18.1 Pyruvate Dehydrogenase Forms Acetyl Coenzyme A from Pyruvate
Glycolysis and Pyruvate Transport:
• Glycolysis occurs in the cytoplasm, while the citric acid cycle takes place in the mitochondria.
• Pyruvate, produced from glycolysis, is transported into mitochondria to undergo aerobic metabolism.
Conversion to Acetyl CoA:
• In mitochondria, pyruvate is converted into acetyl CoA by the pyruvate dehydrogenase complex.
• This conversion links glycolysis to the citric acid cycle.
• The reaction: Pyruvate + CoA + NAD⁺ → Acetyl CoA + CO₂ + NADH + H⁺.
Pyruvate Dehydrogenase Complex:
• Made up of three distinct enzymes: pyruvate dehydrogenase (E1), dihydrolipoyl transacetylase (E2), and dihydrolipoyl dehydrogenase (E3).
• It requires five coenzymes: TPP, lipoic acid, FAD (catalytic coenzymes), CoA, and NAD ⁺ (stoichiometric coenzymes).
Steps of Acetyl CoA Synthesis:
1. Decarboxylation: Pyruvate is decarboxylated by E1 to form hydroxyethyl-TPP.
2. Oxidation: Hydroxyethyl-TPP is oxidized to acetyl group and transferred to lipoamide, forming acetyl-lipoamide.
3. Formation of Acetyl CoA: Acetyl group is transferred to CoA, forming acetyl CoA.
4. Regeneration of Lipoamide: E3 reoxidizes dihydrolipoamide to lipoamide, transferring electrons to NAD ⁺, forming NADH.
Structural Integration:
• The E2 component forms the core of the complex with flexible lipoamide arms.
• Lipoamide arms move substrates efficiently between active sites of E1, E2, and E3.
• This integrated structure increases reaction efficiency and minimizes side reactions.
Overall:
• The conversion of pyruvate to acetyl CoA is crucial for linking glycolysis with citric acid cycle and is essential for carbohydrate metabolism.
• The process involves a highly organized and efficient enzyme complex with multiple active sites and coenzymes.
The pyruvate dehydrogenase complex is a large, highly integrated complex of three distinct enzymes. There are some potential advantages of
such a multienzyme complex with respect to the isolated enzyme components.
Question 1: What are those advantages?
a. Increased reaction rates for each separate reaction
b. Minimised side reactions
c. Higher affinity for substrates
d. Increased overall reaction rate
Answer: A multienzyme complex can carry out the coordinated catalysis of a complex reaction. The intermediates in the reaction remain bound
to the complex and are passed from one enzyme component to the next, which increases the overall reaction rate and minimizes side reactions.
In case of isolated enzymes, the reaction intermediates would have to diffuse randomly between enzymes. The reaction rates of the individual
reactions are not affected, nor is the affinity of the enzymes for the substrate.
The pyruvate dehydrogenase complex is a large, highly integrated complex of three distinct enzymes, with their own cofactors. Enzymes:
pyruvate dehydrogenase component (E1), dihydrolipoyl transacetylase (E2), and dihydrolipoyl dehydrogenase (E3). Cofactors: coenzyme A,
FAD, thiamine pyrophosphate (TPP), and NAD+.
Question 2A: Combine enzymes and cofactors.
Answer: pyruvate dehydrogenase component (E1) → thiamine pyrophosphate (TPP); dihydrolipoyl transacetylase (E2) → coenzyme A
; dihydrolipoyl dehydrogenase (E3) → FAD and NAD+.
Question 2B: What are the roles of the cofactors coenzyme A, FAD, thiamine pyrophosphate (TPP), NAD+, and lipoamide.
Answer: coenzyme A → accepts the acetyl group from acetyl-lipoamide, FAD → oxidation of dihydrolipoamide, thiamine pyrophosphate (TPP)
→ decarboxylation of pyruvate, NAD+ → oxidation of FADH2, and lipoamide → oxidizes the hydroxyethyl group.
