Lecture 30: The fate of Pyruvate REVISIT!! Flashcards
26/11/2024
Does glycolysis require oxygen?
NOOO
How is pyruvate oxidised?
It has a carbon dioxide removed and is hence converted into acetyl-CoA
Did Harden and Young find out that inorganic phosphate is a limiting factor for photosynthesis?
Yes
Do we have to worry about running out of ATP?
No, because there is a constant interconversion between ATP and ADP
What happens if cofactors, like NAD+, run out?
Then glycolysis stops
What is done to pyruvate to restore the supply of NAD+?
What can pyruvate be converted to?
Ehanol in yeast cells, lactate in the muscles, acetyl coA(which can be further oxidised) under aerobic conditions in citric acid cycle
How is NAD+ regenerated aerobically?
- Pyruvate us oxidised to acetyl coA in the citric acid cycle.
- In the electron transport chain, NADH transfers electrons to Oxygen , forming water and regenerating NAD+.
-
Is it true that the regenration of NAD+ requires reduction of another compound?
Yes
What do anaerobic conditions lead to ?
fermentation
Define fermentation
“An ATP producing process in which organic compounds act both as electron donors and as electron acceptors”
(Glucose oxidised to pyruvate (glucose donates e-)
)
( Pyruvate reduced to ?? (pyruvate accepts e-)
)
when is lactate the fermentation product?
In animal muscle, red blood cells, some cancer cells, lactobacillus:
when is ethanol the fermentation product?
In plants and yeast
In which organisms can the fermentation product be: Acetate and other carboxylic acids, hydrogen etc?
In Various other microorganisms
How often are animal and plant cells usually anaerobic?
Animal and plant cells usually temporarily anaerobic
When are animal cells usually anerobic?
During strenuous exercise
Are cancer cells usually anaerobic?
Yes
Why are red blood cells anaerobic?Because they don;t have a mitochondria
Give some examples of environments that are permannetly anaerobic
Bacteria in marine and lake sediments, marshes, animal gut
What was the early atmosphere like?
Anaerobic, so glycolysis could be run
Why do we need to further oxidise pyruvate in the citric acid cycle?
Because this is the only way to release all of glucose’s energy, as the majority of pyruvate’s energy will still be stored in pyruvate at the end of glycolysis.
So which produces more ATP anaerobic respiration or aerobic respiration?
Aerobic respiration
Where does the citric acid cycle take place?
In the mitochondrial matrix
How does pyruvate reach the mitochondrial matrix?
It is cotransported there with H+ ions
What happens after pyruvate is cotransported into the mitochondria matrix?
It is decarboxylated, then converted into acetyl coA. This uses up a NAD+ and regenerates an NADH
Why were original studies of the movement of pyruvate from the cytosol to the mitochondrial matrix conducted on liver and on pigeons?
Because they’re both incredibly rich in mitochondria
After pyruvate is converted into acetly coA, what can happen?
The acetyl can then enter the citric acid cycle and couple with 4C oxaloacetate to form 6C Citrate
What happens after ocaloacetate is converted into citrate?
2 Carbon dioxide’s are lost, and oxaloacetate is constantly reformed, and the cycle repeats over and over again
What happens when fatty acids are broken down into acetyl coA?
They’re oxidised in exacrtly the same way that acetly coA from glucose is (integration point between the 2 main sources of energy, fatty acids and sugars)
Is it true that 2 carbon dioxide’s are released for each acetyl group?
Yes
What is the reactive group of coenzyme A?
The SH group
Draw out the structure of coenzyme A
What part of coenzyme A links to the acetate group?
The SH group
Draw out the structure of acetyl coA
Draw out the reaction for the conversion of pyruvate to acetyl CoA
How do we know that the reaction for the conversion of pyruvate to acetyl CoA is irreversible?
Because the ΔG«0
(no going back to glucose)
What catalyses the conversion of pyruvate to acetyl CoA?
pyruvate dehydrogenase multienzyme complex
Is it true that Pyruvatedehydrogenase complex is very big?
Yes, it is 4-10 x106 kDa
i.e. larger than ribosomes
How many different enzymes does the pyruvate dehydrogenase complex consist of?
