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Biochemistry > Carbohydrate metabolism (lecture 24) > Flashcards

Carbohydrate metabolism (lecture 24) Flashcards

1
Q

What are the 3 major pathways in the oxidation of glucose?

A

Glycolysis
Citric acid cycle
Oxidative phosphorylation

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2
Q

Where does glycolysis happen?

A

Cytosol

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3
Q

Where does the citric acid cycle happen?

A

Mitochondria

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4
Q

Where does oxidative phosphorylation happen?

A

Mitochondria

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5
Q

What is glycolysis?

A

The use of carbohydrates as fuel

Conversion of glucose to pyruvate

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6
Q

Why is glycolysis important?

A

Gives us a ready supply of energy that can be used quickly

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7
Q

What are the 3 stages of glycolysis?

A 
1. Investment or priming 
Glucose --> fructose-1,6-bisphosphate 
2. Splitting 
Fructose-1,6-bisphosphate --> 2 glycerol-3-phosphates  
3. Energy conservation or yield
Glycerol-3-phosphate --> pyruvate
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8
Q

What happens in the investment stage?

A
  1. Glucose –> glucose-6-phosphate
  2. Glucose-6-phosphate –> fructose-6-phosphate
  3. Fructose-6-phosphate –> fructose-1,6-bisphosphate
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9
Q

Glucose –> glucose-6-phosphate

A

Uses enzyme hexokinase
Hexokinase = kinase that phosphorylates hexose sugar to create ATP
Creates a molecule with a large charge - gets repelled by the cell membrane so can no longer escape the cell
ATP is complexed to a magnesium ion - complexes with 2/3 phosphates & protects them from hydrolysis
When ADP is produced Mg+ flies off

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10
Q

Glucose-6-phosphate –> fructose-6-phosphate

A

Uses the enzyme glucose-6-phosphate isomerase
converts the aldopyranose to a ketofuranose

  1. Molecule breaks open
  2. Produces an aldehyde side chain - not stable
  3. Quickly becomes a ketone
  4. Oxygen in the ketone group heads towards the other oxygen
  5. End up with a 5 membered ring - less stable

Almost a mirror image
Doesn’t require ATP

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11
Q

Fructose-6-phosphate –> fructose-1,6-bisphosphate

A

ATP needed
Enzyme is phosphofructokinase - an allosteric enzyme
Irreversible step
ATP bound to Mg+
Mirror image is formed
2 phosphate groups try to pull the molecule apart

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12
Q

What does bisphosphate mean?

A

2 phosphate groups but on different carbons

Would be diphosphate if they were on the same carbon

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13
Q

What happens in splitting?

A

When the single 6-cabon molecule is split into 2 3-carbon molecules
Aldolase breaks the molecule into 2
Produces 2 products which are isomers

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14
Q

What are the 2 products (isomers) produced by splitting?

A
  • Dihydroxyacetone-phosphate
  • Glyceraldehyde-3-phosphate

Dihydroxyacetone-phosphate has 2 possible fates

  1. Can be used in the synthesis of fatty acids
  2. Be converted into its other isomer

Triosephosphate isomerase converts DHAT to GA3P

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15
Q

What happens in the yield stage?

A

Pyruvate is formed

  1. Glyceraldehyde-3-phosphate –> 1,3-bisphosphoglycerate
  2. 1,3-bisphosphoglycerate –> 3-phosphoglycerate
  3. 3-phosphoglycerate –> 2-phosphoglycerate
  4. 2-phosphoglycerate –> phosphoenolpyruvate
  5. phosphoenolpyruvate –> pyruvate
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16
Q

Yield stage

1) Glyceraldehyde-3-phosphate –> 1,3-bisphosphoglycerate

A

‘The magic step’
Doesn’t require ATP
Catalysed by GAPDH - glyceraldehyde-3-phosphate dehydrogenase
• Takes NAD+ & generates reducing power (NADH + H+)
• Takes inorganic phosphate to generate 1,3-bisphosphoglycerate

17
Q

Yield stage

2) 1,3-bisphosphoglycerate –> 3-phosphoglycerate

A

Production of the first ATP by substrate level phosphorylation

1,3-bisphosphoglycerate is a highly charged & a high energy molecule
• Phosphoglycerate kinase catalyses the joining of the phosphate group with ADP to form ATP
• 3-phophoglycerate is produced

18
Q

Yield stage

3) 3-phosphoglycerate –> 2-phosphoglycerate

A

Transfer of phosphate to the central carbon
Catalysed by phosphoglycerate mutase
• Mutases move phosphate groups

19
Q

Yield stage

4) 2-phosphoglycerate –> phosphoenolpyruvate

A

Increases the energy potential of the molecule by removal of water
Catalysed by enolase
Generates phosphoenolpyruvate – highly charged molecule

20
Q

Yield stage

5) phosphoenolpyruvate –> pyruvate

A

Second energy producing reaction
Phosphoenolpyruvate converted to pyruvate by pyruvate kinase
ATP is produced

21
Q

What is the overall net gain from glycolysis?

A

Net gain of 2 ATP for every glucose

2 ATP used in the investment stage
• 1 to generate glucose-6-phosphate
• 1 to generate fructose-1,6-bisphosphate

4 ATP generated (2 per G-3-P)
• 1 molecule of glyceraldehyde-3-phosphate then generates 1 ATP by substrate level phosphorylation from GAPDH
• 1 ATP from substrate level phosphorylation from pyruvate kinase

Biochemistry (16 decks)

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