Metabolism of Monosaccharides and Disaccharides

Question 1

What is the most common monosaccharide consumed by humans?

Answer 1

Glucose

• Fructose and galactose also occur in significant amounts in the diet
• Galactose: important component of structural carbohydrates

Question 2

What monosaccharide makes up 10% of western diet calories?

Answer 2

Fructose

• Major source is sucrose
• Also found as a free monosaccharide in fruits, honey and high-fructose corn syrup, soft drinks, food
• Does not promote insulin secretion

Question 3

What is the function of fructokinase or hexokinase?

Answer 3

For fructose to enter the pathways of intermediary metabolism, it must first be phosphorylated by this enzyme

• Found in liver, kidneys and small intestine
• Uses ATP
• Cleaved by aldolase B -> DHAP + glyceraldehyde
• DHAP can be used in glycolysis/gluconeogenesis and glyceraldehyde can be metabolized by many pathways

Question 4

What is aldose reductase?

Answer 4

Glucose can be reduced by aldose reductase -> sorbitol (then reduced to fructose)
-Found in lens, retina, Schwann cells of peripheral nerves, liver, kidney, placenta, RBCs and cells of the ovaries and seminal vesicles

Question 5

What is sorbitol dehydrogenase?

Answer 5

Sorbitol can be reduced by sorbitol dehydrogenase -> fructose

• Found in liver, ovaries, seminal vesicles
• Sperm cells: fructose = major carb energy source

Question 6

What happens in cases of uncontrolled diabetes if there is a deficiency of sorbitol dehydrogenase?

Answer 6

Elevated [glucose] -> aldose reductase to produce an increase in the amount of sorbitol, which cannot efficiently pass through cell membranes and remains trapped inside the cell

• Deficiency of sorbitol dehydrogenase -> sorbitol accumulates in these cells, causing strong osmotic effects and cell swelling due to water influx and retention
• ->Cataract formation, peripheral neuropathy and microvascular problems leading to nephropathy and retinopathy

Question 7

What is the major dietary source of galactose?

Answer 7

Lactose from milk products

Question 8

Like fructose, galactose must be ____ before it can be further metabolized.

Answer 8

Phosphorylated

-Transport into cells is also not insulin dependent

Question 9

What is galactokinase?

Answer 9

Enzyme that phosphorylates galactose -> galactose 1-P

Question 10

What must happen for galactose 1-P to enter the glycolytic pathway?

Answer 10

First converted to UDP-galactose in an exchange reaction with UDP-glucose
-By galactose 1-phosphate uridylyltransferase (GALT)

Question 11

What is UDP-hexose 4-epimerase?

Answer 11

For UDP-galactose to enter the mainstream of glucose metabolism, it must first be isomerized to its C-4 epimer, UDP-glucose, by this enzyme

Question 12

What is classic galactosemia?

Answer 12

GALT is severely deficient -> galactose 1-P and galactose accumulate -> shunted into side pathways such as that of galacitol production

• treatment: removal of galactose and lactose from diet
• newborn screening available

Question 13

UDP-galactose can serve as the donor of galactose units in what synthetic pathways?

Answer 13

Synthesis of lactose, glycoproteins, glycolipids, and glycosaminoglycans

Question 14

What is lactose?

Answer 14

Disaccharide of galactose and glucose; galactosyl beta(1->4)-glucose

• “Milk sugar” made by lactating mammary glands
• Dietary sources: milk and other dairy products

Question 15

Where is lactose synthesized and by what enzyme?

Answer 15

Synthesized in Golgi by lactose synthase (UDP-galactose:glucose galactosyl-transferase), which transfers galactose from UDP-galactose to glucose

Question 16

What are the 2 subunits of lactose?

Answer 16

1) Protein A (galactosyltransferase): found in most cells where it synthesizes N-acetyllactosamine
- has high Km for glucose (low affinity)
2) Protein B (alpha-lactalbumin): found only in lactating mammary glands; synthesis stimulated by prolactin
- has low Km for glucose (high affinity)