CHAPTER 34: TRICARBOXYLIC ACID CYCLE
Chapter 34: Tricarboxylic Acid Cycle
The TCA cycle (also called the Citric Acid or Krebs Cycle), occurs in the mitochondrial matrix, and is a major integration center for coordinating various pathways of carbohydrate, lipid, and protein metabolism. It plays a major role in transamination and deamination reactions, gluconeogenesis from lactate, amino acids, and propionate, and indirectly, lipogenesis. Although several of these processes are carried out in many different tissues, the liver is the major organ in which all occur to a significant extent. The repercussions, therefore, are profound when, for example, large numbers of hepatocytes are damaged or replaced by connective tissue (as in acute hepatitis and cirrhosis, respectively). Few, if any, genetic abnormalities of TCA cycle enzymes have been reported, thus indicating that such abnormalities are undoubtedly incompatible with life.
The overall reaction catalyzed by the TCA cycle is a sum of the individual reactions
Acetyl-CoA + 3 NAD+ + FAD + GDP + Pi + 2 H2O —> 2 CO2 + 3 NADH + FADH2 + GTP + CoA.SH
The TCA cycle begins by combining a two-carbon molecule of acetyl-CoA with a four carbon dicarboxylate, oxaloacetate (OAA), resulting in the formation of a six carbon tricarboxylate, citrate. The source of acetyl-CoA for this reaction can be pyruvate generated through anaerobic glycolysis, ketogenic amino acids, fatty acids, or ketone bodies. The TCA cycle becomes a terminal furnace for the oxidation of acetyl-CoA if citrate is converted to isocitrate, since subsequent reactions that generate succinyl-CoA release two carbon atoms as CO2. However, unlike acetyl-CoA, OAA can be regenerated by the TCA cycle.
The enzyme catalyzing reaction #1 of the TCA cycle is citrate synthase (also called condensing enzyme). This reaction is exothermic, and physiologically irreversible in mitochondria. Citrate synthase is inhibited by high mitochondrial concentrations of ATP, citrate, and long-chain fatty acyl-CoA. Since both acetyl-CoA and OAA are impermeable to mitochondrial membranes (like NAD+ and NADH), these molecules cannot diffuse out of mitochondria (directly) to serve as substrates for cytoplasmic reactions, or enzyme regulators. However, citrate is permeable to mitochondrial membranes, and when it diffuses into the cytoplasm it inhibits phosphofructokinase (PFK) activity, and can be converted back to acetyl-CoA and OAA through the activity of citrate cleavage enzyme (or citrate lyase). The acetyl-CoA so generated can now serve as a substrate for lipid biosynthesis.
The second reaction sequence in the TCA cycle requires the aconitase (aconitate hydratase) enzyme, which converts citrate to isocitrate. This enzyme contains ferrous iron (Fe++) in the form of an iron-sulfur (Fe:S) protein complex. The conversion takes place in two steps, dehydration to cis-aconitate (not shown), then rehydration to isocitrate. This reaction sequence is inhibited by fluoroacetate and fluoroacetamide, plant poisons and rodenticides which, in the form of fluoroacetyl-CoA, condense with OAA to form fluorocitrate. This compound further inhibits aconitase, causing citrate accumulation. Fluoroacetate and fluoroacetamide are thus suicide inhibitors since the cell converts them from their inactive states to an active inhibitory unit (fluorocitrate). These poisonous compounds are odorless, tasteless, and water-soluble. Dogs, cats, pigs and other animals that eat deceased fluoroacetate- or fluoroacetamide-poisoned rats or mice may meet a similar fate.
Isocitrate next undergoes dehydrogenation (reaction #3) in the presence of isocitrate dehydrogenase (ICD) to form oxalosuccinate (structure not shown). Two different ICDs have been described. One, which is NAD+ -specific, is found only in mitochondria where it is associated with the TCA cycle, and the other, which is NADP+ -specific, is found in the cytoplasm associated with the outer mitochondrial membrane. Cytoplasmic ICD is important in ruminant adipocytes, where it assists in providing NADPH for lipogenesis.
Following oxalosuccinate formation, there is a second reaction catalyzed by ICD which involves decarboxylation, and thus formation of a-ketoglutarate (a-KG=). Either Mn++ or Mg++ is an important prosthetic component of this decarboxylation process. This rate-limiting reaction sequence in the TCA cycle, catalyzed by ICD, is considered to be physiologically irreversible, and is allosterically inhibited by ATP and NADH, and activated by ADP and NAD+. These compounds affect the Km of ICD for its primary substrate, isocitrate. Niacin, water-soluble vitamin B3 in the form of NAD+, is required in three dehydrogenase-catalyzed reactions of the TCA cycle (reactions #3, #4, and #8).
