CITRIC ACID CYCLE AND GLYOXYLATE CYCLE
Reactions of pyruvate oxidation and citric acid cycle
Steps aerobic respiration occurs in presence of oxygen and highly efficient for producing ATP
1. Glycolysis: 1 glucose broken into 2 pyruvate molecules producing small amount ATP and
NADH in cytoplasm
2. Pyruvate oxidation: pyruvate enters mitochondria and converted into acetyl-CoA
releasing carbon dioxide and reducing NAD+ to NADH
3. Citric acid cycle (CAC & Kreb cycle): acetyl-CoA enters citric acid cycle in mitochondria
leading to release of CO2 and production of NADH and FADH2
4. Electron transport chain (ETC): NADH and FADH2 donate high energy electrons to the ETC
in inner mitochondrial membrane generating electrochemical gradient
5. Chemiosmosis: flow of proteins back into mitochondrial matrix through ATP synthase
drives synthesis of ATP from ADP + Pi -> ATP
6. Oxygen as final electron acceptor: oxygen combines with protons at end of ETC forming
water and allowing ETC to run efficiently
Pyruvate oxidation: (2 x 3C) 3C pyruvate oxidised -> (2 x 2C) 2C acetyl-CoA via pyruvate
dehydrogenase complex and enters the citric acid cycle
Citric acid cycle or Kreb cycle: 2C acetyl-CoA fuses with 4C oxalacetate -> 6C citrate via citrate
synthase -> isocitrate via aconitase -> α-Ketoglutarate via isocitrate dehydrogenase -> succinyl-CoA
via α-Ketoglutarate dehydrogenase -> succinate via succinate-CoA synthetase -> fumarate via
succinate dehydrogenase -> malate via fumarase -> 4C oxaloacetate via malate hydrogenase
GTP, NADH and FADH reducing equivalents are produced
,Step 1: glycolysis
1. Which phase of glycolysis involves the consumption of ATP to convert glucose into glucose-6-
phosphate?
2. How many molecules of ATP are invested during the energy investment phase of glycolysis for
the breakdown of one molecule of glucose?
3. Which enzyme catalyses the conversion of glucose to glucose-6-phosphate in glycolysis?
4. During glycolysis, one molecule of glucose is converted into how many molecules of pyruvate?
5. In glycolysis, which enzyme catalyses the conversion of fructose-6-phosphate to fructose-1,6-
bisphosphate?
6. The conversion of 1,3-bisphosphoglycerate to 3-phosphoglycerate in glycolysis is coupled with
the production of?
7. Which phase of glycolysis involves the production of ATP, NADH, and pyruvate?
8. The enzyme glyceraldehyde-3-phosphate dehydrogenase (GAPDH) is involved in which reaction
of glycolysis?
9. How many molecules of NADH are produced in glycolysis by the reduction of NAD+?
10. The conversion of phosphoenolpyruvate (PEP) to pyruvate in glycolysis is catalysed by which
enzyme?