1|Page
,The enzymes and their cofactors:
1. Pyruvate dehydrogenase:
• Cofactor: thiamine pyrophosphate (TPP)
• Role: decarboxylation of pyruvate
2. Dihydrolipoyl transacetylase:
• Cofactor: coenzyme A
• Role: accepts the acetyl group from acetyl-lipoamide
• Cofactor: lipoamide (note: lipoamide takes part in the reaction of E1 and E3 as well, it is bound to E2 however)
• Role: oxidation of the hydroxyethyl group
3. Dihydrolipoyl dehydrogenase:
• Cofactor: FAD
• Role: oxidation of dihydrolipoamide
• Cofactor: NAD+
• Role: oxidation of FADH2
The pyruvate dehydrogenase complex is a large, highly integrated complex of three distinct enzymes:
1. pyruvate dehydrogenase with cofactor thiamine pyrophosphate (TPP)
2. dihydrolipoyl transacetylase with cofactors lipoamide and coenzyme A
3. dihydrolipoyl dehydrogenase with cofactors NAD+ and FAD
One of these cofactors has the role of conveying intermediates to the different enzyme components.
Question 2C: Which of these five cofactors has that role?
a. Thiamine pyrophosphate
b. Lipoamide
c. Coenzyme A
d. NAD+
e. FAD
Answer: Lipoamide, cofactor of the second component of the enzyme complex, the dihydrolipoyl transacetylase, provides a long, flexible arm
that shuttles intermediates between the different enzymes of the enzyme complex.
18.2 The Pyruvate Dehydrogenase Complex Is Regulated by Two Mechanisms
Mechanisms of Regulation:
• The pyruvate dehydrogenase complex (PDC) is regulated by allosteric interactions and covalent modifications.
• The conversion of pyruvate to acetyl CoA is irreversible in animals and commits carbon atoms of glucose to either:
1. Oxidation to CO2 by the citric acid cycle.
2. Incorporation into fatty acids.
Product Inhibition:
• High concentrations of acetyl CoA and NADH inhibit the PDC.
• Acetyl CoA inhibits the transacetylase component (E2).
• NADH inhibits the dihydrolipoyl dehydrogenase (E3).
• This inhibition spares glucose, as most pyruvate is derived from glucose via glycolysis.
Covalent Modification:
• The PDC is regulated by phosphorylation and dephosphorylation.
• Pyruvate dehydrogenase kinase phosphorylates and deactivates the E1 component.
• Pyruvate dehydrogenase phosphatase dephosphorylates and activates the E1 component.
• Both the kinase and phosphatase are associated with the E2-E3-BP complex and are regulated.
2|Page
,Regulation in Muscle:
• At rest: High NADH/NAD⁺, acetyl CoA/CoA, and ATP/ADP ratios stimulate kinase activity,
deactivating the PDC.
• During exercise: Increased ADP and pyruvate levels inhibit kinase activity, activating PDC.
• Calcium ions (Ca2+) stimulate phosphatase activity, enhancing PDC activity during muscle
contraction.
Hormonal Regulation:
• In the liver, epinephrine increases Ca2+ concentration, activating the phosphatase.
• Insulin stimulates the phosphatase in tissues capable of fatty acid synthesis (e.g., liver and adipose tissue), increasing the conversion of
pyruvate to acetyl CoA.
Overall:
• The regulation of the pyruvate dehydrogenase complex ensures efficient energy production and metabolic flexibility depending on the
cell's energy needs and hormonal signals.
Question 3A: These questions deal with regulation of the pyruvate dehydrogenase complex. If the levels of insulin rise, what will happen to the
pyruvate dehydrogenase complex in liver cells?
a. Activated
b. Inhibited
c. No effect
Answer: Insulin stimulates the phosphatase of the pyruvate dehydrogenase complex leading to activation of the complex.
Question 3B: If the ratio NADH/NAD+ decreases, what will happen to the pyruvate dehydrogenase complex?
a. Activated
b. Inhibited
c. No effect
Answer: The complex is activated when NADH decreases in favour of NAD + accumulation and when in addition pyruvate is present. The enzyme
is inhibited by its reaction products NADH and acetyl CoA.
Question 3C: How does pyruvate activate the complex in muscle cells during muscle contraction?
a. By stimulating a specific phosphatase
b. By phosphorylating a compound of the complex
c. By inhibiting the kinase that inactivates the pyruvate dehydrogenase complex
Answer: In muscle, both ADP and pyruvate activate the pyruvate dehydrogenase complex by inhibiting the kinase that inactivates the complex.