Next, a-KG= undergoes oxidative decarboxylation (reaction #4) in a manner analogous to the conversion of pyruvate to acetyl-CoA. The a-KG= oxidative decarboxylation reaction, catalyzed by a-KG= dehydrogenase, also requires identical cofactors to those present in the pyruvate dehydrogenase catalyzed reaction (e.g., thiamin (vitamin B1 which forms a thiamin diphosphate prosthetic group), niacin (a source of NAD+ ), and coenzyme A.SH (CoA.SH; vitamin B5). The equilibrium of this reaction is so much in favor of succinyl-CoA formation, that it is also considered to be physiologically irreversible (like reactions #1 and #3). As in the case of pyruvate decarboxylation, arsenite (As) and mercuric (Hg) ions inhibit a=KG decarboxylation by binding to an -SH group in the dehydrogenase complex, thus preventing its reoxidation to a disulfide. Physiologic inhibitors of this reaction include high mitochondrial levels of NADH, ATP, and the reaction product, succinyl-CoA.
To continue with the TCA cycle, succinyl-CoA is next converted to succinate by the enzyme succinate thiokinase (also called succinyl-CoA synthetase, reaction #5). This is the only example in the TCA cycle of the generation of a high-energy phosphate (either ATP or GTP) at the substrate level. In liver cells, for example, where the gluconeogenic pathway is active, GTP produced in mitochondria through this reaction can be used by PEP carboxykinase to convert OAA to PEP on its way to becoming glucose. However, in most cells ATP will be the high energy phosphate carrier derived from this reaction. Either Mn++ or Mg++ are prosthetic groups for succinate thiokinase, and the water-soluble vitamin B5 (pantothenic acid), which is a part of CoA.SH, is the cofactor attached to "active" carboxylic acid residues like succinyl-CoA.
Succinate is next dehydrogenated to fumarate, catalyzed by the enzyme succinate dehydrogenase (reaction #6). This is the only reaction in the TCA cycle involving transfer of hydrogen from a substrate to a flavoprotein (FAD), without the participation of NAD+ . In addition to FAD, this enzyme, like aconitase above, contains iron-sulfur (Fe:S) protein. The water-soluble vitamin B2 (riboflavin), is used to form flavin adenine dinucleotide (FAD), and the succinate dehydrogenase complex is physiologically inhibited by high mitochondrial concentrations of OAA, as well as by malonate (in vitro). Malonate is structurally similar to succinate.
Fumarase (fumarate hydratase) next catalyzes addition of H2O to fumarate, thus yielding malate (reaction #7). Malate is then converted to OAA by malate dehydrogenase, a reversible reaction requiring vitamin B3 (niacin) as NAD+ (reaction #8). Although the equilibrium of this reaction strongly favors malate formation, net flux is usually toward OAA formation since this compound, together with the other product of the reaction (NADH), is removed continuously in further reactions. However, during gluconeogenesis in liver tissue, OAA synthesis from pyruvate favors malate formation, which then exits mitochondria and reforms OAA in the cytoplasm. Thus, reversal of this reaction becomes important in the hepatic dicarboxylic acid cycle. Three molecules of NADH and one of FADH2 are produced for each molecule of acetyl-CoA catabolized in one revolution of the TCA cycle. These reducing equivalents are next passed on to the respiratory chain in the inner mitochondrial membrane, providing sufficient O2 is available. During passage along the chain, each NADH molecule generates three high-energy phosphate bonds in the form of ATP, and each FADH2 generates two. A further high energy bond in the form of GTP or ATP is produced during conversion of succinyl-CoA to succinate, thus bringing the total to 12 ATPs generated for each turn of the cycle. Conversion of pyruvate to acetyl-CoA generates another NADH, and considering that 1 mol of glucose yields 2 mol of pyruvate, the mitochondrial (aerobic) portion of glucose oxidation (from 2 molecules of pyruvate through acetyl-CoA) yields 30 ATP equivalents. This, added to the 8 ATP equivalents derived (both directly and indirectly) from the cytoplasmic phase, brings the total potential aerobic high energy phosphate bond production from glucose oxidation to 38 ATP equivalents (assuming that pyruvate is passed through acetyl-CoA, and not OAA).