3
What are the 3 enzymes that the pyruvate dehydrogenase complex consists of?
pyruvate dehydrogenase component (E1)
dihydrolipoyl transactetylase (E2)
dihydrolipoyl dehydrogenase (E3)
What is the function of E1?
decarboxylation of pyruvate
What is the function of E2?
Transfer of the acetyl group to CoA
What is the function of E3?
Reoxidation + the regeneration of the oxidised form of lipoamide
Describe E1: Pyruvate dehydrogenase component
- Derived from vitamin B1
- Found in the outer seed coats of cereals incl. rice.
- Deficiency in man
results in “beri-beri” - Not simple process, requires TPP cofactor, which particpates in the decarboxylation of
pyruvate - Hydrogen is lost, resulting in a negative C
Describe E2: Dihydrolipyol transacetylase
Acetyl group is picked off TPP and transferred to CoA ADD MORE DETAIL
As disulfide has been reduced, it has to be re-oxidised
Describe E3: Dihydrolipyol dehydrogenase
Reoxidation of disulphide in lipoamide
Reoxidation of FADH2 generates 1 NADH
Which reactions are catalysed by the pyruvate dehydrogenase
multi-enzyme complex
1)
What is the equation that represents the conversion of pyruvate to acetyl-CoA?
Pyruvate + CoA-SH + NAD+ —–> Acetyl-CoA + CO2 + NADH
What is a multienzyme complex?
A group of 2 or more non-covalently associated enzymes that catalyse two or more sequential steps in a metabolic pathway
What are the advantages of multienzyme complexes?
- Product of the first reaction in the sequence remains attached
- Serves directly as a substrate for the second reaction
- Therefore rate of second reaction not limited by diffusion
- Can channel intermediates between successive enzymes, thereby minimising side reactions
- The reactions may be coordinately regulated
How is pyruvate dehydrogemase controlled?
By phosphorylation. Kinase phosphorylates the enzyme. Kinase is stimulated by high energy status (AcetylCoA, ATP and NADH
) and inhibited by low energy status (ADP and NAD+
)
The Sparker Effect
Who discovered the citric acid cycle?
Szent-Györgyi and Krebs (both got Nobel prizes)
Describe the sparker effect
- Mince liver to release its enzymes, add pyruvate, and measure oxygen consumption- should find that not much happens
- Then add the trace of an organic acid with the pyruvate and mince liver, Should find that oxygen consumption is higher.
- Realised that something must be being oxidised. The organic acids weren’t used up so it could not be them .
- The interpretations from this were that; organic acids work catalytically and one molecule of the organic acid can spark oxidation of an infinite amount of pyruvate
- They repeated the experiment and realised that succinate accumulated and the same thing happened if a trace amount of fumurate was added. This proved that the pathwya was cyclic
What can organic acids include?
Organic acids: citrate, fumarate, malate or succinate
What is malonate a potent inhibitir for?
Malonate was found to be a potent inhibitor of respiration in all animal tissues
What are the other names of the citric acic cycle?
Krebs cycle
Tricarboxylic Acid Cycle TCA cycle
What are the principle negative regulators of the citric acid cycle?
ATP and NADH are the principal negative regulators . The need for energy and for carbon skeletons is the main positive regulator
If oxaloacetate is depleted from the citric acid cycle, what will happen?
The cycle will stop
How is the withdrawal of citric acid cycle intermediates rectified?
Achieved by anaplerotic (ana pleros: to fill up, replenish) reactions. In humans, the main anaplerotic reaction introduces more oxaloacetate into the cycle, via carboxylation of pyruvate: Pyruvate+ CO2+ATP+H2O 🡪 oxaloacetate+ADP+Pi+2H+
(Catalysed by pyruvate carboxylase )
is is true that ‘Glucogenic amino acids can be used in anaplerotic reactions if needed
Ketogenic amino acids can not’?
Yes
is it true that Citric acid cycle is a shared pathway for breakdown of carbohydrates, fats and amino acids?
Yes
is it true that the citric acid cycle mainly generates reduced coenzymes
?
Yes
why are intermediates sometimes taken out of the citric acid cycle?
For biosynthesis i.e amino acid synthesis
Do fats make sugar?
No
What does each cycle of the citric acid/krebs cycle produce?
3 NADH
FADH2
ATP
Per glucose molecule, which produces 2 pyruvates
6 NADH
2 FADH2
2ATP
