Fructose and Galactose

Question 1

What is taken up by SGLT1?

Answer 1

• Galactose and glucose (Na+ dependent transporter)

Question 2

What is fructose taken up by?

What kind of diffusion is involved?

Answer 2

GLUT-5

- Facilitated diffusion

Question 3

What is the chemical formula for glucose and its isomers (galactose, fructose)?

Answer 3

C6 H12 O6

Question 4

What transporter transports Glucose, galactose and fructose from the cell to the interstitium?

Answer 4

GLUT2

Question 5

What is the major source of fructose from?

Answer 5

Sucrose

Question 6

What breaks down Fructose to Fructose-1-Phosphate?

Answer 6

Fructokinase (found in liver)

- Requires ATP

Question 7

What enzyme splits the 5 Carbon F1P into 2 3 carbon molecules; Glyceraldehyde and Dihydroxyacetone Phosphate?

Answer 7

Aldolase B

Question 8

What enzyme converts Glyceraldehyde to Glyceraldehyde-3-phosphate?

Answer 8

Triokinase

Question 9

What 2 molecules can fructose-6-phosphate be converted into which enters them into the glycolysis pathway?

Answer 9

• Glyceraldehyde-3-phosphate (via Glyceraldehyde, triokinase)
• Dihydroxyacetone Phosphate

Enzyme: Aldolase B

Question 10

Why is the metabolism of fructose faster than that of glucose?

Answer 10

Fructose bypasses the rate limiting step of glycolysis (Phosphofructokinase 1)

Question 11

What can hexokinase covert Fructose into?

Answer 11

F6P (only a small amount)

- Only really relevent if deficient in other enzymes

Question 12

What is essential fructosuria due to a deficiency in?

Answer 12

Fructokinase

(Fructose cannot be converted to

Question 13

What is hereditary Fructose Intolerance due to a deficiency in?

Answer 13

Aldolase B

Question 14

What does hereditary Fructose Intolerance (aldolase B deficiency) result in?

Answer 14

• Build-up of F1P
• Depletion of ATP
• Decrease of Gluconeogenesis and glycogen breadown
• Hypoglycemia/vomitting
• Hepatomegaly (glycogen not broken down) -> Liver Failure

Question 15

How may hereditary Fructose Intolerance (aldolase B deficiency) present?

Answer 15

Presents just after weaning from breast milk (no fructose in the milk)

• Failure to thrive
• Hypoglycemia (seizures)
• Enlarged liver -> failure
• Part of newborn screening panel

Question 16

How is Hereditary Fructose Intolerance (aldolase B deficiency) treated?

Answer 16

• Avoid fructose, sucrose, sorbitol

Question 17

Describe the Polyol pathway?

Answer 17

Glucose -> Sorbitol (NADPH-> NADP+)

Sorbitol -> Fructose (NAD+ -> NADH)

Question 18

What enzyme converts Glucose to Sorbitol?

Answer 18

Aldose Reductase (Polyol pathway)
- NADPH -> NAP+ also

Question 19

What enzyme converts Sorbitol to Fructose?

Answer 19

Sorbitol dehydrogenase
- NAD+ -> NADH

Part of polyol pathway

Question 20

Why is sorbitol to be avoided in those with Hereditary Fructose Intolerance?

Answer 20

It can be converted to fructose via sorbitol dehydrogenase (polyol pathway)

Question 21

What is galactose converted into in order to be involved in glycolysis/gluconeogenesis or glycogen synthesis?

Answer 21

• Galactose 1-Phosphate
• > Glucose 1-Phosphate -> Glycogen
• Glucose 1 Phosphate Glucose 6-phosphate (ccan be used for glycolysis or glucose)

Question 22

What enzyme converts galactose into galactose 1-Phosphate?

Answer 22

Galactokinase (ATP required)

Question 23

What enzyme convers Galactose 1-Phosphate to glucose 1-Phosphate?

Answer 23

Galactose 1-Phosphate Uridyltransferase (GALT)

• Also requires UDP-Glucose

Question 24

What does GALT require in order to convert Galactose 1-Phosphate to Glucose 1-Phosphate?

Answer 24

UDP-Glucose

Question 25

What is classic galactosemia due to a deficiency in?

Answer 25

Galactose 1-Phosphate Uridyltransferase (GALT)

Question 26

What molecules builds up in classic galactosemia ?

Answer 26

Galactose 1-phosphate accumulates in cells and liver -> damage - Galactitol accumulates in cells (polyol pathway, galactose broken down to it via aldose reductase)

Question 27

How is classic galactosemia inherited?

Answer 27

Aut Recessive

Question 28

What substance leads to cataracts in classic galactosemia?

Answer 28

Galactitol

Question 29

When does classic galctosemia present, how does it?

Answer 29

- First few days of life, after milk consumption Liver accumulates galactose/galacitol - LF, jaundice, hepatomegaly - Failure to thrive Cataracts (due to galactitol)

Question 30

How is classic galactosemia diagnosed?

Answer 30

Screening | - GALT enzyme activity assay

Question 31

How is classic galactosemia treated?

Answer 31

Avoid galactose, no milk

Question 32

What is galactokinase deficiency? | What does it lead to?

Answer 32

- Milder form of galactosemia - Galactose not taken up by cells - Accumulates in blood and urine - Cataracts as a child/young adult (vision problems)

Question 33

What does galactokinase break down?

Answer 33

Galactose into Galactose 1-Phosphate | requires ATP

Question 34

Why are there not liver problems in galactokinase deficiency but there are with classic galactosemia?

Answer 34

- Liver is not depleted of phosphate in glactokinase deficiency