UDP-Apiose Synthase (UAS, AXS) converts UDP-GlcA in the presence of NAD+ to UDP — Api. The enzyme decarboxylates UDP-GlcA to form a UDP-4ketopentose intermediate and the release ofCO2, and then catalyzes rearrangement ofthe sugar skeleton to form UDP-Api

(476,477). In vitro, the enzyme forms both UDP-Xyl and UDP-Api (403). We believe that the formation of UDP-Api was not confirmed satisfactorily since the product is readily degraded to cyclic apiose1,2-phosphate. Functional genes encoding AXS were isolated from tobacco (358), Arabidopsis and potato (Guyet and Bar-Peled, unpublished) (478). Two isoforms exist in Arabidopsis (At1g08200 and At2g27860) and are predicted to be in the cytosol. The “dual function” of the enzyme in generating both UDP-Xyl and UDP-Api was assayed by NMR spectroscopy (Guyet and Bar-Peled, unpublished). NMR time course assays for the conversion of UDP-GlcA into UDP-pentose using recombinant potato UAS, confirms explicitly that UDP-Api is made first. The analysis indicates that in vitro, UDP-Xyl synthesis lags behind UDP-Api. Mutation in UDP-Api synthase in Nicotiana benthamiana is lethal as a consequence of the lack of RG-II (358). To unambiguously determine if the enzyme is bifunctional and contributes to UDP-Xyl synthesis, a mutation in the three cytosolic UXS (Type C) genes must be carried out.