BIOCHEMISTRY ENTRY OF OTHER CARBOHYDRATES INTO GLYCOLYSIS DR SAMEER FATANI.
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Transcript of BIOCHEMISTRY ENTRY OF OTHER CARBOHYDRATES INTO GLYCOLYSIS DR SAMEER FATANI.
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BIOCHEMISTRY
ENTRY OF OTHER CARBOHYDRATES
INTO GLYCOLYSIS
DR SAMEER FATANI
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ENTRY OF FRUCTOSE, GALACTOSE, AND MANNOSE
INTO GLYCOLYSIS
In most organisms hexoses other than glucose can undergo glycolysis after conversion to a phosphorylated derivatives.
D-fructose. In muscle and kidney:D-fructose, present in many free form in many fruits and formed by hydrolysis of sucrose in small intestine vertebrates, is phosphorylated by HEXOKINASE:
Fructose + ATP Mg2+ fructose 6-phosphate + ADP hexokinase
In the liver:Fructose enters glycolysis by different pathway. The liver enzyme fructokinase catalyzes the phosphorylation of fructose at C-1 rather than C-6:
Fructose + ATP Mg2+ fructose 1-phosphate + ADP fructokinase
Then Fructose 1-phosphate fructose 1-phosphate aldolase Dihydroxyacetone phosphate + Glyceraldehyde
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Dihydroxyacetone phosphate triose phosphate isomerse
glyceraldehyde 3-phosphate (intermediate in glycolysis)
And
Glyceraldehyde + ATP Mg2+ glyceraldehyde 3-phosphate + ADP triose kinase
Thus both products of fructose 1-phosphate hydrolysis
enter the glycolytic pathway as glyceraldehyde 3-phosphate.
D-galactose A product of hydrolysis of the disaccharide lactose (milk sugar), passes in the blood from the intestine to the liver, where it is first phosphorylated at C-1 in the presence of ATP, by the enzyme galactokinase:
Galactose + ATP Mg2+ galactose 1-phosphate + ADP galactokinase
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The galactose 1-phosphate is then converted to its epimer at C-4, glucose 1-phosphate, by a set of reactions in which uridne diphosphate (UDP) functions as a coenzyme-like carrier of hexose groups. The epimerization involves first the oxidation of the C-4 –OH group to a ketone, then reduction of the ketone to an –OH, with inversion of the configuration at C-4. NAD is the cofactor for both the oxidation and the reduction.
Defects in any of the three enzymes in this pathway cause galactosemia in humans.galactokinase-dificinecy galactosemia: characterized by high galactose concentrations are found in blood and urine. (for rest back to book, L4-p537).
D-mannose Released in the digestion of various polysaccharides and glycoproteins of foods, can be phosphorylated at C-6 b hexokinase:
Mannose + ATP Mg2+ mannose 6-phosphate + ADP
hexokinase
Mannose 6-phosphate is isomerized by phospho-mannose isomeraze to yield fructose 6-phosphate, an intermediate of glycolysis.
Mannose 6-phosphate isomerization fructose 6-phosphate phospho-mannose isomeraze
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