Chapter 10: The Citrate Cycle Flashcards
How does the citrate cycle capture energy
Redox reactions
What are two biomolecules that the citrate cycle generates?
- NADH and FADH2
Why is the Citrate cycle so important?
It is central to aerobic metabolism and ATP production
What is the primary function of Acetyl CoA? What does each Citrate cycle yield.
- The primary function is to oxidize acetyl CoA
Each cycle yields:
- Transfer of 8 electrons
- Generates 3 NADH
- Generates 1 FADH2
- 1 GTP (ATP equivalent)
- Each cycle results in production of 10 ATP
- Mostly via oxidative phosphorylation of 3NADH and 1 FADH2
What converts Pyruvate into acetyl-CoA?
Pyruvate dehydrogenase
What five coenzymes are required for the pyruvate dehydrogenase complex?
- NAD+
- FAD
- CoA
- Thiamine pyrophosphate (TPP)
- Alpha-lipoic acid
What part of NAD+ and NADP+ is reduced?
Nicotinamide
What is NAD+ derived from?
Niacin (vitamin B3)
What is FAD derived from?
Riboflavin (vitamin B5)
What is CoA derived from?
Vitamin B5
- Generated by Pantothenic acid
What is Thiamine pyrophosphate (TPP) derived from?
Vitamin B1
- Required for pyruvate dehydrogenase
What does α-lipoic acid do?
- Provides reactive disulfide that participates in redox reactions
- Accepts and transfers acetyl group to CoA
- Not a vitamin
What are the five steps in pyruvate dehydrogenase?
1.Pyruvate binds and is decarboxylated causing the formation of hydroxyethyl-TPP
2. Acetyl group transferred to lipoamide
3. Acetyl group transfered to CoA
4. Dihydroxylipoamide oxidized to lipoamide
5. FAD oxidized forming FADH2 which can reduce NAD+ to NADH
Describe the general structure of PDH protein complex
- 22 E1 subunits
- 60 E2 subunits
- 6 E3 subunits
How does Arsenic affect Pyruvate dehydrogenase?
- Irreversibly blocks catalytic activity of lipoamide-containing enzymes
Describe the steps of E1, E2, and E3 in the ball and chain PDH
- E2 ball and chain move lipoamide domain between E1 and E2 catalytic site
- Acetyl group from TPP in E1 to the CoA substrate in the E2 catalytic site
What are negative and positive regulators of PDH?
Positive:
- NAD+
- CoA
- ADP
Negative:
- NADH
- Acetyl- CoA
- ATP
How many reactions are in the citrate cycle?
8
How favorable or unfavorable are the equations in the citrate cycle?
- 3 reactions have a large -ΔG°’ and thus are key regulators
- 4 reactions have a ΔG°’ close to 0
- 1 reaction has a large +ΔG°’
What happens in step 1 of the citrate cycle?
Reaction: Condensation
Enzyme: Citrate synthase
Reactants: Oxaloacetate, Acetyl-CoA, and H2O
Product: Citrate
(Highly Favorable)
What is the mechanism behind citrate synthase?
- Asp375 deptrotenates acetyl-CoA -> enolate intermediate (stabilized by H bond of His274)
- Nu attack by enolate in on oxaloacetate -> citryl-CoA + deprotonation of His320
- Water is added which helps hydrolyze citryl-CoA thioester
How does conformation affect binding in citrate synthase?
- Open conformation promotes oxo binding
- Closed conformation promotes acetyl-CoA binding
- Prevents acetyl-CoA thioester hydrolysis
What is step 2 in the citrate cycle?
Reaction: dehydration-hydration
Enzyme: Aconitase
Reactants: Citrate and H2O
Product: (intermediate cis-aconitate) Isocitrate
What is the function of the Fe-S cluster enzyme?
- Facilitates the removal of OH group from citrate