25-01-22 - Citric Acid Cycle and Respiratory Chain Flashcards
Learning outcomes
- Describe the conversion of pyruvate to acetyl coenzyme A and role of acetyl-coA in entering the citric acid cycle
- Explain where in the cell the citric acid cycle and oxidative phosphorylation take place
- Describe the steps in the citric acid cycle which generate reduced coenzymes, energy and metabolic waste
- Explain the role of the electron transport chain in oxidising coenzymes and generating a hydrogen ion gradient for the synthesis of ATP
- Know the number of products generated for each stage of cellular respiration and the overall net yield of ATP, for the breakdown of one molecule of glucose
What occurs during glycolysis?
What occurs after glycolysis in aerobic conditions?
What occurs in anaerobic conditions?
- During glycolysis, glucose is oxidated to generate pyruvate, with a net of 2 ATP+ 2 NADH + H+
- After glycolysis, and in aerobic conditions, pyruvate and 2 NADH + H+ is transported to the mitochondria
- In anaerobic conditions, pyruvate is reduced to lactate and NAD+ is regenerated to enable glycolysis to continue (short term)
What are the 3 steps in a quick overview of cellular respiration?
Which phases are aerobic?
1) Glycolysis
• Generation of pyruvate to form acetyl CoA
• Fatty acids and some amino acids can also be used to generate Acetyl CoA
2) Citric Acid Cyle
• Redox reactions to harness energy via electron carriers (NAD+ and FAD), producing CO2
3) Oxidative phosphorylation
• Oxidation of coenzymes
• Electron transfer and reduction of O2 and ATP synthesis via ADP phosphorylation
• Stages 2 and 3 are aerobic phases of cellular respiration, where O2 is consumed and CO2 is produced
Where does Acetyl CoA synthesis occur?
What are the 2 stages to the synthesis of Acetyl CoA synthesis?
What is Acetyl CoA?
How much energy does hydrolysis of one of its bonds produce?
- Acetyl CoA synthesis occurs in the mitochondrial matrix
- Acetyl CoA synthesis steps:
1) Pyruvate (and fatty acids/amino acids) are degraded into acetyl groups
2) Acetyl groups are added to coenzyme A (CoA) to form Acetyl CoA
- Acetyl CoA is a very high energy compound
- Hydrolysis of the thioester bond present in it results in ΔG = -33kh/mol, which is more than ATP hydrolysis at -30.5kj/mol
What are the mitochondria the site of?
What do number of cristae in mitochondria depend on?
What reactions take place in the mitochondria?
How are ionic gradients generated in mitochondria?
What is this key to?
- The mitochondria are the site of eukaryotic oxidative metabolism
- The number of cristae present in mitochondria depend on the metabolic requirements of the cell
- All reactions onwards, from metabolism of pyruvate take place in the matrix of the mitochondria
- Ionic gradients can be generated due to the controlled impermeability of the inner membrane of the mitochondria to most ions and metabolites
- This is key to ATP synthesis
What is the citric acid cycle?
What needs to happen for the citric acid cycle to begin?
What then happens to the compound formed?
What 4 things does each cycle form?
What is regenerated at the end of each cycle?
- The citric acid cycle is the common pathway by which all fuel molecules are broken down to CO2 and H2O
- Before the citric acid cycle begins, with 2 carbon acetyl CoA is condense with 4-carbon oxaloacetate to generate the 6-carbon citrate
- This citrate is then broken down in stages to harness energy via electron carriers (NAD+ and FAD)
• Each cycle of the citric acid cycle forms:
1) 2 x CO2
2) 1 x GTP
3) 3 X NADH + H+
4) 1 X FADH2
• Plus 1 x CO2 and 1 X NADH + H+ generated per pyruvate converted to acetyl CoA
• At the end of each cycle, the 4-carbon oxaloacetate is regenerated, which can then be used for another cycle
What is the first stage of the citric acid cycle?
What type of reaction is this?
What is this energy used for?
What is liberated from this reaction?
What is it used for?
- The first stage of the citric acid cycle is the condensation of the acetyl group (2-carbon) of acetyl CoA with the keto acid oxaloacetate (4 caron) by citrate synthase to generate citrate
- This is a highly exergonic reaction due to the thioester bond in the Acetyl CoA’s large -ΔG
- This energy is used to drive the cycle forward
- Coenzyme A is liberated from this reaction, and can then be used to generate more acetyl CoA, or can be used on intermediates during the citric acid cycle
What 4 steps occur in the 2nd stage of the citric acid cycle?
• Steps of the 2nd stage of the citric acid cycle:
1) Citrate (6C) is altered to form Isocitrate (5C)
2) Isocitrate is changed to Alpha Ketoglutarate (5C), which reduces NAD+ to NADH and releases CO2
3) Alpha Ketoglutarate can then be altered to Succinyl CoA (4C) by the addition of coenzyme A, which reduces NAD+ to NADH and releases CO2
4) Succinyl CoA can be converted to Succinate when Coenzyme A leaves. The high energy thioester bond of succinyl CoA on its conversion to Succinate. This is substrate level phosphorylation
What is FAD?
Where is FAD found in the citric acid cycle?
What are the steps in the third stage of the citric acid cycle?
