Lecture 8 - Pyruvate metabolism in Mitochondria and Tricarboxylic acid cycle | Plant Metabolism - Botany PDF Download

 Page 1


 
 
        
 
 
 
 
 
 
 
 
 
 
Discipline: Botany 
Paper: Plant Metabolism 
Lesson: Pyruvate metabolism in Mitochondria and 
Tricarboxylic Acid Cycle   
Lesson Developer: Dr. Manju A. Lal  
Department of Botany,  
Kirori Mal College, University of Delhi 
 
 
 
 
 
 
 
 
 
Page 2


 
 
        
 
 
 
 
 
 
 
 
 
 
Discipline: Botany 
Paper: Plant Metabolism 
Lesson: Pyruvate metabolism in Mitochondria and 
Tricarboxylic Acid Cycle   
Lesson Developer: Dr. Manju A. Lal  
Department of Botany,  
Kirori Mal College, University of Delhi 
 
 
 
 
 
 
 
 
 
Pyruvate metabolism in mitochondria and Tricarboxylic Acid Cycle 
 
Institute o f Lifelong Learning, University of Delhi, 2 
 
Table of contents 
? Introduction 
? Oxidative decarboxylation of pyruvate 
? Enzymes and cofactors involved 
? Pyruvate dehydrogenase complex (PDH) 
? Mechanism of the reaction 
? Regulation of the enzyme activity 
? Tricarboxylic acid cycle 
? Discovery of the cycle 
? Enzymes and the pathway 
? Carbon balance of the cycle 
? Energetics of the cycle 
? Amphibolic role of the cycle 
? Role of TCA cycle in anabolism 
? Role of TCA cycle in catabolism 
? Anaplerotic reactions  
? Regulation of TCA cycle 
? Unique features of TCA cycle in plants 
? Summary 
? Exercises 
? Glossary 
? References 
? Web links 
 
 
 
 
 
 
 
 
Page 3


 
 
        
 
 
 
 
 
 
 
 
 
 
Discipline: Botany 
Paper: Plant Metabolism 
Lesson: Pyruvate metabolism in Mitochondria and 
Tricarboxylic Acid Cycle   
Lesson Developer: Dr. Manju A. Lal  
Department of Botany,  
Kirori Mal College, University of Delhi 
 
 
 
 
 
 
 
 
 
Pyruvate metabolism in mitochondria and Tricarboxylic Acid Cycle 
 
Institute o f Lifelong Learning, University of Delhi, 2 
 
Table of contents 
? Introduction 
? Oxidative decarboxylation of pyruvate 
? Enzymes and cofactors involved 
? Pyruvate dehydrogenase complex (PDH) 
? Mechanism of the reaction 
? Regulation of the enzyme activity 
? Tricarboxylic acid cycle 
? Discovery of the cycle 
? Enzymes and the pathway 
? Carbon balance of the cycle 
? Energetics of the cycle 
? Amphibolic role of the cycle 
? Role of TCA cycle in anabolism 
? Role of TCA cycle in catabolism 
? Anaplerotic reactions  
? Regulation of TCA cycle 
? Unique features of TCA cycle in plants 
? Summary 
? Exercises 
? Glossary 
? References 
? Web links 
 
 
 
 
 
 
 
 
Pyruvate metabolism in mitochondria and Tricarboxylic Acid Cycle 
 
Institute o f Lifelong Learning, University of Delhi, 3 
 
Learning outcomes 
In this lesson you will learn about: 
1. Pyruvate metabolism in the mitochondria by oxidative decarboxylation. You will 
learn about the steps of the reactions, the enzymes and cofactors involved and also 
about the regulation of the enzymes. 
2. Tricarbxylic acid cycle: the acetyl-Coenzyme A, which is produced in the above said 
reaction, enters TCA cycle. You will learn about the discovery of the cycle, reactions 
of the cycle, the enzymes and regulation of the cycle. 
3.  Anabolic and catabolic roles of the TCA cycle. 
4. ‘Filling up’ pathways, which are responsible for replenishing the intermediates of the 
cycle. 
 
 
Introduction 
In the previous lesson you have studied that glucose is converted to pyruvate in the 
cytosol by a metabolic process known as glycolysis. There is no requirement of oxygen in 
the glycolytic process. However, further fate of pyruvate is determined by the availability 
of oxygen. In case oxygen is not available, pyruvate is either converted to lactic acid or 
ethyl alcohol by the fermentation process in the cytosol itself.  However, in presence of 
oxygen, glycolytic NADH is oxidized by the electron transport chain (located in the inner 
mitochondrial membrane), and pyruvate is transported to mitochondria for further 
metabolism. Because of the presence of porins, the outer membrane of mitochondria is 
freely permeable to many solutes (having the size up to 10,000 daltons) but not to 
proteins. The inner mitochondrial membrane has restricted permeability. There are 
transporters present in the inner mitochondrial membrane, which allow the passage of 
selective molecules across the membrane, so that the compartmentalization of the 
chemical environment inside the mitochondria can be regulated, and the optimal 
requirement for various metabolic reactions, occurring inside the organelle, can be 
maintained. Pyruvate translocator, present in the inner mitochondrial membrane, 
transports pyruvate inside the mitochondria from the cytosol in exchange of OH- ions.  
Page 4


