5.3. Metabolisme Disakarida
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Metabolism of disaccharides:Fructose and Galactose
Harliansyah, PhDDept. of Biochemistry, FKUY
2014
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Fructose MetabolismDiets containing large amounts of sucrose (a disaccharide of glucose and fructose) can utilize the fructose as a major source of energy.
Fructose is found in foods containing sucrose (fruits), high-fructose corn syrups, and honey.
The pathway to utilization of fructose differs inmuscle and liver.
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In liver, dietary fructose is converted to Fructose-1-P by fructokinase (also in kidney and intestine).Then, by the action of Fructose-1-P adolase (aldolase B ), Fructose-1-P is converted to DHAP and glyceraldehyde.
Glyceraldehyde is conveted to glyceraldehyde-3-P by triose kinase which together with DHAP may undergo:
1. Combine together and converted into glucose (main pathway)
2. They may be oxidized in glycolysis
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The utilization of fructose by fructokinase then aldolase bypass the steps of glucokinase and PFK-1 which are activated by insulin
This explains why fructose disappears from blood more rapidly than glucose in diabetic subjects
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The affinity of aldolase B for fructose-1-P is much poorer than that of fructose-1,6-biphosphate, thus fructose-1-P accumulates in fructokinase-expressing tissues. Thus, aldolase B is the rate-limiting enzyme for fructose metabolism (not glucose).
Muscle which contains only hexokinase can phosphorylate fructose to F6P which is a direct glycolytic intermediate However, hexokinase has a very low affinity to fructose compared to glucose, So it is not a significant pathway for fructose metabolism.
Unless it is present in very high concentration in blood.
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In the testes (seminal vesicles), fructose is converted into glucose through sorbitol formation by aldolase reductase and sorbitol dehydrogenase
Deficiency of fructose correlates with male infertility
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CARBOHYDRATES
HMP Shunt
Glycolysis
oxidized
Energy
Structure
function
TCAC
releasing Gluco-
neogenesis
substrates
STARCH
Monosaccharides
Glycogen
as
areOrganic
compounds
Polyhydroxyl alcohols
Disaccharides
Polysaccharides
derivatives
classified as
precursors of
Amylases and Oligosaccharidas
es
hydrolyzed
Hexose Transporter
sAbsorbed by