Question 3D: By what mechanism is muscle pyruvate dehydrogenase also activated?
a. Phosphorylation of the component E1 of the complex
b. Stimulation of the phosphatase of the pyruvate dehydrogenase complex by Ca 2+
Answer: Stimulation of the phosphatase of the pyruvate dehydrogenase complex by Ca 2+.
SUMMARY: Chapter 18
18.1 Pyruvate Dehydrogenase Forms Acetyl Coenzyme A from Pyruvate
Most fuel molecules enter the citric acid cycle as acetyl CoA. The link between glycolysis and the citric acid cycle is the oxidative decarboxylation
of pyruvate to form acetyl CoA. In eukaryotes, this reaction and those of the cycle take place inside mitochondria, in contrast with glycolysis,
which takes place in the cytoplasm. The enzyme complex catalyzing this reaction, the pyruvate dehydrogenase complex, consists of three distinct
enzyme activities.
Pyruvate dehydrogenase catalyzes the decarboxylation of pyruvate and the formation of acetyl– lipoamide. Dihydrolipoyl transacetylase forms
acetyl CoA, and dihydrolipoyl dehydrogenase regenerates the active transacetylase. The complex requires five cofactors: thiamine
pyrophosphate, lipoic acid, coenzyme A, NAD+ and FAD.
3|Page
, 18.2 The Pyruvate Dehydrogenase Complex Is Regulated by Two Mechanisms
The irreversible formation of acetyl CoA from pyruvate is the regulatory point for the entry of glucose-derived pyruvate into the citric acid cycle.
The pyruvate dehydrogenase complex is allosterically inhibited by acetyl CoA and NADH. The activity of the pyruvate dehydrogenase complex is
stringently controlled through reversible phosphorylation by an associated kinase and phosphatase. High concentrations of ATP and NADH
stimulate the kinase, which phosphorylates and inactivates the complex. ADP and pyruvate inhibit the kinase, whereas Ca2+ stimulates the
phosphatase, which dephosphorylates and thereby activates the complex.
The importance of the pyruvate dehydrogenase complex to metabolism, especially to catabolism in the central nervous system, is illustrated by
beriberi. Beriberi is a neurological condition that results from a deficiency of thiamine, the vitamin precursor of thiamine pyrophosphate. The lack
of TPP impairs the activity of the pyruvate dehydrogenase component of the pyruvate dehydrogenase complex. Arsenite and mercury are toxic
because of their effects on the complex. These chemicals bind to the lipoic acid coenzyme of dihydrolipoyl dehydrogenase, inhibiting the activity
of this enzyme.
Quick quiz: List some of the advantages of organizing into a single large complex the enzymes that catalyze the formation of acetyl CoA from
pyruvate.
Answer:
1. The reaction is facilitated by having the active sites in proximity.
2. The reactants do not leave the enzyme until the final product is formed. Constraining the reactants minimizes loss due to diffusion and
minimizes side reactions.
3. All the enzymes are present in the correct amounts.
4. Regulation is more efficient because the regulatory enzymes— the kinase and phosphatase—are part of the complex.
19.1 The Citric Acid Cycle Consists of Two Stages
Overview of the Cycle:
• The two-carbon acetyl group (from acetyl-CoA) condenses with oxaloacetate (a four-carbon
molecule) to form citrate (six carbons).
• Citrate undergoes oxidation, releasing two CO₂ molecules and generating high-energy electrons.
• A four-carbon compound remains, which is further oxidized to regenerate oxaloacetate,
continuing the cycle.
Key Features of the Cycle:
• No direct ATP production → The cycle primarily captures high-energy electrons.
• Does not require oxygen directly → Instead, electrons are transferred to NADH and FADH₂.
• NADH and FADH₂ drive ATP synthesis → They donate electrons to the electron transport chain, creating a proton gradient used for ATP
production.
Two Stages of the Cycle:
1. First Stage:
• Acetyl group joins oxaloacetate → Citrate forms.
• Two CO₂ molecules are released.
2. Second Stage:
• The remaining four-carbon molecule undergoes reactions to regenerate oxaloacetate, ensuring the cycle continues.
Final Energy Yield:
• Electrons from NADH and FADH₂ produce ~9 ATP molecules through oxidative phosphorylation.