Step 2: pyruvate oxidation
3C pyruvate + HS-CoA + NAD+ -> 2C acetyl-CoA + CO2 + NADH via pyruvate dehydrogenase
complex (PDC) where acetyl-CoA enters the TCA cycle
➔ Pyruvate oxidation occurs in mitochondria
➔ Pyruvate transported into mitochondria
➔ Each pyruvate molecule loses 1C atom as CO2 through decarboxylation
➔ Remaining 2C compound is converted to acetyl-CoA
➔ NAD+ reduced to ANDH by accepting a high energy electron
➔ Acetyl-CoA enters citric acid cycle
Conversion of pyruvate to acetyl-CoA via pyruvate dehydrogenase complex (PDC)
3 enzymes work together in tightly regulated complex converting pyruvate to activated
acetyl-CoA and fed into citric acid cycle for energy extraction during aerobic respiration
PDC complex activity highly regulated to ensure control of energy production and prevent
unnecessary waste of resources
,Pyruvate dehydrogenase (E1):
➔ Decarboxylation of pyruvate oxidation removes carboxyl group from pyruvate
releasing carbon dioxide and formation of acetyl group
Dihydrolipoamide acetyltransferase (E2):
➔ Transfer of acetyl group from pyruvate to coenzyme A (CoA) forming activated acetyl-
CoA containing a lipoamide cofactor helping in the transfer of the acetyl group
Dihydrolipoamide dehydrogenase (E3):
➔ Regenerates lipoamide cofactor of E2 allowing pyruvate dehydrogenase complex to
undergo multiple catalytic cycles by transferring electrons from lipoamide cofactor to
NAD+ reducing to NADH
Reaction 1: pyruvate reacts with TPP carbanion of pyruvate dehydrogenase E1 forming
additional product that undergoes decarboxylation giving hydroxyethyl-TPP (HETPP)
Reaction 2: hydroxyethyl group transferred by E1 to lipoamide swinging arm of
dihydrolipoamide acetyltransferase E2 resulting in oxidation of 2C fragment to acetyl group
and reduction of lipoamide disulfide to dihydrolipoamide with acetyl bound
Reaction 3: acetyl group transferred to CoA-SH producing acetyl-CoA and dihydrolipoamide
Reaction 4: dihydrolipoamide dehydrogenase E3 reoxidises the reduced lipoamide swinging
arm by transferring 2e- to an E3 cys-cys disulfide bond
Reaction 5: E3 catalyses the transfer of electrons from cys sulfhydryl groups to NAD +
regenerating the oxidised form of E3, releasing reduced NADH and tightly bound FAD is an
intermediate electron carrier
OVERALL STEPS
1. Pyruvate entry: 3C pyruvate molecule produced in glycolysis is transported from
cytoplasm into mitochondria
2. Decarboxylation: in mitochondria, pyruvate undergoes decarboxylation losing 1C
atom in form of CO2 resulting in a 2C compound as acetyl-CoA
, 3. Dehydrogenation: high energy electron transferred to electron carrier as NAD+
converting to NADH during decarboxylation
4. Acetyl-CoA formation: remaining 2C acetyl group combined with coenzyme A (CoA)
molecule forming activated CoA
Acetyl-CoA is fuel for citric acid cycle
3 enzymes involved in pyruvate dehydrogenase complex
Pyruvate dehydrogenase enzyme E1 catalysed the carboxylation of pyruvate
Dihydrolipoamide enzyme transfer acetyl group from lipoic acid to CoA
Fate of acetyl-CoA and citric acid cycle
Acetyl-CoA fed intro citric acid cycle for energy extraction
➔ Acetyl-CoA entry: cycle begins when acetyl-CoA derived from metabolic processes
combines with oxaloacetate forming citrate
➔ Condensation of activated 2C acetyl-CoA with 4C acceptor forming 6C molecule
➔ Oxidative decarboxylation produces activated 4C product as succinyl-CoA and 2 CO2
➔ Hydrolysis of succinyl-CoA drives GTP or ATP synthesis