SUMMARY
The Tricarboxylic Acid (TCA) cycle, also known as the Citric Acid or Krebs Cycle, is a crucial metabolic pathway that occurs in the mitochondrial matrix. It serves as a central hub for coordinating various pathways of carbohydrate, lipid, and protein metabolism. The liver is the primary organ where these processes occur to a significant extent. The TCA cycle begins by combining acetyl-CoA with oxaloacetate to form citrate. Acetyl-CoA can be derived from various sources such as pyruvate, fatty acids, or ketone bodies. The enzyme citrate synthase catalyzes this reaction and is inhibited by high levels of ATP, citrate, and long-chain fatty acyl-CoA. Citrate can diffuse out of mitochondria and inhibit phosphofructokinase activity in the cytoplasm, leading to the production of acetyl-CoA for lipid biosynthesis. The TCA cycle continues with the conversion of citrate to isocitrate by the enzyme aconitase. Isocitrate is then converted to alpha-ketoglutarate by isocitrate dehydrogenase. This reaction is NAD+-specific in mitochondria and NADP+-specific in the cytoplasm. Alpha-ketoglutarate undergoes oxidative decarboxylation to form succinyl-CoA, which is then converted to succinate by succinate thiokinase. Succinate is dehydrogenated to fumarate by succinate dehydrogenase, and fumarate is hydrated to malate by fumarase. Malate is then converted back to oxaloacetate by malate dehydrogenase. Throughout the TCA cycle, reducing equivalents in the form of NADH and FADH2 are generated, which are then passed on to the respiratory chain to produce ATP. In total, one turn of the TCA cycle can generate 12 ATP equivalents. When considering the conversion of pyruvate to acetyl-CoA, the total potential ATP production from glucose oxidation is 38 ATP equivalents.
OUTLINE
The TCA cycle (also called the Citric Acid or Krebs Cycle) occurs in the mitochondrial matrix and is a major integration center for coordinating various pathways of carbohydrate, lipid, and protein metabolism.
The TCA cycle plays a major role in transamination and deamination reactions, gluconeogenesis from lactate, amino acids, and propionate, and indirectly, lipogenesis.
The liver is the major organ in which all these processes occur to a significant extent.
Few, if any, genetic abnormalities of TCA cycle enzymes have been reported, indicating that such abnormalities are incompatible with life.
Acetyl-CoA + 3 NAD+ + FAD + GDP + Pi + 2 H2O —> 2 CO2 + 3 NADH + FADH2 + GTP + CoA.SH
Acetyl-CoA combines with oxaloacetate (OAA) to form citrate.
Citrate is converted to isocitrate by the enzyme aconitase.
Isocitrate is dehydrogenated to form oxalosuccinate, which is then decarboxylated to form α-ketoglutarate (α-KG) by isocitrate dehydrogenase (ICD).
α-KG undergoes oxidative decarboxylation to form succinyl-CoA by α-KG dehydrogenase.
Succinyl-CoA is converted to succinate by succinate thiokinase, generating either ATP or GTP.
Succinate is dehydrogenated to form fumarate by succinate dehydrogenase.
Fumarate is hydrated to form malate by fumarase.
Malate is converted back to oxaloacetate by malate dehydrogenase.
Citrate synthase is inhibited by high mitochondrial concentrations of ATP, citrate, and long-chain fatty acyl-CoA.
Citrate inhibits phosphofructokinase (PFK) activity in the cytoplasm.
Aconitase is inhibited by fluoroacetate and fluoroacetamide, which can be converted to fluorocitrate, a suicide inhibitor.
Isocitrate dehydro
QUESTIONS
Qcard 1:
Question: What is the TCA cycle?
Answer: The TCA cycle, also known as the Citric Acid or Krebs Cycle, is a major integration center for coordinating various pathways of carbohydrate, lipid, and protein metabolism.
Qcard 2:
Question: Where does the TCA cycle occur?
Answer: The TCA cycle occurs in the mitochondrial matrix.
Qcard 3:
Question: What role does the TCA cycle play in metabolism?
Answer: The TCA cycle plays a major role in transamination and deamination reactions, gluconeogenesis, and indirectly, lipogenesis.
Qcard 4:
Question: What happens when hepatocytes are damaged or replaced by connective tissue?
Answer: The repercussions are profound as the liver is the major organ where all the metabolic processes of the TCA cycle occur.
