Citric Acid Cycle Steps

Question 1

1) Citrate Synthase

Answer 1

This catalyses condensation of acetyl-CoA and ocaloacetate, forming citrate.

Question 2

1) Reactive Pathway of Citrate Synthase

Answer 2

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.

Question 3

1) Why is it important the Oxaloacetate cleft closes?

Answer 3

So solvents cannot reach it

Question 4

2) Aconitase

Answer 4

Catalyses isomerization of citrate and isocitrate, with a cis-isocitrate intermediate.

Question 5

2) Reactive pathway of Aconitase

Answer 5

A proton/OH group are removed from citrate, where aconitase FE-S cluster facilitatse OH group elmination; cis-Isocitrate is then rehydrated,

Question 6

2) How does aconitase ensure the correct of citrates stereoisomers are formed?

Answer 6

Two carboxymethyl groups substituent to its central C, so water can only be eliminated from one of the carboxymethyl arms(the lower one)

Question 7

Chiral

Answer 7

Something asymmetric in such a way that it and its mirror image are NOT superimposable

Question 8

2) Why is citrate prochiral?

Answer 8

Because although it is achiral(lacking a stereocenter), it can be converted to being chiral (isocitrate)

Question 9

2) Why is the prochirality of Citrate important?

Answer 9

It allows aconitase to distinguish between the two arms.

Question 10

Iron-Sulfur Cluster

Answer 10

An arrangement of four sulfur and four iron atoms.

Question 11

3) NAD+ dependent isocitrate dehydrogenase

Answer 11

This catalyses oxidative decarboxylation of isocitrate to a-ketoglutarate

Question 12

3) Reaction pathway of ICD?

Answer 12

Catalysis of oxidation of isocitrate to an intermediate, oxalosuccinate, followed by decarboxylation to a second intermediate, then formation of a-ketoglutarate.

Question 13

4) a- Ketoglutarate Dehydrogenase

Answer 13

Catalyses oxidative decarboxylation of a-Ketoglutarate, producing second CO2, forming succinyl COA.

Question 14

4) What is the reaction pathway of AKGD?

Answer 14

It is a ME, with identical reaction pathway to PDC.

Question 15

5) Succinyl-CoA thiokinase

Answer 15

Couples high-energy S-COA cleavage to synthesis of high-energy NTP, forming succinate.

Question 16

5) What are the three steps of Succinyl-CoA thiokinase?

Answer 16

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.

Question 17

5) How is energy conserved through Succinyl-CoA thiokinase reaction pathway?

Answer 17

Formation of high energy compounds

Question 18

5) What is the culmination of the first 5 steps?

Answer 18

One acetyl oxidised to two CO2, with formation of 2 NADH and one GTP

Question 19

6) Succinate Dehydrogenase

Answer 19

This causes dehydrogenation of succinate to fumarate

Question 20

What is the only membrane bound CAC enzyme?

Answer 20

Succinate Dehydrogenase

Question 21

6) What is succinate dehydrogenase dependent on?

Answer 21

Reoxidation of FADH2, as it is dependent on it.

Question 22

6) How does succinate dehydrogenase reoxidise FADH2?

Answer 22

Passing its electrons to mitochondrial ETC.

Question 23

7) Fumarase

Answer 23

Catalyses hydration of fumarate, forming malate

Question 24

8) Malate Dehydrogenase

Answer 24

This regnerates oxaloacetate, oxidising the OH group of malate.

Question 25

8) What is the DeltaG of malate dehydrognease?

Answer 25

+29.7kJ

Question 26

8) How is malate dehydrogenase drive forwards, despites its positve DeltaG?

Answer 26

The exergonic nature of citrate synthase in the first step,, with a negative -31.5 kJ, due to thioester bond cleavage.

Question 27

What properties are identified to know how a metabolic pathway is controlled?

Answer 27

The enzymes catalyzing the rate-determining steps, the effectors of the enzymes and the concentrations of these effecetors.

Question 28

What is required for identifying rate determining steps?

Answer 28

Determining DeltaG of each of its reactions from concentration of substrates and prodcuts.

Question 29

Which enzymes regulate CAC?

Answer 29

Citrate Synthase, Isocitrate Dehydrogenase and the AKDC

Question 30

What is metabolite flux through the CAC proportional to?

Answer 30

O2 consumption

Question 31

How is CAC metabolite flux proportional to O2 consumpton?

Answer 31

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.

Question 32

What are CAC enzyme regulation dependent on?

Answer 32

Substrate availability
Product inhibition
Competetive feedback inhibtion by intermediates further down the cycle.

Question 33

What are the most critical regulators of the CAC?

Answer 33

Acetyl- CoA, Oxaloacetate and NADH.

Question 34

Why is citrate synthase rate variable by substrate concentration?

Answer 34

Because ACOA and O are present in the mitochondria in concentratsion that do not saturate CS.

Question 35

What does Oxaloacetate concentration fluctuate with?

Answer 35

NADH/NAD+ ratio

Question 36

How does Aconitase distinguish between the two arms of citrate?

Answer 36

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.