➔ Regeneration of 4C acceptor oxaloacetate 4C and production of reducing equilvanets
Step 3: citric acid cycle
Citric acid cycle reaction in mitochondrial matrix when acetyl-CoA enters derived from
metabolic processes combining with oxaloacetate to form citrate
➔ Isomerisation and decarboxylation: citrate isomerised into isocitrate undergoing
decarboxylation losing CO2 forming α-Ketoglutarate
➔ 2nd decarboxylation: α-Ketoglutarate loses another CO2 producing succinyl-CoA
➔ Succinyl-CoA converted to succinate generating GTP from GDP (GDP + Pi -> GTP)
➔ Succinate oxidised to fumarate and FADH2 formed
➔ Fumarate hydrated to malate
➔ Malate oxidised to oxaloacetate generating NADH
End 1 cycle: oxaloacetate regenerated producing ATP, NADH and FADH2
Condensation reaction -> reaction 1
➔ Acetyl-CoA + oxalacetate + H2O -> citrate + CoA
➔ Condensation of 2C acetyl-CoA with 4C oxaloacetate producing 6C citrate
➔ Catalysed by citrate synthase
➔ Highly exergonic
Reactions of pyruvate oxidation and citric acid cycle
Steps aerobic respiration occurs in presence of oxygen and highly efficient for producing ATP
1. Glycolysis: 1 glucose broken into 2 pyruvate molecules producing small amount ATP and
NADH in cytoplasm
2. Pyruvate oxidation: pyruvate enters mitochondria and converted into acetyl-CoA
releasing carbon dioxide and reducing NAD+ to NADH
3. Citric acid cycle (CAC & Kreb cycle): acetyl-CoA enters citric acid cycle in mitochondria
leading to release of CO2 and production of NADH and FADH2
4. Electron transport chain (ETC): NADH and FADH2 donate high energy electrons to the ETC
in inner mitochondrial membrane generating electrochemical gradient
5. Chemiosmosis: flow of proteins back into mitochondrial matrix through ATP synthase
drives synthesis of ATP from ADP + Pi -> ATP
6. Oxygen as final electron acceptor: oxygen combines with protons at end of ETC forming
water and allowing ETC to run efficiently
Pyruvate oxidation: (2 x 3C) 3C pyruvate oxidised -> (2 x 2C) 2C acetyl-CoA via pyruvate
dehydrogenase complex and enters the citric acid cycle
Citric acid cycle or Kreb cycle: 2C acetyl-CoA fuses with 4C oxalacetate -> 6C citrate via citrate
synthase -> isocitrate via aconitase -> α-Ketoglutarate via isocitrate dehydrogenase -> succinyl-CoA
via α-Ketoglutarate dehydrogenase -> succinate via succinate-CoA synthetase -> fumarate via
succinate dehydrogenase -> malate via fumarase -> 4C oxaloacetate via malate hydrogenase
GTP, NADH and FADH reducing equivalents are produced
,Step 1: glycolysis
1. Which phase of glycolysis involves the consumption of ATP to convert glucose into glucose-6-
phosphate?
2. How many molecules of ATP are invested during the energy investment phase of glycolysis for
the breakdown of one molecule of glucose?
3. Which enzyme catalyses the conversion of glucose to glucose-6-phosphate in glycolysis?
4. During glycolysis, one molecule of glucose is converted into how many molecules of pyruvate?
5. In glycolysis, which enzyme catalyses the conversion of fructose-6-phosphate to fructose-1,6-
bisphosphate?
6. The conversion of 1,3-bisphosphoglycerate to 3-phosphoglycerate in glycolysis is coupled with
the production of?
7. Which phase of glycolysis involves the production of ATP, NADH, and pyruvate?
8. The enzyme glyceraldehyde-3-phosphate dehydrogenase (GAPDH) is involved in which reaction
of glycolysis?
9. How many molecules of NADH are produced in glycolysis by the reduction of NAD+?
10. The conversion of phosphoenolpyruvate (PEP) to pyruvate in glycolysis is catalysed by which
enzyme?
