Metabolism of Hexoses Other than Glucose

Question 1

What kind of carbohydrates are mannose, glucose, galactose, and fructose?

Answer 1

Hexoses

Question 2

Where does galactose enter glycolysis at?

Answer 2

It is converted to glucose 6-phosphate and begins glycolysis there.

Question 3

Where do mannose and fructose (in muscle) enter glycolysis at?

Answer 3

They are converted to fructose 6-phosphate and enters glycolysis there.

Question 4

Where does fructose enter glycolysis at in the liver?

Answer 4

It is converted to glyceraldehyde 3-phosphate and enters glycolysis there.

Question 5

What are the primary sources of fructose?

Answer 5

1) Fruit

2) Sucrose (disaccharide of glucose and fructose)

Question 6

What enzyme converts fructose into fructose-6-phosphate in muscle?

Answer 6

Hexakinase

Question 7

What enzyme catalyzes the conversion of fructose to fructose-1-phosphate in liver?

Answer 7

Fructokinase

Question 8

True or false: Glucokinase in the liver will react with fructose?

Answer 8

False

Question 9

Which type of aldolase is specific for fructose-1,6-bisphosphate?

Answer 9

Type A in muscle

Question 10

Which type of aldolase in the liver will use both FBP and fructose 1-phosphate as substrates?

Answer 10

Type B

Question 11

What are the two options for fructose in the liver after it has been converted to glyceraldehyde?

Answer 11

1) Glyceraldehyde kinase converts it to GAP (which allows it to enter glycolysis)
2) It is converted to glycerol by ADH, then to glycerol-3-phosphate by glycerol kinase, then to DHAP by glycerol phosphate dehydrogenase, then to GAP by triose phosphate isomerase (where it enters glycolysis).

Question 12

True or false: Both pathways in the liver from glyceraldehyde to GAP have the same net energy cost.

Answer 12

True

Question 13

True or false: Both pathways in the liver from glyceraldehyde to GAP produce ATP.

Answer 13

False

Question 14

Which of the two pathways in the liver from glyceraldehyde to GAP oxidize/reduce NADH?

Answer 14

The longer pathway

Question 15

Which of the two pathways in the liver from glyceraldehyde to GAP produce glycerol-3-phosphate and glycerol?

Answer 15

The longer pathway?

Question 16

What can glycerol-3-phosphate become the backbone for?

Answer 16

Glycerophospholipids and triacylglycerols

Question 17

What happens to fructose if there are high concentrations of ATP?

Answer 17

It will be converted to fat.

Question 18

True or false: Fructose is converted to fructose-1-phosphate by phosphofructokinase in the liver.

Answer 18

False; fructose bypasses PFK in the liver. Fructose kinase takes it to F1P.

Question 19

What is fructose intolerance caused by?

Answer 19

Deficiency of Type B aldolase

Question 20

Why does a deficiency of Type B aldolase cause when fructose is consumed?

Answer 20

1) F1P accumulates and depletes liver stores of Pi.
2) Reduced ATP causes liver damage.
3) Increased F1P inhibits glycogen phosphorylase (breaks down glycogen) and fructose-1,6-bisphosphate (for gluconeogenesis).
4) This causes severe hypoglycemia.

Question 21

How is fructose ingestion inhibited in someone with a deficiency of Type B aldolase?

Answer 21

They develop a strong distaste for anything sweet.

Question 22

Where does galactose come from?

Answer 22

Hydrolysis of lactose (galactose and glucose)

Question 23

What is the difference between galactose and glucose?

Answer 23

They differ only in configuration at carbon 4.

Question 24

True or false: Hexokinase converts galactose to glucose-6-phosphate.

Answer 24

False; it works on glucose, fructose, and mannose, but not galactose.

Question 25

What are the 4 steps of the epimerization that convert galactose to glucose-6-phosphate?

Answer 25

1) Galactokinase converts galactose to galactose 1-phosphate
2) Galactose-1-phosphate uridyl transferase converts galactose-1-phosphate to UDP-galactose.
3) UDP-galactose-4-epimerase converts UDP-galactose to glucose-1-phosphate.
4) Phosphoglucomutase converts G1P to G6P, which enters glycolysis.

Question 26

What is the disease that occurs from the inability to convert galactose to glucose?

Answer 26

Galactosemia

Question 27

What are the symptoms of galactosemia?

Answer 27

1) Failure to thrive
2) Mental retardation
3) Death
4) Cataracts

Question 28

What usually causes galactosemia?

Answer 28

Deficiency of Gal-1-P uridyl transferase

Question 29

What does galactosemia cause to happen in the lens of the eye?

Answer 29

High [galactitol] (from high blood levels of galactose), leading to cataracts.

Question 30

What is the treatment for galactosemia?

Answer 30

Galactose-free diet (which can reverse all symptoms except mental retardation).

Question 31

Where does one’s required galactose come from if one is on a galactose-free diet?

Answer 31

Glucose

Question 32

Where does mannose come from?

Answer 32

Digestion of polysaccharides and glycoproteins

Question 33

Where does mannose enter glycolysis?

Answer 33

As fructose-6-phosphate

Question 34

What enzyme converts mannose into mannose-6-phosphate?

Answer 34

Hexokinase

Question 35

What enzyme converts mannose-6-phosphate to fructose-6-phosphate?

Answer 35

Phosphomannose isomerase.