TCA Cycle

Question 1

Describe step 1 of the TCA cycle. Is it reversible? Does it use energy?

Answer 1

Acetyl-CoA and oxaloacetate react to form citrate which is catalyzed via citrate synthase.
This condensation reaction is irreversible and utilizes the energy from cleaving the thiolester bond of acetyl-CoA to drive the reaction forward.

Question 2

Describe step 2 of the TCA cycle. Is it reversible? Does it use energy?

Answer 2

• Citrate is dehydrolyse by aconitase to form cis-aconitate (intermediate).
• Cis-aconitate is hydrolysed by aconitase to form isocitrate.
  The purpose of this, is to form a secondary carbon (bearing the carboxyl group) so that is able to undergo oxidation on the next step.
  It does not use energy.

Question 3

Describe step 3 of the TCA cycle. Is it reversible? Does it use energy?

Answer 3

• Isocitrate is first converted into oxalosuccinate (intermediate) by isocitrate dehydrogenase. This reaction is driven by the reduction of NAD+ into NADH.
• Oxalosuccinate undergoes decarboxylation which allows C3 to fully oxidized and form a-KG.
  This reaction is irreversible.

Question 4

Describe step 4 of the TCA cycle. Is it reversible? Does it use energy?

Answer 4

aKG is oxidized into succinyl-CoA by aKG dehydrogenase.
This reaction is driven forward by the reduction of NAD+ to NADH.
This reaction is irreversible.

Note: the COO- is replaced by CoA-SH. This releasing CO2 as a byproduct.

Question 5

Describe step 5 of the TCA cycle. Is it reversible? Does it use energy?

Answer 5

• Succinyl-CoA synthetase phosphorylates succinyl-CoA to release CoA and form succinate-phosphate.

Succinyl-CoA synthase transfers the phosphate group onto ADP/GDP to generate ATP/GTP and succinate.

Question 6

Remember -> aKG dehydrogenase is similar to the PDH complex meaning that it utilizes all five factors (FAD, NAD, TTP, lipoic acid and CoA-SH).

Answer 6

got it

Question 7

Which enzyme is shared between the TCA cycle and ETC?

Answer 7

succinate dehydrogenase

Question 8

Describe step 6 of the TCA cycle. Is it reversible? Does it use energy?

Answer 8

Succinate is oxidized into fumarate by succinate dehydrogenase. This reaction is coupled with the reduction of FAD into FADH2.

Question 9

Describe step 7 of the TCA cycle. Is it reversible? Does it use energy?

Answer 9

Fumarate is hydrolyzed into L-malate by fumarase

Question 10

Describe step 8 of the TCA cycle. Is it reversible? Does it use energy?

Answer 10

L-malate is oxidized into oxaloacetate by L-malate dehydrogenase. This reaction is coupled with the reduction of NAD+ to NADH.

Question 11

What factors activate/inhibit citrate synthase. Does it make sense?

Answer 11

activated by AMP
inhibited by ATP, citrate, succinyl-CoA, and NADH

Question 12

What factors activate/inhibit isocitrate dehydrogenase. Does it make sense?

Answer 12

inhibited by ATP and activated by AMP and Ca2+

Question 13

When our muscles need energy Ca2+ levels are generally found in high concentrations. Which enzymes are activated by Ca2+ levels?

Answer 13

PK dehydrogenase, aKG dehydrogenase, and isocitrate dehydrogenase

Question 14

Practice Question: Which enzyme of the citric cycle is capable of substrate level phosphorylation?

Answer 14

Succinyl-CoA synthetase