4|Page
Glycolysis and Pyruvate Transport:
• Glycolysis occurs in the cytoplasm, while the citric acid cycle takes place in the mitochondria.
• Pyruvate, produced from glycolysis, is transported into mitochondria to undergo aerobic metabolism.
Conversion to Acetyl CoA:
• In mitochondria, pyruvate is converted into acetyl CoA by the pyruvate dehydrogenase complex.
• This conversion links glycolysis to the citric acid cycle.
• The reaction: Pyruvate + CoA + NAD⁺ → Acetyl CoA + CO₂ + NADH + H⁺.
Pyruvate Dehydrogenase Complex:
• Made up of three distinct enzymes: pyruvate dehydrogenase (E1), dihydrolipoyl transacetylase (E2), and dihydrolipoyl dehydrogenase (E3).
• It requires five coenzymes: TPP, lipoic acid, FAD (catalytic coenzymes), CoA, and NAD ⁺ (stoichiometric coenzymes).
Steps of Acetyl CoA Synthesis:
1. Decarboxylation: Pyruvate is decarboxylated by E1 to form hydroxyethyl-TPP.
2. Oxidation: Hydroxyethyl-TPP is oxidized to acetyl group and transferred to lipoamide, forming acetyl-lipoamide.
3. Formation of Acetyl CoA: Acetyl group is transferred to CoA, forming acetyl CoA.
4. Regeneration of Lipoamide: E3 reoxidizes dihydrolipoamide to lipoamide, transferring electrons to NAD ⁺, forming NADH.
Structural Integration:
• The E2 component forms the core of the complex with flexible lipoamide arms.
• Lipoamide arms move substrates efficiently between active sites of E1, E2, and E3.
• This integrated structure increases reaction efficiency and minimizes side reactions.
Overall:
• The conversion of pyruvate to acetyl CoA is crucial for linking glycolysis with citric acid cycle and is essential for carbohydrate metabolism.
• The process involves a highly organized and efficient enzyme complex with multiple active sites and coenzymes.
The pyruvate dehydrogenase complex is a large, highly integrated complex of three distinct enzymes. There are some potential advantages of
such a multienzyme complex with respect to the isolated enzyme components.
Question 1: What are those advantages?
a. Increased reaction rates for each separate reaction
b. Minimised side reactions
c. Higher affinity for substrates
d. Increased overall reaction rate
Answer: A multienzyme complex can carry out the coordinated catalysis of a complex reaction. The intermediates in the reaction remain bound
to the complex and are passed from one enzyme component to the next, which increases the overall reaction rate and minimizes side reactions.
In case of isolated enzymes, the reaction intermediates would have to diffuse randomly between enzymes. The reaction rates of the individual
reactions are not affected, nor is the affinity of the enzymes for the substrate.
The pyruvate dehydrogenase complex is a large, highly integrated complex of three distinct enzymes, with their own cofactors. Enzymes:
pyruvate dehydrogenase component (E1), dihydrolipoyl transacetylase (E2), and dihydrolipoyl dehydrogenase (E3). Cofactors: coenzyme A,
FAD, thiamine pyrophosphate (TPP), and NAD+.
Question 2A: Combine enzymes and cofactors.
Answer: pyruvate dehydrogenase component (E1) → thiamine pyrophosphate (TPP); dihydrolipoyl transacetylase (E2) → coenzyme A
; dihydrolipoyl dehydrogenase (E3) → FAD and NAD+.
Question 2B: What are the roles of the cofactors coenzyme A, FAD, thiamine pyrophosphate (TPP), NAD+, and lipoamide.
Answer: coenzyme A → accepts the acetyl group from acetyl-lipoamide, FAD → oxidation of dihydrolipoamide, thiamine pyrophosphate (TPP)
→ decarboxylation of pyruvate, NAD+ → oxidation of FADH2, and lipoamide → oxidizes the hydroxyethyl group.
