as substrates. We will focus more on current developments and on our own research on those enzymes during the last decade or so
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