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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
Author(s) -
Mølhøj Michael,
Verma Rajeev,
Reiter WolfDieter
Publication year - 2003
Publication title -
the plant journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 3.058
H-Index - 269
eISSN - 1365-313X
pISSN - 0960-7412
DOI - 10.1046/j.1365-313x.2003.01841.x
Subject(s) - glucuronate , biochemistry , chemistry , xylose , atp synthase , stereochemistry , enzyme , biosynthesis , uridine diphosphate , fermentation
Summary 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.