The TCA Cycle Flashcards
What does TCA stand for in ‘TCA cycle’.
Tricarboxylic acid
What are TWO alternative names for the TCA cycle?
- The Krebs Cycle
- The Citric Acid Cycle (CAC)
What is the net yield of the TCA cycle?
(i.e. from two turns of the cycle)
- 2 ATP (generated from 2 GTP)
- 6 NADH
- 2 FADH2
4 CO2 molecules are given off as byproducts during the cycle.
The intermediates of the TCA cycle are important. Why?
Some act as precursors for other (biosynthetic) pathways.
How many electrons are liberated from the oxidation of glucose to CO2?
24 e-
These are ‘captured’ in NADH and FADH2.
Where does the TCA cycle occur?
The mitochondrial matrix.
All but one of the TCA cycle enzymes is found here too.
What is the TCA cycle?
The aerobic oxidation of 2 acetyl-CoA to 2 CO2.
How many enzyme catalysed reactions make up the TCA cycle?
8
How many of the eight reactions of the TCA cycle are oxidation reactions, and which steps are these?
4: steps 3, 4, 6, & 8.
During these NAD+ (or FAD in one) is reduced.
Which step of the TCA cycle produces GTP?
(and ultimately ATP)
Step 5
Which steps of the TCA cycle are irreversible and the driving force behind it?
Steps 1, 3, & 4.
These are coupled with non-spontaneous reactions to drive them in the forward direction.
Thus, these steps and their enzymes are highly regulated.
Why is the TCA cycle necessary to fully oxidise acetyl-CoA?
Oxidation of acetyl groups requires a C-C cleavage (i.e. either α- or β-cleavage), which acetyl-CoA DOES NOT contain or is capable of in its original form.
The cycle involves converting acteyl-CoA into citrate by combining it with oxaloacetate in the first step, and then going from there.
Identify:
The enzyme, substrate, end product, and any other important byproducts/reactants of the TCA cycle reaction I.
- Citrate synthase
- Condensation reaction combining acetyl-CoA and oxaloacetate to form citrate.
Citrate synthase is an allosteric enzyme that is inhibited by NADH and succinyl-CoA.
Identify:
The enzyme, substrate, end product, and any other important byproducts/reactants of the TCA cycle reaction II.
- Aconitase
- Isomerisation of citrate to isocitrate (via a cis-aconitate intermediate).
Citrate is a 3° alcohol and so is a poor substrate for oxidation - thus it is converted into isocitrate in this step.
Because of citrate’s COOH arms, this is a stereospecific reaction.
Identify:
The enzyme, substrate, end product, and any other important byproducts/reactants of the TCA cycle reaction III.
- Isocitrate dehydrogenase
- Oxidative decarboxylation in which two hydrides are extracted from isocitrate to form oxalosuccinate intermediate.
- This then undergoes β-decarboxylation which removes the COOH from the βC as CO2 to form α-ketoglutarate.
Isocitrate dehydrogenase requires a cofactor and is a point of allosteric regulation.
Identify:
The enzyme, substrate, end product, and any other important byproducts/reactants of the TCA cycle reaction IV.
- α-ketoglutarate dehydrogenase
- Oxidative decarboxylation of α-ketoglutarate to form succinyl-CoA.
α-KG DH is almost identical to the PDH complex, and also has 5 coenzymes.
The high energy thioester bond in succinyl-CoA has a high potential energy for hydrolysis, which is important in the next step of the cycle.
Identify:
The enzyme, substrate, end product, and any other important byproducts/reactants of the TCA cycle reaction V.
- Succinyl-CoA synthetase
- Hydrolysis of the unstable thioester bond of succinyl-CoA to form succinate.
- Coupled with GTP synthesis.
The phosphate added to GDP to form GTP is then taken by ADP to form ATP (substrate-level phosphorylation).
Identify:
The enzyme, substrate, end product, and any other important byproducts/reactants of the TCA cycle reaction VI.
- Succinate dehydrogenase
- Redox/dehydration reaction whereby succinate is oxidised to trans, unsaturated fumarate.
- Coupled with reduction of FAD to FADH2.
SDH is special as it is the only enzyme of the TCA cycle that is bound to the inner mitochondrial membrane and participates in the ETC as well.
Malonate is a competitive inhibitor of SDH, and naturally occurs within cells.
Identify:
The enzyme, substrate, end product, and any other important byproducts/reactants of the TCA cycle reaction VII.
Identify:
The enzyme, substrate, end product, and any other important byproducts/reactants of the TCA cycle reaction VIII.
Name the TWO key allosteric inhibitors of citrate synthase.
NADH & succinyl-CoA
How is citrate recognised in the active site of an enzyme?
Its COOH arms are stereochemically different.
Why must citrate be converted to isocitrate in the TCA cycle?
Citrate is a 3° alcohol, which is a poor substrate for oxidation.
What is the purpose of the iron-sulphur clusters bound to cysteine residues in the structure of aconitase?
(i.e. its mechanistic role in the TCA cycle)
These allow aconitase to adjust the orientation of citrate by coordinating OH & COOH groups, and act as a Lewis acid/e- carrier via the iron.
This facilitates the isomerisation reaction, as OH is NOT a good leaving group.
What is the ‘trojan horse inhibitor’ of the TCA cycle, and which enzyme does it affect?
Fluoroacetate (1080) which covalently binds aconitase and blocks the TCA cycle.
It has a similar structure to pyruvate and so gets converted into FA-CoA earlier in the cycle.
Which enzyme of the TCA cycle is almost identical to the PDH complex of the prepatory stage?
α-ketoglutarate dehydrogenase
Name the THREE enzymes that make up the α-ketoglutarate dehydrogenase complex.
- E1: α-ketoglutarate dehydrogenase
- E2: Dihydrolipoyl transsuccinylase
- E3: Dihydrolipoyl dehydrogenase
E3 is identical to the PDH complex.
True or False:
The α-ketoglutarate dehydrogenase complex has the exact same five cofactors as the PDH complex.
True
These are TPP, lipoic acid, NAD+, FAD, and CoA.
What is the purpose of the final three steps in the TCA cycle?
To regenerate oxaloacetate.
Which enzyme of the TCA cycle is the only one bound to the inner mitochondrial membrane and also participates in the ETC?
Succinate dehydrogenase (SDH)
