| Figure 25.8 Conversion of UDP-Glc to UDP-glucuronic acid (UDP-GlcUA) and UDP-xylose. Note that oxidation of UDP-Glc is a two-step reaction, from alcohol to aldehyde, then to an acid. Both reactions are catalyzed by UDP-Glc dehydrogenase. |
UDP-Glc is the precursor of other essential sugars, such as glucuronic acid and xylose , which are required for proteoglycan biosynthesis. The reactions that lead to the formation of these sugars are outlined in Figure 25.8. Oxidation of UDP-Glc by UDP-Glc dehydrogenase leads to the activated form of glucuronic acid, i.e. UDP-glucuronic acid (UDP-GlcUA). This nucleotide is the donor of glucuronic acid both for the formation of proteoglycans (see Chapter 27) and for conjugation and detoxification reactions that occur in the liver. In that organ, glucuronic acid is conjugated to steroid hormones, bilirubin (a degradation product of heme), and many drugs. The conjugation reaction increases the water solubility of hydrophobic compounds, facilitating their excretion in urine. UDP-GlcUA undergoes a decarboxylation reaction to form UDP-xylose, the activated form of xylose, the pentose sugar that serves as the link between
protein and glycan in proteoglycans. UDP-GlcUA is also a precursor of ascorbate (vitamin C) in most mammals, except primates and guinea pigs.
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