 
 
        
 
 
 
 
 
 
 
 
 
 
Discipline: Botany 
Paper: Plant Metabolism 
Lesson: Pyruvate metabolism in Mitochondria and 
Tricarboxylic Acid Cycle   
Lesson Developer: Dr. Manju A. Lal  
Department of Botany,  
Kirori Mal College, University of Delhi 
 
 
 
 
 
 
 
 
 
Pyruvate metabolism in mitochondria and Tricarboxylic Acid Cycle 
 
Institute o f Lifelong Learning, University of Delhi, 2 
 
Table of contents 
? Introduction 
? Oxidative decarboxylation of pyruvate 
? Enzymes and cofactors involved 
? Pyruvate dehydrogenase complex (PDH) 
? Mechanism of the reaction 
? Regulation of the enzyme activity 
? Tricarboxylic acid cycle 
? Discovery of the cycle 
? Enzymes and the pathway 
? Carbon balance of the cycle 
? Energetics of the cycle 
? Amphibolic role of the cycle 
? Role of TCA cycle in anabolism 
? Role of TCA cycle in catabolism 
? Anaplerotic reactions  
? Regulation of TCA cycle 
? Unique features of TCA cycle in plants 
? Summary 
? Exercises 
? Glossary 
? References 
? Web links 
 
 
 
 
 
 
 
 
Pyruvate metabolism in mitochondria and Tricarboxylic Acid Cycle 
 
Institute o f Lifelong Learning, University of Delhi, 3 
 
Learning outcomes 
In this lesson you will learn about: 
1. Pyruvate metabolism in the mitochondria by oxidative decarboxylation. You will 
learn about the steps of the reactions, the enzymes and cofactors involved and also 
about the regulation of the enzymes. 
2. Tricarbxylic acid cycle: the acetyl-Coenzyme A, which is produced in the above said 
reaction, enters TCA cycle. You will learn about the discovery of the cycle, reactions 
of the cycle, the enzymes and regulation of the cycle. 
3.  Anabolic and catabolic roles of the TCA cycle. 
4. ‘Filling up’ pathways, which are responsible for replenishing the intermediates of the 
cycle. 
 
 
Introduction 
In the previous lesson you have studied that glucose is converted to pyruvate in the 
cytosol by a metabolic process known as glycolysis. There is no requirement of oxygen in 
the glycolytic process. However, further fate of pyruvate is determined by the availability 
of oxygen. In case oxygen is not available, pyruvate is either converted to lactic acid or 
ethyl alcohol by the fermentation process in the cytosol itself.  However, in presence of 
oxygen, glycolytic NADH is oxidized by the electron transport chain (located in the inner 
mitochondrial membrane), and pyruvate is transported to mitochondria for further 
metabolism. Because of the presence of porins, the outer membrane of mitochondria is 
freely permeable to many solutes (having the size up to 10,000 daltons) but not to 
proteins. The inner mitochondrial membrane has restricted permeability. There are 
transporters present in the inner mitochondrial membrane, which allow the passage of 
selective molecules across the membrane, so that the compartmentalization of the 
chemical environment inside the mitochondria can be regulated, and the optimal 
requirement for various metabolic reactions, occurring inside the organelle, can be 
maintained. Pyruvate translocator, present in the inner mitochondrial membrane, 
transports pyruvate inside the mitochondria from the cytosol in exchange of OH- ions.  
Pyruvate metabolism in mitochondria and Tricarboxylic Acid Cycle 
 
Institute o f Lifelong Learning, University of Delhi, 4 
 
 
Figure: Pyruvate is transported into mitochondria where further metabolism takes place. 
Source:http://cnx.org/contents/db89c8f8-a27c-4685-ad2a-
19d11a2a7e2e@13.11:66/Principles_of_Biology (cc) 
 
Oxidative decarboxylation of pyruvate 
In mitochondria, pyruvate is converted to acetyl- Coenzyme A by a reaction known as 
oxidative decarboxylation, since the terminal carboxylic group of pyruvate is lost as 
carbon-dioxide (decarboxylation) and oxidation of pyruvate also occurs, which is coupled 
to reduction of NAD
+ 
to NADH. The reaction can be written as: 
 
Pyruvate dehydrogenase complex 
CH
3
COCOO-+ CoA- SH + NAD
+
 ---------------- ?CH
3
COCoA 
+ CO
2
 + NADH 
 
 
Page 5


 
 
        
 
 
 
 
 
 
 
 
 
 
Discipline: Botany 
Paper: Plant Metabolism 
Lesson: Pyruvate metabolism in Mitochondria and 
Tricarboxylic Acid Cycle   
Lesson Developer: Dr. Manju A. Lal  
Department of Botany,  
Kirori Mal College, University of Delhi 
 
 
 
 
 
 
 
 
 