How is FAD reoxidised?
- FAD is a coenzyme formed from the vitamin riboflavin (vitamin B2)
- FAD is bound to enzyme succinate dehydrogenase, which is the only citric acid cycle enzyme bound to the inner mitochondrial membrane
• Third stage of citric acid cycle:
1) Succinate (4C) is converted to Fumarate (4C), while FAD is reduced to FADH2
2) Water us then added to Fumarate to form Malate (4C)
3) Malate is then converted to oxaloacetate (4C) while NAD+ is reduced to form NADH
• FAD is reoxidised via the electron transport chain
What does each turn of the citric acid form?
What happens to reduced coenzymes?
• Each cycle of the citric acid cycle forms:
5) 2 x CO2
6) 1 x GTP
7) 3 X NADH + H+
8) 1 X FADH2
- Plus 1 x CO2 and 1 X NADH + H+ generated per pyruvate converted to acetyl CoA
- Reduced coenzymes NADH + H+ and FADH2 now enter into the electron transport chain
What are the 4 substances whose levels will dictate whether enzymes are inhibited in the citric acid cycle?
What can intermediates in the citric acid cycle be used for?
• Inhibition of enzymes involved in these stages of the citric acid cycle by levels of:
1) ATP
2) Acetyl CoA
3) NADH
4) CO2
Many of the intermediates of the citric acid cycle are precursors for other biosynthetic pathways:
1) Fatty acids
2) Purine/pyrimidines
3) Amino acids
4) Glucose
What is the citric acid cycle?
What occurs during the citric acid cycle?
How much energy can these coenzymes generate when delivered to the electron transport chain?
What is the metabolic waste generated by the citric acid cycle?
- The citric acid cycle is a stepwise chemical transformation of substrates
- During the citric acid cycle substrates are oxidised to harness molecular energy via the reduction of coenzymes
• When delivered to the electron transport chain:
1) Energy from each molecule of NADH + H+ will generate about 2.5 ATP
2) Energy from each molecule of FADH2 will generate about 1.5 ATP
• The citric acid cycle generates metabolic waste in the form of carbon dioxide
Describe the 6 stages of the electron transport chain and oxidative phosphorylation
1) Reduced coenzymes NADH and FADH2 deliver electrons to complexes 1 and 2
2) Electron transfer occurs, and each complex is reduced, then oxidised (redox)
3) The energy released pumps H+ into the intermembranous space of the mitochondria from the matrix
4) Coenzyme Q and Cytochrome C (another coenzyme) transfer electrons
5) Complex 4 combines 2H+, 2e- and ½ O2 to form H2O
6) ATP is synthesised from ADP and Pi when protons flow back down the proton gradient and into the matrix of the mitochondria through ATP synthase
What are the 4 complexes in the electron transport chain?
What redox reaction do they each perform?
How is energy released?
1) Complex 1 – NADH-Q reductase
• Oxidises NADH + H+
• Reduces coenzyme Q
2) Complex 2 – Succinate-Q-reductase
• Oxidises FADH2
• Reduces coenzyme Q
3) Complex 3 – Q-cytochrome C oxidoreductase
• Oxidises coenzyme Q
• Reduces cytochrome C
4) Complex 4 – Cytochrome C oxidase
• Oxidises Cytochrome C
• Reduces O2 to H2O
• 2H+ + 2e_ + ½ O2 = H2O
• Energy is released in a stepwise manner as electrons are passed along the complexes and Protons (H+) are pumped into the intermembranous space
What are 2 electron transport chain inhibitors?
How do both of these poisons work?
• Electron transport chain inhibitors:
1) Cyanide
2) Carbon Monoxide
Both of these poisons bind to the Cytochrome C oxidase
• They bind to the iron in the enzyme, which prevents the electron transport chain from working, halting ATP production
What 2 things does the proton gradient in the electron transport chain create?
What do both of these things cause?
Where is the only free permeable region?
• The proton gradient in the electron transport chain creates:
1) A pH gradient – H+ concentration in the matrix is lower than in the intermembraneous space
2) A voltage across the membrane
Both of these conditions strongly attract H+ back inside the matrix
• The only free permeable region is Complex 5, where ATP synthases are located (molecular rotary motors)
How many calories are used per hour at rest?
How many calories can be generated form 1 mole of glucose in the presence of oxygen?
How much energy is retained?
What are the 5 stages of energy flow during cellular respiration?
- The average person uses abut 100 kcal/hour a t rest
- When O2 is available, cellular respiration generates about 680 kcal of energy per 1 mole of glucose
- About 260 kcal is captured in ATP bonds, while the rest is liberated as heat
- This is an energy capture of about 35%
• Energy flow during cellular respiration:
1) Glucose
2) NADH + H+
3) Electron transport chain
4) Proton gradient energy
5) ATP
What is the net gain of ATP at each point of cellular respiration?
What is the overall yield of ATP per molecule of glucose?
1) Glycolysis
• 2 ATP invested
• 4 ATP produced
• Net of 2 ATP
2) Citric acid cycle
• 2 ATP produced
3) ETC and Oxidative Phosphorylation
• 28 ATP
- Yield of 32 ATP – 2 ATP as average shuttle cost
- Typical yield of 30 ATP per molecule of glucose