Qcard 5:
Question: What is the overall reaction catalyzed by the TCA cycle?
Answer: Acetyl-CoA + 3 NAD+ + FAD + GDP + Pi + 2 H2O —> 2 CO2 + 3 NADH + FADH2 + GTP + CoA.SH
Qcard 6:
Question: How does the TCA cycle begin?
Answer: The TCA cycle begins by combining acetyl-CoA with oxaloacetate, resulting in the formation of citrate.
Qcard 7:
Question: What can be the source of acetyl-CoA for the TCA cycle?
Answer: Acetyl-CoA can be generated from pyruvate, ketogenic amino acids, fatty acids, or ketone bodies.
Qcard 8:
Question: What is the enzyme that catalyzes the first reaction of the TCA cycle?
Answer: The enzyme is citrate synthase, also known as condensing enzyme.
Qcard 9:
Question: How is citrate involved in lipid biosynthesis?
Answer: Citrate can be converted back to acetyl-CoA and oxaloacetate through the activity of citrate cleavage enzyme, which generates acetyl-CoA for lipid biosynthesis.
Qcard 10:
Question: What is the second reaction sequence in the TCA cycle?
Answer: The second reaction sequence involves the conversion of citrate to isocitrate, catalyzed by the enzyme aconitase.
The TCA cycle is a major integration center for coordinating various pathways of carbohydrate, lipid, and protein metabolism.
Overview of the TCA cycle
Reactions and enzymes involved in the TCA cycle
Regulation and control of the TCA cycle
Metabolic implications of TCA cycle dysfunction
Occurs in the mitochondrial matrix
Plays a major role in transamination and deamination reactions, gluconeogenesis, and lipogenesis
Liver is the major organ where these processes occur
Overall reaction of the TCA cycle
Formation of citrate from acetyl-CoA and oxaloacetate
Citrate synthase and its regulation
Conversion of citrate to isocitrate by aconitase
Isocitrate dehydrogenase and its role in dehydrogenation and decarboxylation
Oxidative decarboxylation of alpha-ketoglutarate
Succinyl-CoA formation and its role in substrate-level phosphorylation
Conversion of succinate to fumarate by succinate dehydrogenase
Fumarase and its role in hydration of fumarate
Conversion of malate to oxaloacetate by malate dehydrogenase
Inhibition and activation of enzymes in the TCA cycle
Allosteric regulation of isocitrate dehydrogenase
Physiological inhibitors of alpha-ketoglutarate dehydrogenase
Role of NAD+ and niacin in TCA cycle reactions
Genetic abnormalities of TCA cycle enzymes
Impact of liver damage on TCA cycle processes
Importance of TCA cycle in energy production (ATP generation)
Note: The mind map is not exhaustive and only covers the main points of Chapter 34: Tricarboxylic Acid Cycle.
Read and understand the overall reaction catalyzed by the TCA cycle.
Familiarize yourself with the role of the TCA cycle in coordinating various pathways of metabolism.
Study the significance of the liver in TCA cycle processes.
Take note of the absence of genetic abnormalities in TCA cycle enzymes.
Focus on the first reaction of the TCA cycle catalyzed by citrate synthase.
Understand the inhibitory factors of citrate synthase.
Learn about the permeability of citrate and its role in cytoplasmic reactions.
Explore the connection between citrate and lipid biosynthesis.
Dive into the second reaction sequence of the TCA cycle involving aconitase.
Study the steps of aconitase's conversion of citrate to isocitrate.
Discover the inhibitory effects of fluoroacetate and fluoroacetamide on aconitase.
Understand the consequences of consuming fluoroacetate or fluoroacetamide-poisoned animals.
Focus on the dehydrogenation of isocitrate in the third reaction of the TCA cycle.
Differentiate between the NAD+-specific and NADP+-specific isocitrate dehydrogenases.
Explore the role of cytoplasmic isocitrate dehydrogenase in ruminant adipocytes.
Recognize the importance of niacin (vitamin B3) in dehydrogenase-catalyzed reactions.
Study the oxidative decarboxylation of α-ketoglutarate in the fourth reaction of the TCA cycle.
Compare the cofactors required for α-ketoglutarate dehydrogenase and pyruvate dehydrogenase reactions.
Understand the physiological irreversibility of the α-ketoglutarate decarboxylation reaction.
Take note of the physiologic inhibitors of this reaction.
Note: It is important to review and revise the material covered each day to reinforce understanding and retention. Additionally, practicing with related questions and problems can enhance comprehension and application of the concepts.
