Structural highlights
Function[AXS1_ARATH] Catalyzes the conversion of UDP-D-glucuronate to a mixture of UDP-D-apiose and UDP-D-xylose. D-Apiose (3-C-hydroxymethyl-d-erythrose) is the only plant cell wall monosaccharide with a branched carbon skeleton and is found in rhamnogalacturonan II (RG-II), apiogalacturonan, and several apioglycosides.[1] Publication Abstract from PubMedD-Apiose is a C-branched pentose sugar important for plant cell wall development. Its biosynthesis as UDP-D-apiose involves decarboxylation of the UDP-D-glucuronic acid precursor coupled to pyranosyl-to-furanosyl sugar ring contraction. This unusual multistep reaction is catalyzed within a single active site by UDP-D-apiose/UDP-D-xylose synthase (UAXS). Here, we decipher the UAXS catalytic mechanism based on crystal structures of the enzyme from Arabidopsis thaliana, molecular dynamics simulations expanded by QM/MM calculations, and mutational-mechanistic analyses. Our studies show how UAXS uniquely integrates a classical catalytic cycle of oxidation and reduction by a tightly bound nicotinamide coenzyme with retro-aldol/aldol chemistry for the sugar ring contraction. They further demonstrate that decarboxylation occurs only after the sugar ring opening and identify the thiol group of Cys100 in steering the sugar skeleton rearrangement by proton transfer to and from the C3'. The mechanistic features of UAXS highlight the evolutionary expansion of the basic catalytic apparatus of short-chain dehydrogenases/reductases for functional versatility in sugar biosynthesis. Deciphering the enzymatic mechanism of sugar ring contraction in UDP-apiose biosynthesis.,Savino S, Borg AJE, Dennig A, Pfeiffer M, de Giorgi F, Weber H, Dubey KD, Rovira C, Mattevi A, Nidetzky B Nat Catal. 2019 Nov 25;2(12):1115-1123. doi: 10.1038/s41929-019-0382-8. PMID:31844840[2] From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine. References
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