1 fructose metabolism
1.1 fructolysis
1.2 metabolism of fructose dhap , glyceraldehyde
1.3 synthesis of glycogen dhap , glyceraldehyde 3-phosphate
1.4 synthesis of triglyceride dhap , glyceraldehyde 3-phosphate
fructose metabolism
all 3 dietary monosaccharides transported liver glut2 transporter. fructose , galactose phosphorylated in liver fructokinase (km= 0.5 mm) , galactokinase (km = 0.8 mm). contrast, glucose tends pass through liver (km of hepatic glucokinase = 10 mm) , can metabolised anywhere in body. uptake of fructose liver not regulated insulin. however, insulin capable of increasing abundance , functional activity of glut5 in skeletal muscle cells.
fructolysis
the initial catabolism of fructose referred fructolysis, in analogy glycolysis, catabolism of glucose. in fructolysis, enzyme fructokinase produces fructose 1-phosphate, split aldolase b produce trioses dihydroxyacetone phosphate (dhap) , glyceraldehyde [1]. unlike glycolysis, in fructolysis triose glyceraldehyde lacks phosphate group. third enzyme, triokinase, therefore required phosphorylate glyceraldehyde, producing glyceraldehyde 3-phosphate. resulting trioses identical obtained in glycolysis , can enter gluconeogenic pathway glucose or glycogen synthesis, or further catabolized through lower glycolytic pathway pyruvate.
metabolism of fructose dhap , glyceraldehyde
the first step in metabolism of fructose phosphorylation of fructose fructose 1-phosphate fructokinase, trapping fructose metabolism in liver. fructose 1-phosphate undergoes hydrolysis aldolase b form dhap , glyceraldehydes; dhap can either isomerized glyceraldehyde 3-phosphate triosephosphate isomerase or undergo reduction glycerol 3-phosphate glycerol 3-phosphate dehydrogenase. glyceraldehyde produced may converted glyceraldehyde 3-phosphate glyceraldehyde kinase or further converted glycerol 3-phosphate glycerol 3-phosphate dehydrogenase. metabolism of fructose @ point yields intermediates in gluconeogenic pathway leading glycogen synthesis fatty acid , triglyceride synthesis.
synthesis of glycogen dhap , glyceraldehyde 3-phosphate
the resultant glyceraldehyde formed aldolase b undergoes phosphorylation glyceraldehyde 3-phosphate. increased concentrations of dhap , glyceraldehyde 3-phosphate in liver drive gluconeogenic pathway toward glucose , subsequent glycogen synthesis. appears fructose better substrate glycogen synthesis glucose , glycogen replenishment takes precedence on triglyceride formation. once liver glycogen replenished, intermediates of fructose metabolism directed toward triglyceride synthesis.
figure 6: metabolic conversion of fructose glycogen in liver
synthesis of triglyceride dhap , glyceraldehyde 3-phosphate
carbons dietary fructose found in both free fatty acid , glycerol moieties of plasma triglycerides. high fructose consumption can lead excess pyruvate production, causing buildup of krebs cycle intermediates. accumulated citrate can transported mitochondria cytosol of hepatocytes, converted acetyl coa citrate lyase , directed toward fatty acid synthesis. in addition, dhap can converted glycerol 3-phosphate, providing glycerol backbone triglyceride molecule. triglycerides incorporated very-low-density lipoproteins (vldl), released liver destined toward peripheral tissues storage in both fat , muscle cells.
figure 7: metabolic conversion of fructose triglyceride in liver
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