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