1|Page
,The enzymes and their cofactors:
1. Pyruvate dehydrogenase:
• Cofactor: thiamine pyrophosphate (TPP)
• Role: decarboxylation of pyruvate
2. Dihydrolipoyl transacetylase:
• Cofactor: coenzyme A
• Role: accepts the acetyl group from acetyl-lipoamide
• Cofactor: lipoamide (note: lipoamide takes part in the reaction of E1 and E3 as well, it is bound to E2 however)
• Role: oxidation of the hydroxyethyl group
3. Dihydrolipoyl dehydrogenase:
• Cofactor: FAD
• Role: oxidation of dihydrolipoamide
• Cofactor: NAD+
• Role: oxidation of FADH2
The pyruvate dehydrogenase complex is a large, highly integrated complex of three distinct enzymes:
1. pyruvate dehydrogenase with cofactor thiamine pyrophosphate (TPP)
2. dihydrolipoyl transacetylase with cofactors lipoamide and coenzyme A
3. dihydrolipoyl dehydrogenase with cofactors NAD+ and FAD
One of these cofactors has the role of conveying intermediates to the different enzyme components.
Question 2C: Which of these five cofactors has that role?
a. Thiamine pyrophosphate
b. Lipoamide
c. Coenzyme A
d. NAD+
e. FAD
Answer: Lipoamide, cofactor of the second component of the enzyme complex, the dihydrolipoyl transacetylase, provides a long, flexible arm
that shuttles intermediates between the different enzymes of the enzyme complex.
18.2 The Pyruvate Dehydrogenase Complex Is Regulated by Two Mechanisms
Mechanisms of Regulation:
• The pyruvate dehydrogenase complex (PDC) is regulated by allosteric interactions and covalent modifications.
• The conversion of pyruvate to acetyl CoA is irreversible in animals and commits carbon atoms of glucose to either:
1. Oxidation to CO2 by the citric acid cycle.
2. Incorporation into fatty acids.
Product Inhibition:
• High concentrations of acetyl CoA and NADH inhibit the PDC.
• Acetyl CoA inhibits the transacetylase component (E2).
• NADH inhibits the dihydrolipoyl dehydrogenase (E3).
• This inhibition spares glucose, as most pyruvate is derived from glucose via glycolysis.
Covalent Modification:
• The PDC is regulated by phosphorylation and dephosphorylation.
• Pyruvate dehydrogenase kinase phosphorylates and deactivates the E1 component.
• Pyruvate dehydrogenase phosphatase dephosphorylates and activates the E1 component.
• Both the kinase and phosphatase are associated with the E2-E3-BP complex and are regulated.
2|Page
,Regulation in Muscle:
• At rest: High NADH/NAD⁺, acetyl CoA/CoA, and ATP/ADP ratios stimulate kinase activity,
deactivating the PDC.
• During exercise: Increased ADP and pyruvate levels inhibit kinase activity, activating PDC.
• Calcium ions (Ca2+) stimulate phosphatase activity, enhancing PDC activity during muscle
contraction.
Hormonal Regulation:
• In the liver, epinephrine increases Ca2+ concentration, activating the phosphatase.
• Insulin stimulates the phosphatase in tissues capable of fatty acid synthesis (e.g., liver and adipose tissue), increasing the conversion of
pyruvate to acetyl CoA.
Overall:
• The regulation of the pyruvate dehydrogenase complex ensures efficient energy production and metabolic flexibility depending on the
cell's energy needs and hormonal signals.
Question 3A: These questions deal with regulation of the pyruvate dehydrogenase complex. If the levels of insulin rise, what will happen to the
pyruvate dehydrogenase complex in liver cells?
a. Activated
b. Inhibited
c. No effect
Answer: Insulin stimulates the phosphatase of the pyruvate dehydrogenase complex leading to activation of the complex.
Question 3B: If the ratio NADH/NAD+ decreases, what will happen to the pyruvate dehydrogenase complex?
a. Activated
b. Inhibited
c. No effect
Answer: The complex is activated when NADH decreases in favour of NAD + accumulation and when in addition pyruvate is present. The enzyme
is inhibited by its reaction products NADH and acetyl CoA.
Question 3C: How does pyruvate activate the complex in muscle cells during muscle contraction?
a. By stimulating a specific phosphatase
b. By phosphorylating a compound of the complex
c. By inhibiting the kinase that inactivates the pyruvate dehydrogenase complex
Answer: In muscle, both ADP and pyruvate activate the pyruvate dehydrogenase complex by inhibiting the kinase that inactivates the complex.
