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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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