Structure and mechanism of human UDP-xylose synthase: evidence for a promoting role of sugar ring distortion in a three-step catalytic conversion of UDP-glucuronic acid.
UDP-xylose synthase (UXS) catalyzes decarboxylation of UDP-D-glucuronic acid to UDP-xylose. In mammals, UDP-xylose serves to initiate glycosaminoglycan synthesis on the protein core of extracellular matrix proteoglycans. Lack of UXS activity leads to a defective extracellular matrix, resulting in strong interference with cell signaling pathways. We present comprehensive structural and mechanistic characterization of the human form of UXS. The 1.26-Å crystal structure of the enzyme bound with NAD(+) and UDP reveals a homodimeric short-chain dehydrogenase/reductase (SDR), belonging to the NDP-sugar epimerases/dehydratases subclass. We show that enzymatic reaction proceeds in three chemical steps via UDP-4-keto-D-glucuronic acid and UDP-4-keto-pentose intermediates. Molecular dynamics simulations reveal that the D-glucuronyl ring accommodated by UXS features a marked (4)C(1) chair to B(O,3) boat distortion that facilitates catalysis in two different ways. It promotes oxidation at C(4) (step 1) by aligning the enzymatic base Tyr(147) with the reactive substrate hydroxyl and it brings the carboxylate group at C(5) into an almost fully axial position, ideal for decarboxylation of UDP-4-keto-D-glucuronic acid in the second chemical step. The protonated side chain of Tyr(147) stabilizes the enolate of decarboxylated C(4) keto species ((2)H(1) half-chair) that is then protonated from the Si face at C(5), involving water coordinated by Glu(120). Arg(277), which is positioned by a salt-link interaction with Glu(120), closes up the catalytic site and prevents release of the UDP-4-keto-pentose and NADH intermediates. Hydrogenation of the C(4) keto group by NADH, assisted by Tyr(147) as catalytic proton donor, yields UDP-xylose adopting the relaxed (4)C(1) chair conformation (step 3).
Institute of Biotechnology and Biochemical Engineering, Graz University of Technology, A-8010 Graz, Austria.
Thomas
T
Eixelsberger
Sabine
S
Sykora
Sigrid
S
Egger
Michael
M
Brunsteiner
Kathryn L
KL
Kavanagh
Udo
U
Oppermann
Lothar
L
Brecker
Bernd
B
Nidetzky
D002262:Carboxy-Lyases / Q000737:chemistry* / Q000378:metabolism; D002384:Catalysis; D018360:Crystallography, X-Ray; D006801:Humans; D020836:Protein Structure, Quaternary; D017434:Protein Structure, Tertiary; D013329:Structure-Activity Relationship; D014535:Uridine Diphosphate Glucuronic Acid / Q000737:chemistry*
D014535:Uridine Diphosphate Glucuronic Acid; D002262:Carboxy-Lyases; C023257:UDPglucuronate decarboxylase