Proteoglycans act as receptors for growth factors and are essential for cell proliferation, migration and adhesion. Disrupting proteoglycan biosynthesis can attenuate tumor growth and progression, thus controlling proteoglycan biosynthesis is a promising strategy for treating cancer. The biosynthesis of most proteoglycans begins with the covalent attachment of xylose to the hydroxyl of a serine. Human UDP-xylose synthase (hUXS) produces the essential nucleotide sugar donor that initiates proteoglycan synthesis. The hUXS mechanism is divided into sequential oxidation and reduction reactions that recycle a bound NAD+ cofactor. In the first step, hUXS uses the NAD+ cofactor to oxidize UDP-GlcA to a reaction intermediate, UDP-α-D-4-keto-xylose. Next, it uses the NADH produced in the first step to reduce the intermediate to UDP-Xyl.
The man o’ war (mow) mutation in zebrafish UXS disrupts proteoglycan biosynthesis, resulting in severe craniofacial defects. Because the disruption of proteoglycan biosynthesis can attenuate tumor growth and progression, understanding the molecular basis by which the mow mutation inactivates UXS may facilitate the development of new strategies to control proteoglycan synthesis. Zebrafish and human UXS share an overall sequence identity of 93%. We have previously shown that the mow mutation (R236H) in hUXS prevents the synthesis of UDP-Xyl. The mow mutation induces tetrameric hUXS to reorganize into a new, hexameric structure. Surprisingly, the mow-induced hexamer is almost identical to the hexamers formed by the deeply divergent UXS homologs from Staphylococcus aureus and Helicobacter pylori (21% and 16% sequence identity, respectively). The persistence of a latent hexamer-building interface in the human enzyme suggests that the ancestral UXS may have been a hexamer.