The biosynthesis of the branched-chain sugar D-apiose in plants:: functional cloning and characterization of a UDP-D-apiose/UDP-D-xylose synthase from Arabidopsis

D-Apiose is a plant-specific branched-chain monosaccharide found in rhamnogalacturonan II (RG-II), apiogalacturonan, and several apioglycosides. Within RG-II, D-apiose serves as the binding site for borate, which leads to the formation of cross-links within the wall. Biochemical studies in duckweed and parsley have established that uridine 5'-diphospho-D-apiose (UDP-D-apiose) is formed from UDP-D-glucuronate by decarboxylation and re-arrangement of the carbon skeleton, leading to ring contraction and branch formation. The enzyme catalyzing this reaction also forms UDP-D-xylose by decarboxylation of UDP-D-glucuronate, and has therefore been named UDP-D-apiose/UDP-D-xylose synthase. Using a bioinformatics approach, we identified a candidate gene (AXS1) for this enzyme in Arabidopsis and functionally expressed its cDNA in Escherichia coli. The recombinant enzyme catalyzed the conversion of UDP-D-glucuronate to a mixture of UDP-D-apiose and UDP-D-xylose with a turnover number of 0.3 min(-1). AXS1 required NAD+ for enzymatic activity, and was strongly inhibited by UDP-D-galacturonate. It was highly expressed in all plant organs consistent with a function in synthesizing an essential cell wall precursor. Database searches indicated the presence of closely related sequences in a variety of crop plants. The cloning of the AXS1 gene will help to investigate the biosynthesis of RG-II, and permit insights into the mechanism by which D-apiose and other branched monosaccharides are formed.