Control of glycolysis & the fates of pyruvate (lecture 25) Flashcards
Why do metabolic pathways need to be tightly regulated?
- To link supply with demand (storage & release)
- To allow cells/organism to respond to environmental changes (eg. Temperature, diet, microenvironment)
- To maintain a constant internal environment (homeostasis)
- To enable different tissues to interact (eg. Liver, adipose, muscle)
How is glycolysis controlled?
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
How do you control the activity of an enzyme?
Conformational change
• Allosteric control – non-covalent binding/release of effector molecule
Covalent modification (eg. phosphorylation)
How do you control the accessibility of the enzyme to its substrate?
By compartmentalisation of enzymes
By compartmentalisation of substrates
What is the different between the cytoplasm and the cytosol?
Cytoplasm = everything but the nucleus
Cytosol = cytoplasm – organelles = the soluble part of the cell
What are the 3 major controls of glycolysis?
Hexokinase
Phosphofructokinase
Pyruvate kinase
What is a major indicator of a low energy state in the cytosol in glycolysis?
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
What happens when too much fructose-6-phosphate is made than can be used in the glycolysis pathway?
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
What does adenylate kinase do?
Shuffles around phosphate groups
What are the 2 positive modulators of PFK?
AMP
Fructose-2,6-bisphosphate
What is feedforward stimulation in the glycolysis pathway?
Avoids any log jams
If there’s lots of fructose-1,6-bisphosphate, pyruvate kinase gets stimulated
Avoids build-up of intermediates
When does glycolysis get turned on?
When energy levels in the cell are low
Glycolysis enzymes get activated
When does glycolysis get turned off?
When there are high energy levels in the cell
Where does ATP act as a negative modulator in the glycolysis pathway?
Substrate inhibition of
1) PFK
2) Pyruvate kinase
What happens when intermediates start to build up in the glycolysis pathway?
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
What are the negative modulators of PFK in the glycolysis pathway?
ATP
Low pH
Citrate
Why is hexokinase not heavily modulated?
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
What is the fate of pyruvate?
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
Pyruvate dehydrogenase structure
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
How does pyruvate dehydrogenase work?
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+
What disorders are caused by a vitamin B1 deficiency?
Wernicke-Korsakoff encephalopathy
Beri-beri
What is beri-beri?
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
Pyruvate conversion to fatty acids
When ATP levels are high, acetyl CoA diverted to a more efficient storage molecule
Pyruvate conversion to amino acids
During anabolic growth pyruvate is aminated to non-essential amino acids
Pyruvate conversion to lactate
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
Pyruvate conversion to ethanol
Yeast and some bacteria