Chapter 34: Tricarboxylic Acid Cycle
The TCA cycle (also called the Citric Acid or Krebs Cycle), occurs in the mitochondrial matrix, and is a major integration center for coordinating various pathways of carbohydrate, lipid, and protein metabolism. It plays a major role in transamination and deamination reactions, gluconeogenesis from lactate, amino acids, and propionate, and indirectly, lipogenesis. Although several of these processes are carried out in many different tissues, the liver is the major organ in which all occur to a significant extent. The repercussions, therefore, are profound when, for example, large numbers of hepatocytes are damaged or replaced by connective tissue (as in acute hepatitis and cirrhosis, respectively). Few, if any, genetic abnormalities of TCA cycle enzymes have been reported, thus indicating that such abnormalities are undoubtedly incompatible with life.
The overall reaction catalyzed by the TCA cycle is a sum of the individual reactions
Acetyl-CoA + 3 NAD+ + FAD + GDP + Pi + 2 H2O —> 2 CO2 + 3 NADH + FADH2 + GTP + CoA.SH
The TCA cycle begins by combining a two-carbon molecule of acetyl-CoA with a four carbon dicarboxylate, oxaloacetate (OAA), resulting in the formation of a six carbon tricarboxylate, citrate. The source of acetyl-CoA for this reaction can be pyruvate generated through anaerobic glycolysis, ketogenic amino acids, fatty acids, or ketone bodies. The TCA cycle becomes a terminal furnace for the oxidation of acetyl-CoA if citrate is converted to isocitrate, since subsequent reactions that generate succinyl-CoA release two carbon atoms as CO2. However, unlike acetyl-CoA, OAA can be regenerated by the TCA cycle.
The enzyme catalyzing reaction #1 of the TCA cycle is citrate synthase (also called condensing enzyme). This reaction is exothermic, and physiologically irreversible in mitochondria. Citrate synthase is inhibited by high mitochondrial concentrations of ATP, citrate, and long-chain fatty acyl-CoA. Since both acetyl-CoA and OAA are impermeable to mitochondrial membranes (like NAD+ and NADH), these molecules cannot diffuse out of mitochondria (directly) to serve as substrates for cytoplasmic reactions, or enzyme regulators. However, citrate is permeable to mitochondrial membranes, and when it diffuses into the cytoplasm it inhibits phosphofructokinase (PFK) activity, and can be converted back to acetyl-CoA and OAA through the activity of citrate cleavage enzyme (or citrate lyase). The acetyl-CoA so generated can now serve as a substrate for lipid biosynthesis.
The second reaction sequence in the TCA cycle requires the aconitase (aconitate hydratase) enzyme, which converts citrate to isocitrate. This enzyme contains ferrous iron (Fe++) in the form of an iron-sulfur (Fe:S) protein complex. The conversion takes place in two steps, dehydration to cis-aconitate (not shown), then rehydration to isocitrate. This reaction sequence is inhibited by fluoroacetate and fluoroacetamide, plant poisons and rodenticides which, in the form of fluoroacetyl-CoA, condense with OAA to form fluorocitrate. This compound further inhibits aconitase, causing citrate accumulation. Fluoroacetate and fluoroacetamide are thus suicide inhibitors since the cell converts them from their inactive states to an active inhibitory unit (fluorocitrate). These poisonous compounds are odorless, tasteless, and water-soluble. Dogs, cats, pigs and other animals that eat deceased fluoroacetate- or fluoroacetamide-poisoned rats or mice may meet a similar fate.
Isocitrate next undergoes dehydrogenation (reaction #3) in the presence of isocitrate dehydrogenase (ICD) to form oxalosuccinate (structure not shown). Two different ICDs have been described. One, which is NAD+ -specific, is found only in mitochondria where it is associated with the TCA cycle, and the other, which is NADP+ -specific, is found in the cytoplasm associated with the outer mitochondrial membrane. Cytoplasmic ICD is important in ruminant adipocytes, where it assists in providing NADPH for lipogenesis.
Following oxalosuccinate formation, there is a second reaction catalyzed by ICD which involves decarboxylation, and thus formation of a-ketoglutarate (a-KG=). Either Mn++ or Mg++ is an important prosthetic component of this decarboxylation process. This rate-limiting reaction sequence in the TCA cycle, catalyzed by ICD, is considered to be physiologically irreversible, and is allosterically inhibited by ATP and NADH, and activated by ADP and NAD+. These compounds affect the Km of ICD for its primary substrate, isocitrate. Niacin, water-soluble vitamin B3 in the form of NAD+, is required in three dehydrogenase-catalyzed reactions of the TCA cycle (reactions #3, #4, and #8).
