Glycolysis
Metabolism of non -glucose sugars
Metabolism of non-glucose sugars1. Fructose metabolismn Account for about 10% of daily intake
(sucrose)n Phosphorylated by:
a. Hexokinase to Fructose-6-P (high Fructose level)b. Fructokinase to Fructose-1-P (low Fructose level)
n F1P cleavage by Aldolase B to DHAP and GA(glyceraldehyde)
n Both can join glycolysis pathway.
Metabolism of fructose
FRUCTOSE F1P DHAP + Glyceraldehyde
GAP
ATPATP
ADPADP
fructokinase aldolase B
PYRUVATE
Sorbitol n In presence of high intracellular level of
glucose and NADPH, aldose reductase reduces glucose to alcohol sorbitol (a polyol)
n Sorbitol is oxidized to fructose by sorbitol dehydrogenase
n Sorbitol if not oxidized it can not diffuse outside the cell, so results in swelling and damage of the cell
Sorbitol metabolism
Glucose Sorbitol Fructose
Aldose reductase
Sorbitol dehydrogenase
NADPH+H+
NADP+
NADH+H+
NAD+
Figure : The polyol pathway
Galactose metabolism v Major source is milkv Lactose Glc + Gal
(lactase)v Gal phosphrylated by:
a. hexokinase to Gal-6Pb. Galactokinase to Gal-1P
v It join glycolysis:a. Gal-1P UDP-Gal
(by Gal1-P uridyltransferase)b. UDP-Gal – epimerized to –UDP-Glc
LactoseSynthesized in mammary glands
Ø UDP-Gal + Glucose ----------------- Lactose(lactose synthase)
Ø The enzyme is a dimmer of two proteins A and Bi. Protein A (b-galactosyltransferase)ii. Protein B (a-lactalbumin).
Ø The synthesis of the enzyme is stimulated by prolactin hormone
Ø In absence of prolactin Galactose is used in glycoprotein synthesis
THE END