Question 3D: By what mechanism is muscle pyruvate dehydrogenase also activated?
a. Phosphorylation of the component E1 of the complex
b. Stimulation of the phosphatase of the pyruvate dehydrogenase complex by Ca 2+
Answer: Stimulation of the phosphatase of the pyruvate dehydrogenase complex by Ca 2+.
SUMMARY: Chapter 18
18.1 Pyruvate Dehydrogenase Forms Acetyl Coenzyme A from Pyruvate
Most fuel molecules enter the citric acid cycle as acetyl CoA. The link between glycolysis and the citric acid cycle is the oxidative decarboxylation
of pyruvate to form acetyl CoA. In eukaryotes, this reaction and those of the cycle take place inside mitochondria, in contrast with glycolysis,
which takes place in the cytoplasm. The enzyme complex catalyzing this reaction, the pyruvate dehydrogenase complex, consists of three distinct
enzyme activities.
Pyruvate dehydrogenase catalyzes the decarboxylation of pyruvate and the formation of acetyl– lipoamide. Dihydrolipoyl transacetylase forms
acetyl CoA, and dihydrolipoyl dehydrogenase regenerates the active transacetylase. The complex requires five cofactors: thiamine
pyrophosphate, lipoic acid, coenzyme A, NAD+ and FAD.
3|Page
, 18.2 The Pyruvate Dehydrogenase Complex Is Regulated by Two Mechanisms
The irreversible formation of acetyl CoA from pyruvate is the regulatory point for the entry of glucose-derived pyruvate into the citric acid cycle.
The pyruvate dehydrogenase complex is allosterically inhibited by acetyl CoA and NADH. The activity of the pyruvate dehydrogenase complex is
stringently controlled through reversible phosphorylation by an associated kinase and phosphatase. High concentrations of ATP and NADH
stimulate the kinase, which phosphorylates and inactivates the complex. ADP and pyruvate inhibit the kinase, whereas Ca2+ stimulates the
phosphatase, which dephosphorylates and thereby activates the complex.
The importance of the pyruvate dehydrogenase complex to metabolism, especially to catabolism in the central nervous system, is illustrated by
beriberi. Beriberi is a neurological condition that results from a deficiency of thiamine, the vitamin precursor of thiamine pyrophosphate. The lack
of TPP impairs the activity of the pyruvate dehydrogenase component of the pyruvate dehydrogenase complex. Arsenite and mercury are toxic
because of their effects on the complex. These chemicals bind to the lipoic acid coenzyme of dihydrolipoyl dehydrogenase, inhibiting the activity
of this enzyme.
Quick quiz: List some of the advantages of organizing into a single large complex the enzymes that catalyze the formation of acetyl CoA from
pyruvate.
Answer:
1. The reaction is facilitated by having the active sites in proximity.
2. The reactants do not leave the enzyme until the final product is formed. Constraining the reactants minimizes loss due to diffusion and
minimizes side reactions.
3. All the enzymes are present in the correct amounts.
4. Regulation is more efficient because the regulatory enzymes— the kinase and phosphatase—are part of the complex.
19.1 The Citric Acid Cycle Consists of Two Stages
Overview of the Cycle:
• The two-carbon acetyl group (from acetyl-CoA) condenses with oxaloacetate (a four-carbon
molecule) to form citrate (six carbons).
• Citrate undergoes oxidation, releasing two CO₂ molecules and generating high-energy electrons.
• A four-carbon compound remains, which is further oxidized to regenerate oxaloacetate,
continuing the cycle.
Key Features of the Cycle:
• No direct ATP production → The cycle primarily captures high-energy electrons.
• Does not require oxygen directly → Instead, electrons are transferred to NADH and FADH₂.
• NADH and FADH₂ drive ATP synthesis → They donate electrons to the electron transport chain, creating a proton gradient used for ATP
production.
Two Stages of the Cycle:
1. First Stage:
• Acetyl group joins oxaloacetate → Citrate forms.
• Two CO₂ molecules are released.
2. Second Stage:
• The remaining four-carbon molecule undergoes reactions to regenerate oxaloacetate, ensuring the cycle continues.
Final Energy Yield:
• Electrons from NADH and FADH₂ produce ~9 ATP molecules through oxidative phosphorylation.
4|Page
