Describe step 1 of the TCA cycle. Is it reversible? Does it use energy?
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.
Describe step 2 of the TCA cycle. Is it reversible? Does it use energy?
- 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.
Describe step 3 of the TCA cycle. Is it reversible? Does it use energy?
- 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.
Describe step 4 of the TCA cycle. Is it reversible? Does it use energy?
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.
Describe step 5 of the TCA cycle. Is it reversible? Does it use energy?
- 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.
Remember -> aKG dehydrogenase is similar to the PDH complex meaning that it utilizes all five factors (FAD, NAD, TTP, lipoic acid and CoA-SH).
got it
Which enzyme is shared between the TCA cycle and ETC?
succinate dehydrogenase
Describe step 6 of the TCA cycle. Is it reversible? Does it use energy?
Succinate is oxidized into fumarate by succinate dehydrogenase. This reaction is coupled with the reduction of FAD into FADH2.
Describe step 7 of the TCA cycle. Is it reversible? Does it use energy?
Fumarate is hydrolyzed into L-malate by fumarase
Describe step 8 of the TCA cycle. Is it reversible? Does it use energy?
L-malate is oxidized into oxaloacetate by L-malate dehydrogenase. This reaction is coupled with the reduction of NAD+ to NADH.
What factors activate/inhibit citrate synthase. Does it make sense?
activated by AMP
inhibited by ATP, citrate, succinyl-CoA, and NADH
What factors activate/inhibit isocitrate dehydrogenase. Does it make sense?
inhibited by ATP and activated by AMP and Ca2+
When our muscles need energy Ca2+ levels are generally found in high concentrations. Which enzymes are activated by Ca2+ levels?
PK dehydrogenase, aKG dehydrogenase, and isocitrate dehydrogenase
Practice Question: Which enzyme of the citric cycle is capable of substrate level phosphorylation?
Succinyl-CoA synthetase
