Control of glycolysis & the fates of pyruvate (lecture 25)

Question 1

Why do metabolic pathways need to be tightly regulated?

Answer 1

1. To link supply with demand (storage & release)
2. To allow cells/organism to respond to environmental changes (eg. Temperature, diet, microenvironment)
3. To maintain a constant internal environment (homeostasis)
4. To enable different tissues to interact (eg. Liver, adipose, muscle)

Question 2

How is glycolysis controlled?

Answer 2

Rate limiting enzymes
Enzymes at the start and at branches in the pathway

Controlling the amount of an enzyme = slow
Controlling the activity of an enzyme = fast

Controlling the accessibility of enzyme to substrate

Question 3

How do you control the activity of an enzyme?

Answer 3

Conformational change
• Allosteric control – non-covalent binding/release of effector molecule

Covalent modification (eg. phosphorylation)

Question 4

How do you control the accessibility of the enzyme to its substrate?

Answer 4

By compartmentalisation of enzymes

By compartmentalisation of substrates

Question 5

What is the different between the cytoplasm and the cytosol?

Answer 5

Cytoplasm = everything but the nucleus

Cytosol = cytoplasm – organelles = the soluble part of the cell

Question 6

What are the 3 major controls of glycolysis?

Answer 6

Hexokinase
Phosphofructokinase
Pyruvate kinase

Question 7

What is a major indicator of a low energy state in the cytosol in glycolysis?

Answer 7

AMP

Why???
• AMP still has a phosphate group – still an energetic molecule
• Adenylate kinase can shuffle around phosphate molecules
• AMP is a positive modulator of phosphofructokinase – increases activity

Question 8

What happens when too much fructose-6-phosphate is made than can be used in the glycolysis pathway?

Answer 8

It gets put unto a side pathway which generates fructose-2,6-bisphosphate

Build-up of fructose-2,6-bisphosphate also acts as a positive modulator of PFK

Question 9

What does adenylate kinase do?

Answer 9

Shuffles around phosphate groups

Question 10

What are the 2 positive modulators of PFK?

Answer 10

AMP

Fructose-2,6-bisphosphate

Question 11

What is feedforward stimulation in the glycolysis pathway?

Answer 11

Avoids any log jams

If there’s lots of fructose-1,6-bisphosphate, pyruvate kinase gets stimulated

Avoids build-up of intermediates

Question 12

When does glycolysis get turned on?

Answer 12

When energy levels in the cell are low

Glycolysis enzymes get activated

Question 13

When does glycolysis get turned off?

Answer 13

When there are high energy levels in the cell

Question 14

Where does ATP act as a negative modulator in the glycolysis pathway?

Answer 14

Substrate inhibition of

1) PFK
2) Pyruvate kinase

Question 15

What happens when intermediates start to build up in the glycolysis pathway?

Answer 15

The cell pH starts to fall

All intermediates end in ‘ate’ = weak acids

Low pH acts as a negative modulator of PFK

Citrate is also a negative modulator of PFK

Question 16

What are the negative modulators of PFK in the glycolysis pathway?

Answer 16

ATP
Low pH
Citrate

Question 17

Why is hexokinase not heavily modulated?

Answer 17

It carries out the important role of trapping glucose-6-phosphate in the cell

Negative feedback occurs but only to a certain extent – doesn’t push activity to 0

Question 18

What is the fate of pyruvate?

Answer 18

Pyruvate enters the citric acid cycle

• It’s shuttled into the mitochondria by mitochondrial transporters
• Pyruvate converted to acetyl CoA by pyruvate dehydrogenase
• Oxidative decarboxylation
• 2 molecules of CO2 removed & 2 molecules of acetyl CoA formed

Question 19

Pyruvate dehydrogenase structure

Answer 19

132 subunit multi enzyme complex

E1 – pyruvate dehydrogenase
E2 – dihydrolipoyl transacetylase
E3 – dihydrolipoyl dehydrogenase

Vitamins required
• B1 as thiamine pyrophosphate
• Riboflavin as FADH2
• Niacin as NAD

Question 20

How does pyruvate dehydrogenase work?

Answer 20

Thiamine pyrophosphate temporarily holds onto the CO2

CO2 passed to thiamine & as pyrophosphate is released, CO2 is released

Hydrogen is first transferred to FAD to FADH2 which is then offloaded to NAD+

Question 21

What disorders are caused by a vitamin B1 deficiency?

Answer 21

Wernicke-Korsakoff encephalopathy

Beri-beri

Question 22

What is beri-beri?

Answer 22

Resembles extreme starvation – actually lots to eat just poor quality

Loss of appetite – cannot convert pyruvate to acetyl CoA when thiamine isn’t available so pyruvate builds up & glycolysis stops

No ATP generated
• Lethargic
• No nervous activity – numbness of limbs & extremities
• Eventually muscle atrophy

Question 23

Pyruvate conversion to fatty acids

Answer 23

When ATP levels are high, acetyl CoA diverted to a more efficient storage molecule

Question 24

Pyruvate conversion to amino acids

Answer 24

During anabolic growth pyruvate is aminated to non-essential amino acids

Question 25

Pyruvate conversion to lactate

Answer 25

Pyruvate is reduced to lactate by lactate dehydrogenase
• Pyruvate is a ketone
• Lactate is a secondary alcohol

Causes a build up of lactic acid in muscles
Short term access to energy

Oxygen needed to get the citric acid cycle started again & pyruvate can be used in that pathway

Lactate can then be converted back to pyruvate with the same enzyme – reversible reaction

Question 26

Pyruvate conversion to ethanol

Answer 26

Yeast and some bacteria