Other carbohydrates such as polysaccharides like dietary starch and glycogen are breakdown their monomer (glucose) during digestion by the action of amylase enzyme, and now glucose absorbed into the cell where it enters glycolysis, but endogenous glycogen, some disaccharides (trehalose, maltose, lactose), and some monosaccharides, which are not the intermediates of the glycolytic pathway, enter in glycolysis with some previous reaction for preparing them to entry.
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Endogenous glycogen entry
Endogenous glycogen is breakdown into its monomer with help of three enzymes, glycogen phosphorylase, glycogen debranching enzyme, and phosphoglucomutase. Glycogen phosphorylase removes glucose 1-phosphate from the non-reducing end, the debranching enzyme removes glucose from the branch point, and phosphoglucomutase change glucose 1-P to glucose 6-P. Now glucose 6-P can enter into glycolysis.
Entry of disaccharides
Trehalose breakdown into glucose monomer by the action of trehalase enzyme, now free glucose can enter into the metabolic pathway
Sucrose enzymatically breakdown into glucose and fructose with the use of water molecule is the hydrolysis of sucrose, free glucose can into the catalytic pathway. And fructose enters after some other enzymatic reaction.
With the help of lactase enzyme lactose breakdown into glucose and galactose, now galactose undergoes some other enzymatic reaction before entry.
Entry of monosaccharides
Fructose enters into glycolysis either by phosphorylation on sixth carbon by hexokinase enzyme to change into fructose 6-P or by phosphorylation on first carbon to change into fructose 1-P by fructokinase enzyme. Now fructose 1-P is breakdown into Dihydoxyacetonephosphate (DHAP) and glyceraldehyde by the action of fructose 1-phosphate aldolase, and then glyceraldehyde is phosphorylated at third carbon converted into glyceraldehyde 3-p by triose kinase with the hydrolysis of ATP. Now DHAP and glyceraldehyde can enter into the glycolytic pathway at the point where dehydrogenation of glyceraldehyde 3-P takes place.
Galactose changes into glucose by three enzymatic reactions, first enzyme galactokinase, second enzyme UDP glucose: galactose 1-P uridylyltransferase, and third is glucose 4-epimerase. In the first reaction, galactokinase enzyme phosphorylates galactose at first carbon changing it into galactose 1-P. Now second enzyme UDP glucose: galactose 1-P uridylyltransferase transfer UDP from UDP glucose to galactose 1-P forming UDP galactose, transfer a phosphate group from galactose to glucose forming glucose1-P. Now third enzyme glucose 4-epimerase catalyzes the epimerization of UDP galactose to UDP glucose, this UDP glucose change into glucose 1-P through the second step of this pathway that is glucose transfer UDP to galactose 1-P. Now, these glucose 1-P molecules change into glucose 6-P by the action of the phosphoglucomutase enzyme.
Mannose enter into glycolytic pathway via a two-step reaction, in the first step hexokinase phosphorylates mannose to forming mannose 6-P by hydrolysis of ATP, then mannose 6-P isomerize into fructose 6-P by the action of phosphomannose isomerase.
Lactose intolerance (lactase deficiency)
It is due to deficiency or malfunctioning of lactase enzyme, without lactase enzyme affected person can’t digest milk sugar (lactose), so undigested lactose can’t be absorbed from the intestine and bacteria convert it into the toxic product, which leads to diarrhea. The affected person can use predigested milk.
Galactosemia (deficiency of galactose metabolizing enzyme)
Galactosemia is due to a deficiency in any of the three enzymes of galactose catabolism, first enzyme galactokinase, second enzyme UDP glucose: galactose 1-P uridylyltransferase and third is glucose 4-epimerase. Galactose concentration is high in blood and urine, and galactose metabolite galactitol, deposit in eye lens causes cataract in infancy.
- Lehninger Principle of Biochemistry by David L. Nelson and Michael M. Cox, 6th edition
- www.bioinfo.org.cn (Feeder pathway for Glycolysis)
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- 1% – https://www.ncbi.nlm.nih.gov/pubmed/3022797
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- 1% – https://facty.com/lifestyle/wellness/all-about-lactase-deficiency-and-lactose-intolerance/
- 1% – https://en.wikipedia.org/wiki/UDP-glucose_4-epimerase
- 1% – https://en.wikipedia.org/wiki/Galactose-1-phosphate_uridylyltransferase
- 1% – https://en.m.wikipedia.org/wiki/Fructolysis
- 1% – http://www.bioinfo.org.cn/book/biochemistry/chapt14/bio4.htm