Next, a-KG= undergoes oxidative decarboxylation (reaction #4) in a manner analogous to the conversion of pyruvate to acetyl-CoA. The a-KG= oxidative decarboxylation reaction, catalyzed by a-KG= dehydrogenase, also requires identical cofactors to those present in the pyruvate dehydrogenase catalyzed reaction (e.g., thiamin (vitamin B1 which forms a thiamin diphosphate prosthetic group), niacin (a source of NAD+ ), and coenzyme A.SH (CoA.SH; vitamin B5). The equilibrium of this reaction is so much in favor of succinyl-CoA formation, that it is also considered to be physiologically irreversible (like reactions #1 and #3). As in the case of pyruvate decarboxylation, arsenite (As) and mercuric (Hg) ions inhibit a=KG decarboxylation by binding to an -SH group in the dehydrogenase complex, thus preventing its reoxidation to a disulfide. Physiologic inhibitors of this reaction include high mitochondrial levels of NADH, ATP, and the reaction product, succinyl-CoA.
To continue with the TCA cycle, succinyl-CoA is next converted to succinate by the enzyme succinate thiokinase (also called succinyl-CoA synthetase, reaction #5). This is the only example in the TCA cycle of the generation of a high-energy phosphate (either ATP or GTP) at the substrate level. In liver cells, for example, where the gluconeogenic pathway is active, GTP produced in mitochondria through this reaction can be used by PEP carboxykinase to convert OAA to PEP on its way to becoming glucose. However, in most cells ATP will be the high energy phosphate carrier derived from this reaction. Either Mn++ or Mg++ are prosthetic groups for succinate thiokinase, and the water-soluble vitamin B5 (pantothenic acid), which is a part of CoA.SH, is the cofactor attached to "active" carboxylic acid residues like succinyl-CoA.
Succinate is next dehydrogenated to fumarate, catalyzed by the enzyme succinate dehydrogenase (reaction #6). This is the only reaction in the TCA cycle involving transfer of hydrogen from a substrate to a flavoprotein (FAD), without the participation of NAD+ . In addition to FAD, this enzyme, like aconitase above, contains iron-sulfur (Fe:S) protein. The water-soluble vitamin B2 (riboflavin), is used to form flavin adenine dinucleotide (FAD), and the succinate dehydrogenase complex is physiologically inhibited by high mitochondrial concentrations of OAA, as well as by malonate (in vitro). Malonate is structurally similar to succinate.
Fumarase (fumarate hydratase) next catalyzes addition of H2O to fumarate, thus yielding malate (reaction #7). Malate is then converted to OAA by malate dehydrogenase, a reversible reaction requiring vitamin B3 (niacin) as NAD+ (reaction #8). Although the equilibrium of this reaction strongly favors malate formation, net flux is usually toward OAA formation since this compound, together with the other product of the reaction (NADH), is removed continuously in further reactions. However, during gluconeogenesis in liver tissue, OAA synthesis from pyruvate favors malate formation, which then exits mitochondria and reforms OAA in the cytoplasm. Thus, reversal of this reaction becomes important in the hepatic dicarboxylic acid cycle. Three molecules of NADH and one of FADH2 are produced for each molecule of acetyl-CoA catabolized in one revolution of the TCA cycle. These reducing equivalents are next passed on to the respiratory chain in the inner mitochondrial membrane, providing sufficient O2 is available. During passage along the chain, each NADH molecule generates three high-energy phosphate bonds in the form of ATP, and each FADH2 generates two. A further high energy bond in the form of GTP or ATP is produced during conversion of succinyl-CoA to succinate, thus bringing the total to 12 ATPs generated for each turn of the cycle. Conversion of pyruvate to acetyl-CoA generates another NADH, and considering that 1 mol of glucose yields 2 mol of pyruvate, the mitochondrial (aerobic) portion of glucose oxidation (from 2 molecules of pyruvate through acetyl-CoA) yields 30 ATP equivalents. This, added to the 8 ATP equivalents derived (both directly and indirectly) from the cytoplasmic phase, brings the total potential aerobic high energy phosphate bond production from glucose oxidation to 38 ATP equivalents (assuming that pyruvate is passed through acetyl-CoA, and not OAA).
