Uronic Acid Pathway

1. 1

   Glucose-1-phosphate + UTP -> UDP-glucose + PPi

   UDP-glucose pyrophosphorylase 2.7.7.9 ST-0301 Irreversible/directional UTP Mg2+
   In Glucose-1-phosphate UTP

   Out UDP-glucose PPi
   › Notes

   Shared entry with glycogenesis.

2. 2

   UDP-glucose + 2 NAD+ + H2O -> UDP-glucuronate + 2 NADH + 2 H+

   UDP-glucose dehydrogenase (UGDH) 1.1.1.22 ST-0302 Irreversible rate-limiting NAD+
   In UDP-glucose NAD+ H2O

   Out UDP-glucuronate NADH H+
   › Notes

   Committed step; supplies UDP-glucuronate for glucuronidation (bilirubin, drugs, steroids) and for glycosaminoglycan/proteoglycan synthesis.

3. 3

   UDP-glucuronate -> L-gulonate -> (L-gulonolactone) -> ascorbate

   Multiple; L-gulonolactone oxidase (GULO) is a pseudogene in primates ST-0303 Irreversible/directional NADPH
   In UDP-glucuronate

   Out L-Gulonate
   › Notes

   Humans, other primates and guinea pigs lack functional GULO, so ascorbate is a dietary vitamin rather than a metabolic product.

4. 4

   L-Gulonate -> 3-keto-L-gulonate -> L-xylulose -> xylitol -> D-xylulose -> xylulose-5-phosphate

   Gulonate dehydrogenase; L-xylulose reductase; xylulokinase ST-0304 Irreversible/directional NADPH NAD+ ATP
   In L-Gulonate

   Out 3-Keto-L-gulonate
   › Notes

   Feeds into the non-oxidative pentose phosphate pathway. Deficiency of L-xylulose reductase causes essential pentosuria (benign).

Showing all 4 steps.