UDP-sugars, activated forms of monosaccharides, are synthesized throughde novoand salvage pathways and serve as substrates for the synthesis of polysaccharides, glycolipids, and glycoproteins in higher plants. A UDP-sugar pyrophosphorylase, designated PsUSP, was purified about 1,200-fold from pea (Pisum sativumL.) sprouts by conventional chromatography. The apparent molecular mass of the purified PsUSP was 67,000 Da. The enzyme catalyzed the formation of UDP-Glc, UDP-Gal, UDP-glucuronic acid, UDP-l-arabinose, and UDP-xylose from respective monosaccharide 1-phosphates in the presence of UTP as a co-substrate, indicating that the enzyme has broad substrate specificity toward monosaccharide 1-phosphates. Maximum activity of the enzyme occurred at pH 6.5–7.5, and at 45 °C in the presence of 2 mmMg2+. The apparentKmvalues for Glc 1-phosphate andl-arabinose 1-phosphate were 0.34 and 0.96 mm, respectively.PsUSPcDNA was cloned by reverse transcriptase-PCR.PsUSPappears to encode a protein with a molecular mass of 66,040 Da (600 amino acids) and possesses a uridine-binding site, which has also been found in a human UDP-N-acetylhexosamine pyrophosphorylase. Phylogenetic analysis revealed that PsUSP can be categorized in a group together with homologues fromArabidopsisand rice, which is distinct from the UDP-Glc and UDP-N-acetylhexosamine pyrophosphorylase groups. Recombinant PsUSP expressed inEscherichia colicatalyzed the formation of UDP-sugars from monosaccharide 1-phosphates and UTP with efficiency similar to that of the native enzyme. These results indicate that the enzyme is a novel type of UDP-sugar pyrophosphorylase, which catalyzes the formation of various UDP-sugars at the end of salvage pathways in higher plants.
Substance transport and accumulation in higher plants. -2 3. Membrane transport of ion, sugar and amino acids.