SUMMARY
The Tricarboxylic Acid (TCA) cycle, also known as the Citric Acid or Krebs Cycle, is a crucial metabolic pathway that occurs in the mitochondrial matrix. It serves as a central hub for coordinating various pathways of carbohydrate, lipid, and protein metabolism. The liver is the primary organ where these processes occur to a significant extent. The TCA cycle begins by combining acetyl-CoA with oxaloacetate to form citrate. Acetyl-CoA can be derived from various sources such as pyruvate, fatty acids, or ketone bodies. The enzyme citrate synthase catalyzes this reaction and is inhibited by high levels of ATP, citrate, and long-chain fatty acyl-CoA. Citrate can diffuse out of mitochondria and inhibit phosphofructokinase activity in the cytoplasm, leading to the production of acetyl-CoA for lipid biosynthesis. The TCA cycle continues with the conversion of citrate to isocitrate by the enzyme aconitase. Isocitrate is then converted to alpha-ketoglutarate by isocitrate dehydrogenase. This reaction is NAD+-specific in mitochondria and NADP+-specific in the cytoplasm. Alpha-ketoglutarate undergoes oxidative decarboxylation to form succinyl-CoA, which is then converted to succinate by succinate thiokinase. Succinate is dehydrogenated to fumarate by succinate dehydrogenase, and fumarate is hydrated to malate by fumarase. Malate is then converted back to oxaloacetate by malate dehydrogenase. Throughout the TCA cycle, reducing equivalents in the form of NADH and FADH2 are generated, which are then passed on to the respiratory chain to produce ATP. In total, one turn of the TCA cycle can generate 12 ATP equivalents. When considering the conversion of pyruvate to acetyl-CoA, the total potential ATP production from glucose oxidation is 38 ATP equivalents.
OUTLINE
The TCA cycle (also called the Citric Acid or Krebs Cycle) occurs in the mitochondrial matrix and is a major integration center for coordinating various pathways of carbohydrate, lipid, and protein metabolism.
The TCA cycle plays a major role in transamination and deamination reactions, gluconeogenesis from lactate, amino acids, and propionate, and indirectly, lipogenesis.
The liver is the major organ in which all these processes occur to a significant extent.
Few, if any, genetic abnormalities of TCA cycle enzymes have been reported, indicating that such abnormalities are incompatible with life.
Acetyl-CoA + 3 NAD+ + FAD + GDP + Pi + 2 H2O —> 2 CO2 + 3 NADH + FADH2 + GTP + CoA.SH
Acetyl-CoA combines with oxaloacetate (OAA) to form citrate.
Citrate is converted to isocitrate by the enzyme aconitase.
Isocitrate is dehydrogenated to form oxalosuccinate, which is then decarboxylated to form α-ketoglutarate (α-KG) by isocitrate dehydrogenase (ICD).
α-KG undergoes oxidative decarboxylation to form succinyl-CoA by α-KG dehydrogenase.
Succinyl-CoA is converted to succinate by succinate thiokinase, generating either ATP or GTP.
Succinate is dehydrogenated to form fumarate by succinate dehydrogenase.
Fumarate is hydrated to form malate by fumarase.
Malate is converted back to oxaloacetate by malate dehydrogenase.
Citrate synthase is inhibited by high mitochondrial concentrations of ATP, citrate, and long-chain fatty acyl-CoA.
Citrate inhibits phosphofructokinase (PFK) activity in the cytoplasm.
Aconitase is inhibited by fluoroacetate and fluoroacetamide, which can be converted to fluorocitrate, a suicide inhibitor.
Isocitrate dehydro
QUESTIONS
Qcard 1:
Question: What is the TCA cycle?
Answer: The TCA cycle, also known as the Citric Acid or Krebs Cycle, is a major integration center for coordinating various pathways of carbohydrate, lipid, and protein metabolism.
Qcard 2:
Question: Where does the TCA cycle occur?
Answer: The TCA cycle occurs in the mitochondrial matrix.
Qcard 3:
Question: What role does the TCA cycle play in metabolism?
Answer: The TCA cycle plays a major role in transamination and deamination reactions, gluconeogenesis, and indirectly, lipogenesis.
Qcard 4:
Question: What happens when hepatocytes are damaged or replaced by connective tissue?
Answer: The repercussions are profound as the liver is the major organ where all the metabolic processes of the TCA cycle occur.
Qcard 5:
Question: What is the overall reaction catalyzed by the TCA cycle?
