Sean - TCA Cycle/ Krebs Cycle Flashcards
What does TCA stand for?
Tricarboxylic acid cycle
What is the function of the TCA cycle?
To produce high energy molecules such as NADH and FADH2 which act as electron carriers in the electron transport chain (ETC)
What happens in the ETC?
This is where most of the cell’s ATP is made
Where does the TCA cycle take place?
Takes place in the mitochondrion (where oxygen is utilised to generate ATP)
What is the first reaction in the TCA cycle?
(6)
Citrate synthesis
The formation of citrate from oxaloacetate and acetyl CoA
Condensation reaction catalysed by citrate synthase
Located in the mitochondria matrix
Essentially irreversible
What is citrate synthesis?
Formation of citrate from oxaloacetate and acetyl CoA
Where does citrate synthesis take place?
The mitochondria matrix
What enzyme catalyses citrate synthesis?
Citrate synthase
What happens in the second step of the TCA cycle?
(5)
Isocitrate synthesis
Reversible reaction
Two step reaction catalysed by aconitase
First step
- citrate is converted to aconitate via dehydration
Second Step
- aconitate is converted to isocitrate via hydration
The isocitrate synthesis reaction is reversible, which direction is favoured and why?
(2)
Forward reaction is favoured (citrate to isocitrate)
This is because isocitrate is quickly used up by step three of the TCA cycle
What enzyme catalyses isocitrate synthesis?
Aconitase
What happens in step three of the TCA cycle?
(6))
Alpha-ketoglutarate synthesis
Catalysed by isocitrate dehydrogenase
2 step reaction with oxalosuccinate as an intermediate
Isocitrate is oxidatively decarboxylated to oxalosuccinate
Hydrogens are given to NAD+ or NADP+
Oxalosuccinate is then converted into a-ketoglutarate
What catalyses a-ketoglutarate synthesis?
Catalysed by isocitrate dehydrogenase
What is the intermediate in a-ketoglutarate synthesis?
Oxalosuccinate
What happens in reaction 4 of the TCA cycle?
(4)
Synthesis of Succinyl-CoA
Oxidation causes a-ketoglutarate to bind with acetyl-CoA to form succinyl-CoA
Hydrogens are donated to NAD+ to form NADH + H+
Catalysed by a-ketoglutarate dehydrogenase complex
What catalyses succinyl-CoA synthesis?
a-ketoglutarate dehydrogenase complex
What must bind to a-ketoglutarate to form succinyl-CoA
Acetyl CoA
What happens in reaction 5 of the TCA cycle?
(5)
Succinate synthesis
Succinyl-CoA is converted to succinate
Cleavage of thioester bond releases energy to drive ATP/GTP synthesis
Coenzyme A is released
Catalysed by succinyl-CoA synthetase
What catalyses succinate synthesis?
Succinyl-CoA synthetase
What must be done to succinyl-CoA to form succinate?
Succinyl-CoA loses it’s CoA and it’s thioester bond is broken
How many types of succinyl-CoA synthetase exist?
Two types exist
- one for ADP
- one for GDP
How can GTP help out ADP?
GTP can donate its terminal phosphoryl group to ADP so it can form ATP
What occurs in reaction 6 of the TCA cycle?
(5)
Fumarate synthesis
Succinate is converted into succinate
This is catalysed by succinate dehydrogenase
FAD is converted into FADH2
FADH2 produced carries electrons to the ETC to produce ATP
What catalyses fumarate synthesis?
Succinate dehydrogenase
What is the energy carrier in fumarate synthesis?
FAD is reduced to FADH2
What happens in reaction 7 of the TCA cycle?
(5)
Malate synthesis
Fumarate is converted into malate
Catalysed by fumarase
Reversible reaction
Hydration reaction
What catalyses the conversion of fumarate into malate?
Fumarase
What type of reaction is malate synthesis?
Hydration reaction
What happens in reaction 8 of the TCA cycle?
(4)
Oxaloacetate synthesis
Malate is oxidised into oxaloacetate
Catalysed by L-Malate dehydrogenase
Reversible reaction
What catalyses oxaloacetate synthesis?
L-Malate dehydrogenase
How many reactions are in the TCA cycle?
Eight
List the different molecules in the TCA cycle, in order
Oxaloacetate + Acetyl CoA
Citrate
Aconitate
Isocitrate
a-Ketoglutarate
Succinyl-CoA
Succinate
Fumarate
Malate
Oxaloacetate (2)