Pyruvate metabolism in mitochondria and Tricarboxylic Acid Cycle 
 
Institute o f Lifelong Learning, University of Delhi, 2 
 
Table of contents 
? Introduction 
? Oxidative decarboxylation of pyruvate 
? Enzymes and cofactors involved 
? Pyruvate dehydrogenase complex (PDH) 
? Mechanism of the reaction 
? Regulation of the enzyme activity 
? Tricarboxylic acid cycle 
? Discovery of the cycle 
? Enzymes and the pathway 
? Carbon balance of the cycle 
? Energetics of the cycle 
? Amphibolic role of the cycle 
? Role of TCA cycle in anabolism 
? Role of TCA cycle in catabolism 
? Anaplerotic reactions  
? Regulation of TCA cycle 
? Unique features of TCA cycle in plants 
? Summary 
? Exercises 
? Glossary 
? References 
? Web links 
 
 
 
 
 
 
 
 
Pyruvate metabolism in mitochondria and Tricarboxylic Acid Cycle 
 
Institute o f Lifelong Learning, University of Delhi, 3 
 
Learning outcomes 
In this lesson you will learn about: 
1. Pyruvate metabolism in the mitochondria by oxidative decarboxylation. You will 
learn about the steps of the reactions, the enzymes and cofactors involved and also 
about the regulation of the enzymes. 
2. Tricarbxylic acid cycle: the acetyl-Coenzyme A, which is produced in the above said 
reaction, enters TCA cycle. You will learn about the discovery of the cycle, reactions 
of the cycle, the enzymes and regulation of the cycle. 
3.  Anabolic and catabolic roles of the TCA cycle. 
4. ‘Filling up’ pathways, which are responsible for replenishing the intermediates of the 
cycle. 
 
 
Introduction 
In the previous lesson you have studied that glucose is converted to pyruvate in the 
cytosol by a metabolic process known as glycolysis. There is no requirement of oxygen in 
the glycolytic process. However, further fate of pyruvate is determined by the availability 
of oxygen. In case oxygen is not available, pyruvate is either converted to lactic acid or 
ethyl alcohol by the fermentation process in the cytosol itself.  However, in presence of 
oxygen, glycolytic NADH is oxidized by the electron transport chain (located in the inner 
mitochondrial membrane), and pyruvate is transported to mitochondria for further 
metabolism. Because of the presence of porins, the outer membrane of mitochondria is 
freely permeable to many solutes (having the size up to 10,000 daltons) but not to 
proteins. The inner mitochondrial membrane has restricted permeability. There are 
transporters present in the inner mitochondrial membrane, which allow the passage of 
selective molecules across the membrane, so that the compartmentalization of the 
chemical environment inside the mitochondria can be regulated, and the optimal 
requirement for various metabolic reactions, occurring inside the organelle, can be 
maintained. Pyruvate translocator, present in the inner mitochondrial membrane, 
transports pyruvate inside the mitochondria from the cytosol in exchange of OH- ions.  
Pyruvate metabolism in mitochondria and Tricarboxylic Acid Cycle 
 
Institute o f Lifelong Learning, University of Delhi, 4 
 
 
Figure: Pyruvate is transported into mitochondria where further metabolism takes place. 
Source:http://cnx.org/contents/db89c8f8-a27c-4685-ad2a-
19d11a2a7e2e@13.11:66/Principles_of_Biology (cc) 
 
Oxidative decarboxylation of pyruvate 
In mitochondria, pyruvate is converted to acetyl- Coenzyme A by a reaction known as 
oxidative decarboxylation, since the terminal carboxylic group of pyruvate is lost as 
carbon-dioxide (decarboxylation) and oxidation of pyruvate also occurs, which is coupled 
to reduction of NAD
+ 
to NADH. The reaction can be written as: 
 
Pyruvate dehydrogenase complex 
CH
3
COCOO-+ CoA- SH + NAD
+
 ---------------- ?CH
3
COCoA 
+ CO
2
 + NADH 
 
 
Pyruvate metabolism in mitochondria and Tricarboxylic Acid Cycle 
 
Institute o f Lifelong Learning, University of Delhi, 5 
 
It is an exergonic reaction with a net standard free energy change of -33.4 kJ mole
-1
 
(?G
0
’ = - 33.4 kJmole
-1
). The reaction occurs in mitochondrial matrix. 
 
Figure: The step of conversion of pyruvate molecule into acetyl coenzyme A is the 
junction between glycolysis and the TCA cycle. It is accomplished by a multi-enzyme 
complex that catalyzes three reactions: 1. Pyruvate is oxidized and carboxyl group 
(COO–) is removed to release CO
2
. 2. The remaining two-carbon compound is oxidized 
and an acetate compound is formed. An enzyme transfers the extracted electrons to 
NAD+, storing energy in the form of NADH. 3. Coenzyme A (CoA) is attached to the 
acetate by an unstable bond and this makes the acetyl group become very reactive. 
Acetyl CoA has a high potential energy will undergoes the TCA cycle to release energy to 
make ATP. 
Source:http://en.wikibooks.org/wiki/Structural_Biochemistry/Pyruvate_Dehydrogenase_
Complex (cc)