Answer: Acetyl-CoA + 3 NAD+ + FAD + GDP + Pi + 2 H2O —> 2 CO2 + 3 NADH + FADH2 + GTP + CoA.SH
Qcard 6:
Question: How does the TCA cycle begin?
Answer: The TCA cycle begins by combining acetyl-CoA with oxaloacetate, resulting in the formation of citrate.
Qcard 7:
Question: What can be the source of acetyl-CoA for the TCA cycle?
Answer: Acetyl-CoA can be generated from pyruvate, ketogenic amino acids, fatty acids, or ketone bodies.
Qcard 8:
Question: What is the enzyme that catalyzes the first reaction of the TCA cycle?
Answer: The enzyme is citrate synthase, also known as condensing enzyme.
Qcard 9:
Question: How is citrate involved in lipid biosynthesis?
Answer: Citrate can be converted back to acetyl-CoA and oxaloacetate through the activity of citrate cleavage enzyme, which generates acetyl-CoA for lipid biosynthesis.
Qcard 10:
Question: What is the second reaction sequence in the TCA cycle?
Answer: The second reaction sequence involves the conversion of citrate to isocitrate, catalyzed by the enzyme aconitase.
The TCA cycle is a major integration center for coordinating various pathways of carbohydrate, lipid, and protein metabolism.
Overview of the TCA cycle
Reactions and enzymes involved in the TCA cycle
Regulation and control of the TCA cycle
Metabolic implications of TCA cycle dysfunction
Occurs in the mitochondrial matrix
Plays a major role in transamination and deamination reactions, gluconeogenesis, and lipogenesis
Liver is the major organ where these processes occur
Overall reaction of the TCA cycle
Formation of citrate from acetyl-CoA and oxaloacetate
Citrate synthase and its regulation
Conversion of citrate to isocitrate by aconitase
Isocitrate dehydrogenase and its role in dehydrogenation and decarboxylation
Oxidative decarboxylation of alpha-ketoglutarate
Succinyl-CoA formation and its role in substrate-level phosphorylation
Conversion of succinate to fumarate by succinate dehydrogenase
Fumarase and its role in hydration of fumarate
Conversion of malate to oxaloacetate by malate dehydrogenase
Inhibition and activation of enzymes in the TCA cycle
Allosteric regulation of isocitrate dehydrogenase
Physiological inhibitors of alpha-ketoglutarate dehydrogenase
Role of NAD+ and niacin in TCA cycle reactions
Genetic abnormalities of TCA cycle enzymes
Impact of liver damage on TCA cycle processes
Importance of TCA cycle in energy production (ATP generation)
Note: The mind map is not exhaustive and only covers the main points of Chapter 34: Tricarboxylic Acid Cycle.
Read and understand the overall reaction catalyzed by the TCA cycle.
Familiarize yourself with the role of the TCA cycle in coordinating various pathways of metabolism.
Study the significance of the liver in TCA cycle processes.
Take note of the absence of genetic abnormalities in TCA cycle enzymes.
Focus on the first reaction of the TCA cycle catalyzed by citrate synthase.
Understand the inhibitory factors of citrate synthase.
Learn about the permeability of citrate and its role in cytoplasmic reactions.
Explore the connection between citrate and lipid biosynthesis.
Dive into the second reaction sequence of the TCA cycle involving aconitase.
Study the steps of aconitase's conversion of citrate to isocitrate.
Discover the inhibitory effects of fluoroacetate and fluoroacetamide on aconitase.
Understand the consequences of consuming fluoroacetate or fluoroacetamide-poisoned animals.
Focus on the dehydrogenation of isocitrate in the third reaction of the TCA cycle.
Differentiate between the NAD+-specific and NADP+-specific isocitrate dehydrogenases.
Explore the role of cytoplasmic isocitrate dehydrogenase in ruminant adipocytes.
Recognize the importance of niacin (vitamin B3) in dehydrogenase-catalyzed reactions.
Study the oxidative decarboxylation of α-ketoglutarate in the fourth reaction of the TCA cycle.
Compare the cofactors required for α-ketoglutarate dehydrogenase and pyruvate dehydrogenase reactions.
Understand the physiological irreversibility of the α-ketoglutarate decarboxylation reaction.
Take note of the physiologic inhibitors of this reaction.
Note: It is important to review and revise the material covered each day to reinforce understanding and retention. Additionally, practicing with related questions and problems can enhance comprehension and application of the concepts.
