This document summarizes several minor carbohydrate metabolic pathways, including those for uronic acids, galactose, fructose, and amino sugars. The uronic acid pathway synthesizes glucuronic acid from glucose via UDP-glucuronic acid. Fructose is metabolized mainly in the liver by fructokinase and aldolase B. Galactose is converted to glucose in the liver. Amino sugars are formed from glucosamine-6-phosphate, which is derived from fructose-6-phosphate and glutamine.

1. Carbohydrate Metabolism
Minor Pathways
R. C. Gupta
Professor and Head
Department of Biochemistry
National Institute of Medical Sciences
Jaipur, India

2. The major pathways for metabolism
of carbohydrates are:
Glycolysis
Glycogenesis
Glycogenolysis
HMP shunt
Gluconeogenesis

3. Besides the major pathways, there
are pathways for metabolism of:
Uronic acids
Galactose
Fructose
Amino sugars etc

4. This pathway synthesizes glucuronic acid
Glucuronic acid is synthesized from
glucose
Initial reactions are identical to those of
glycogenesis
Glucose is converted into UDP-glucose
Uronic acid pathway

6. UDP-Glucose is oxidized to UDP-glucuronic
acid
The reaction is catalysed by UGP-glucose
dehydrogenase

8. UDP-Glucuronic acid is the activated form of
glucuronic acid
UDP-Glucuronic acid is used for:
Synthesis of some mucopoly-
saccharides
Conjugation of bilirubin, steroids
and some drugs

9. The unutilized glucuronic acid is converted
into D-xylulose which enters HMP shunt
In plants and most animals, L-ascorbic acid
is also synthesized by this pathway

10. An intermediate of this pathway, L-gulonic
acid is converted into L-ascorbic acid via L-
gulonolactone and 2-keto-L-gulonolactone
In man, other primates and guinea pigs,
ascorbic acid cannot be synthesized due to
absence of L-gulonolactone oxidase

12. Fructose is present in some fruits
It is also formed by hydrolysis of dietary
sucrose
It is absorbed by intestinal mucosal cells
Metabolism of fructose

13. From intestinal mucosa, fructose reaches
liver via portal blood
Some of it is converted into fructose-6-
phosphate by hexokinase
But the Km of hexokinase for fructose is
very high

14. Most of the fructose is phosphorylated to
fructose-1-phosphate by fructokinase in liver
Fructose-1-phosphate cannot be converted
into fructose-6-phosphate or fructose-1,6-
biphosphate
It is split into glyceraldehyde and dihydroxy-
acetone phosphate by aldolase B in liver

15. Glyceraldehyde is phosphorylated to gly-
ceraldehyde-3-phosphate by triose kinase
Glyceraldehyde-3-phosphate and dihydroxy-
acetone phosphate may be:
Converted into glucose by
gluconeogenesis
Oxidized via the glycolytic
pathway

17. Fructose is required as a source of energy by
spermatozoa
Fructose is synthesized from glucose in
seminal vesicles, and is secreted into
seminal plasma
Among biological fluids, the highest
concentration of fructose is found in seminal
plasma

18. It can be synthesized from glucose by
the sequential actions of aldose
reductase and sorbitol dehydrogenase
Fructose can be synthesized in the body
even if dietary fructose is not available

20. Dietary lactose is the source of
galactose
The only food in which lactose is
present is milk
Lactose is hydrolysed into glucose and
galactose by lactase in the intestine
Metabolism of galactose

21. Glucose and galactose reach liver via
portal blood
Galactose is converted into glucose in the
liver
Galactose is first phosphorylated to
galactose-1-phosphate by galactokinase

22. Galactose-1-phosphate reacts with UDP-
glucose to form glucose-1-phosphate and
UDP-galactose
UDP-Galactose is epimerised to UDP-
glucose

24. Galactose is required
for the synthesis of:
Mucopolysaccharides
Glycolipids
Glycoproteins
Lactose

25. If dietary galactose is not available,
galactose can be synthesized from glucose
UDP-Galactose-4-epimerase can convert
UDP-glucose into UDP-galactose
UDP-Galactose
UDP-Galactose-4-
epimerase, NAD+
UDP-Glucose

26. Metabolism of amino sugars
Amino sugars are present in mucopoly-
saccharides, glycolipids and glycoproteins
The first amino sugar to be formed is
glucosamine-6-phosphate

27. Glucosamine-6-phosphate is formed from
fructose-6-phosphate and glutamine
Glutamine is converted into glutamate,
and fructose-6-phosphate is converted
into glucosamine-6-phosphate

29. Glucosamine-6-phosphate is the parent
compound of other amino sugars
Other amino sugars formed from
glucosamine-6-phosphate are:
N-Acetylglucosamine
N-Acetylgalactosamine
N-Acetylneuraminic acid