Step 2: pyruvate oxidation
3C pyruvate + HS-CoA + NAD+ -> 2C acetyl-CoA + CO2 + NADH via pyruvate dehydrogenase
complex (PDC) where acetyl-CoA enters the TCA cycle
➔ Pyruvate oxidation occurs in mitochondria
➔ Pyruvate transported into mitochondria
➔ Each pyruvate molecule loses 1C atom as CO2 through decarboxylation
➔ Remaining 2C compound is converted to acetyl-CoA
➔ NAD+ reduced to ANDH by accepting a high energy electron
➔ Acetyl-CoA enters citric acid cycle
Conversion of pyruvate to acetyl-CoA via pyruvate dehydrogenase complex (PDC)
3 enzymes work together in tightly regulated complex converting pyruvate to activated
acetyl-CoA and fed into citric acid cycle for energy extraction during aerobic respiration
PDC complex activity highly regulated to ensure control of energy production and prevent
unnecessary waste of resources
,Pyruvate dehydrogenase (E1):
➔ Decarboxylation of pyruvate oxidation removes carboxyl group from pyruvate
releasing carbon dioxide and formation of acetyl group
Dihydrolipoamide acetyltransferase (E2):
➔ Transfer of acetyl group from pyruvate to coenzyme A (CoA) forming activated acetyl-
CoA containing a lipoamide cofactor helping in the transfer of the acetyl group
Dihydrolipoamide dehydrogenase (E3):
➔ Regenerates lipoamide cofactor of E2 allowing pyruvate dehydrogenase complex to
undergo multiple catalytic cycles by transferring electrons from lipoamide cofactor to
NAD+ reducing to NADH
Reaction 1: pyruvate reacts with TPP carbanion of pyruvate dehydrogenase E1 forming
additional product that undergoes decarboxylation giving hydroxyethyl-TPP (HETPP)
Reaction 2: hydroxyethyl group transferred by E1 to lipoamide swinging arm of
dihydrolipoamide acetyltransferase E2 resulting in oxidation of 2C fragment to acetyl group
and reduction of lipoamide disulfide to dihydrolipoamide with acetyl bound
Reaction 3: acetyl group transferred to CoA-SH producing acetyl-CoA and dihydrolipoamide
Reaction 4: dihydrolipoamide dehydrogenase E3 reoxidises the reduced lipoamide swinging
arm by transferring 2e- to an E3 cys-cys disulfide bond
Reaction 5: E3 catalyses the transfer of electrons from cys sulfhydryl groups to NAD +
regenerating the oxidised form of E3, releasing reduced NADH and tightly bound FAD is an
intermediate electron carrier
OVERALL STEPS
1. Pyruvate entry: 3C pyruvate molecule produced in glycolysis is transported from
cytoplasm into mitochondria
2. Decarboxylation: in mitochondria, pyruvate undergoes decarboxylation losing 1C
atom in form of CO2 resulting in a 2C compound as acetyl-CoA
, 3. Dehydrogenation: high energy electron transferred to electron carrier as NAD+
converting to NADH during decarboxylation
4. Acetyl-CoA formation: remaining 2C acetyl group combined with coenzyme A (CoA)
molecule forming activated CoA
Acetyl-CoA is fuel for citric acid cycle
3 enzymes involved in pyruvate dehydrogenase complex
Pyruvate dehydrogenase enzyme E1 catalysed the carboxylation of pyruvate
Dihydrolipoamide enzyme transfer acetyl group from lipoic acid to CoA
Fate of acetyl-CoA and citric acid cycle
Acetyl-CoA fed intro citric acid cycle for energy extraction
➔ Acetyl-CoA entry: cycle begins when acetyl-CoA derived from metabolic processes
combines with oxaloacetate forming citrate
➔ Condensation of activated 2C acetyl-CoA with 4C acceptor forming 6C molecule
➔ Oxidative decarboxylation produces activated 4C product as succinyl-CoA and 2 CO2
➔ Hydrolysis of succinyl-CoA drives GTP or ATP synthesis
➔ Regeneration of 4C acceptor oxaloacetate 4C and production of reducing equilvanets
Step 3: citric acid cycle
Citric acid cycle reaction in mitochondrial matrix when acetyl-CoA enters derived from
metabolic processes combining with oxaloacetate to form citrate
➔ Isomerisation and decarboxylation: citrate isomerised into isocitrate undergoing
decarboxylation losing CO2 forming α-Ketoglutarate
➔ 2nd decarboxylation: α-Ketoglutarate loses another CO2 producing succinyl-CoA
➔ Succinyl-CoA converted to succinate generating GTP from GDP (GDP + Pi -> GTP)
➔ Succinate oxidised to fumarate and FADH2 formed
➔ Fumarate hydrated to malate
➔ Malate oxidised to oxaloacetate generating NADH
End 1 cycle: oxaloacetate regenerated producing ATP, NADH and FADH2
Condensation reaction -> reaction 1
➔ Acetyl-CoA + oxalacetate + H2O -> citrate + CoA
➔ Condensation of 2C acetyl-CoA with 4C oxaloacetate producing 6C citrate
➔ Catalysed by citrate synthase
➔ Highly exergonic
