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USMLE Biochemistry > Fructose and Galactose > Flashcards

Fructose and Galactose Flashcards

1
Q

What is taken up by SGLT1?

A
  • Galactose and glucose (Na+ dependent transporter)
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2
Q

What is fructose taken up by?

What kind of diffusion is involved?

A

GLUT-5

- Facilitated diffusion

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3
Q

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

A

C6 H12 O6

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4
Q

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

A

GLUT2

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5
Q

What is the major source of fructose from?

A

Sucrose

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6
Q

What breaks down Fructose to Fructose-1-Phosphate?

A

Fructokinase (found in liver)

- Requires ATP

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7
Q

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

A

Aldolase B

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8
Q

What enzyme converts Glyceraldehyde to Glyceraldehyde-3-phosphate?

A

Triokinase

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9
Q

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

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

Enzyme: Aldolase B

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10
Q

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

A

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

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11
Q

What can hexokinase covert Fructose into?

A

F6P (only a small amount)

- Only really relevent if deficient in other enzymes

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12
Q

What is essential fructosuria due to a deficiency in?

A

Fructokinase

(Fructose cannot be converted to

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13
Q

What is hereditary Fructose Intolerance due to a deficiency in?

A

Aldolase B

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14
Q

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

A
  • Build-up of F1P
  • Depletion of ATP
  • Decrease of Gluconeogenesis and glycogen breadown
  • Hypoglycemia/vomitting
  • Hepatomegaly (glycogen not broken down) -> Liver Failure
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15
Q

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

A

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
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16
Q

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

A
  • Avoid fructose, sucrose, sorbitol
17
Q

Describe the Polyol pathway?

A

Glucose -> Sorbitol (NADPH-> NADP+)

Sorbitol -> Fructose (NAD+ -> NADH)

18
Q

What enzyme converts Glucose to Sorbitol?

A 
Aldose Reductase (Polyol pathway)
- NADPH -> NAP+ also
19
Q

What enzyme converts Sorbitol to Fructose?

A

Sorbitol dehydrogenase
- NAD+ -> NADH

Part of polyol pathway

20
Q

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

A

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

21
Q

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

A
  • Galactose 1-Phosphate
  • > Glucose 1-Phosphate -> Glycogen
  • Glucose 1 Phosphate Glucose 6-phosphate (ccan be used for glycolysis or glucose)
22
Q

What enzyme converts galactose into galactose 1-Phosphate?

A

Galactokinase (ATP required)

23
Q

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

A

Galactose 1-Phosphate Uridyltransferase (GALT)

  • Also requires UDP-Glucose
24
Q

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

A

UDP-Glucose

25
Q

What is classic galactosemia due to a deficiency in?

A

Galactose 1-Phosphate Uridyltransferase (GALT)

26
Q

What molecules builds up in classic galactosemia ?

A

Galactose 1-phosphate accumulates in cells and liver -> damage

  • Galactitol accumulates in cells (polyol pathway, galactose broken down to it via aldose reductase)
27
Q

How is classic galactosemia inherited?

A

Aut Recessive

28
Q

What substance leads to cataracts in classic galactosemia?

A

Galactitol

29
Q

When does classic galctosemia present, how does it?

A
  • First few days of life, after milk consumption

Liver accumulates galactose/galacitol

  • LF, jaundice, hepatomegaly
  • Failure to thrive

Cataracts (due to galactitol)

30
Q

How is classic galactosemia diagnosed?

A

Screening

- GALT enzyme activity assay

31
Q

How is classic galactosemia treated?

A

Avoid galactose, no milk

32
Q

What is galactokinase deficiency?

What does it lead to?

A
  • Milder form of galactosemia
  • Galactose not taken up by cells
  • Accumulates in blood and urine
  • Cataracts as a child/young adult (vision problems)
33
Q

What does galactokinase break down?

A

Galactose into Galactose 1-Phosphate

requires ATP

34
Q

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

A
  • Liver is not depleted of phosphate in glactokinase deficiency

USMLE Biochemistry (44 decks)

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