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
What is fluoroacetate and what does it do?
- Converted to Fluoroacetyl-CoA then Fluorocitrate
- Potent inhibitor of Aconitase
What is step 3 in the citrate cycle?
Reaction: Oxidative decarboxylation
Enzyme: Isocitrate dehydrogenase
Reactants: Isocitrate, NADP+, and H+
Product: NADPH, (intermediate oxalosuccinate), CO2, and alpha ketoglutarate
Which step is the rate limiting reaction in the citrate cycle?
Step 3 or isocitrate dehydrogenase
What are positive and negative effectors of step 3 in the Citrate cycle?
Positive:
- ADP and CA2+
Negative:
- ATP and NADH
What is step 4 in the citrate cycle?
Reaction: Decarboxylation
Enzyme: α-Ketoglutarate Dehydrogenase (E1, E2, E3)
Reactants: α-Ketoglutarate, CoA, and NAD+
Product: CO2, NADH +H+, and Succinyl-CoA
(Same reaction as Pyruvate dehydrogenase)
(Energetically favorable)
What is step 5 in the citrate cycle?
Reaction: Substrate level phosphorylation
Enzyme: Succinyl-CoA synthetase
Reactants: Succinyl-CoA, Pi, and GDP
Product: CoA, GTP, and Succinate
What is step 6 in the citrate cycle?
Reaction: Oxidation
Enzyme: Succinate dehydrogenase
Reactants: Succinate and FAD
Product: FADH2 and Fumarate
(part of ETC)
What is step 7 in the citrate cycle?
Reaction: Hydration
Enzyme: Fumarase
Reactants: Fumarate, OH-, and H+
Product: (carbanion intermediate) and Malate
(stereospecific L-isomer)
What is step 8 in the citrate cycle?
Reaction: Oxidation
Enzyme: Malate dehydrogenase
Reactants: Malate and NAD+
Product: NADH + H+ and Oxaloacetate
What does one citrate cycle produce
- One cycle:
- 3 NADH
- 1 FADH2
- 1 GTP
- Equals 10 ATP
- 2 carbons enter as Acetyl-CoA
- Leave cycle via decarboxylation as CO2
- Not same carbons as Carbons eliminated in step 1
What are the three main regulatory regulatory enzymes in the citrate cycle?
- Citrate Synthase
- Isocitrate synthase
- α-ketoglutarate
dehydrogenase
How can culture conditions affect the citrate cycle? How can citrate export increase?
- Citrate cycle can be inhibited by culture conditions
- Inhibitions of Citrate lyase results in increase citrate export
The citrate cycle is an ________ pathway.
Amphibolic (both anabolic and catabolic
What is an anaplerotic reaction?
A reaction that replenishes Citrate cycle intermediates from other metabolic pathways
Examples:
Pyruvate carboxylase
Requires biotin
What is biotin?
A CO2 carrier
What does phosphoenolpyruvate carboxylase do?
Replenish Oxaloacetate
What are the Two main functions of the citrate cycle?
Function 1
- Oxidize acetyl-CoA
- Yields 3 NADH, 1 FADH2, 1 GTP
- Can produce 10 ATP
Function 2
- Provides metabolites for other pathways
- Amino acids
- Heme
- Fatty acids
