Citric Acid Cycle Steps Flashcards
1) Citrate Synthase
This catalyses condensation of acetyl-CoA and ocaloacetate, forming citrate.
1) Reactive Pathway of Citrate Synthase
O binds, which CC a two-domain cleft containing O binding site, where binding initiates 18 degree rotation, closing cleft, revealing ACOA binding site, and sealing the O one.
1) Why is it important the Oxaloacetate cleft closes?
So solvents cannot reach it
2) Aconitase
Catalyses isomerization of citrate and isocitrate, with a cis-isocitrate intermediate.
2) Reactive pathway of Aconitase
A proton/OH group are removed from citrate, where aconitase FE-S cluster facilitatse OH group elmination; cis-Isocitrate is then rehydrated,
2) How does aconitase ensure the correct of citrates stereoisomers are formed?
Two carboxymethyl groups substituent to its central C, so water can only be eliminated from one of the carboxymethyl arms(the lower one)
Chiral
Something asymmetric in such a way that it and its mirror image are NOT superimposable
2) Why is citrate prochiral?
Because although it is achiral(lacking a stereocenter), it can be converted to being chiral (isocitrate)
2) Why is the prochirality of Citrate important?
It allows aconitase to distinguish between the two arms.
Iron-Sulfur Cluster
An arrangement of four sulfur and four iron atoms.
3) NAD+ dependent isocitrate dehydrogenase
This catalyses oxidative decarboxylation of isocitrate to a-ketoglutarate
3) Reaction pathway of ICD?
Catalysis of oxidation of isocitrate to an intermediate, oxalosuccinate, followed by decarboxylation to a second intermediate, then formation of a-ketoglutarate.
4) a- Ketoglutarate Dehydrogenase
Catalyses oxidative decarboxylation of a-Ketoglutarate, producing second CO2, forming succinyl COA.
4) What is the reaction pathway of AKGD?
It is a ME, with identical reaction pathway to PDC.
5) Succinyl-CoA thiokinase
Couples high-energy S-COA cleavage to synthesis of high-energy NTP, forming succinate.
5) What are the three steps of Succinyl-CoA thiokinase?
Succinyl-CoA reacts with Pi, forming succinyl phosphate and CoA
The phosphate is transferred from succinyl phosphate to a his residue on the thiokinase
Phosphate is transferred to GDP, forming GTP.
5) How is energy conserved through Succinyl-CoA thiokinase reaction pathway?
Formation of high energy compounds
5) What is the culmination of the first 5 steps?
One acetyl oxidised to two CO2, with formation of 2 NADH and one GTP
6) Succinate Dehydrogenase
This causes dehydrogenation of succinate to fumarate
What is the only membrane bound CAC enzyme?
Succinate Dehydrogenase
6) What is succinate dehydrogenase dependent on?
Reoxidation of FADH2, as it is dependent on it.
6) How does succinate dehydrogenase reoxidise FADH2?
Passing its electrons to mitochondrial ETC.
7) Fumarase
Catalyses hydration of fumarate, forming malate
8) Malate Dehydrogenase
This regnerates oxaloacetate, oxidising the OH group of malate.
8) What is the DeltaG of malate dehydrognease?
+29.7kJ
8) How is malate dehydrogenase drive forwards, despites its positve DeltaG?
The exergonic nature of citrate synthase in the first step,, with a negative -31.5 kJ, due to thioester bond cleavage.
What properties are identified to know how a metabolic pathway is controlled?
The enzymes catalyzing the rate-determining steps, the effectors of the enzymes and the concentrations of these effecetors.
What is required for identifying rate determining steps?
Determining DeltaG of each of its reactions from concentration of substrates and prodcuts.
Which enzymes regulate CAC?
Citrate Synthase, Isocitrate Dehydrogenase and the AKDC
What is metabolite flux through the CAC proportional to?
O2 consumption
How is CAC metabolite flux proportional to O2 consumpton?
As in O2 consumption, NADH reoxidation and ATP production are tightly coupled, so CAC regulates based on feedback mechanisms co-ordinating NADH production with energy expenditure.
What are CAC enzyme regulation dependent on?
Substrate availability
Product inhibition
Competetive feedback inhibtion by intermediates further down the cycle.
What are the most critical regulators of the CAC?
Acetyl- CoA, Oxaloacetate and NADH.
Why is citrate synthase rate variable by substrate concentration?
Because ACOA and O are present in the mitochondria in concentratsion that do not saturate CS.
What does Oxaloacetate concentration fluctuate with?
NADH/NAD+ ratio
How does Aconitase distinguish between the two arms of citrate?
Its central carbons bound groups are in different dimensions of space, so the central carbons hydroxyl will always be in the same plane as the lower arm.
