CHAPTER 35: LEAKS IN THE TCA CYCLE

The TCA cycle is considered to be an amphibolic pathway, having both oxidative and biosynthetic functions. Its oxidative functions involve the complete combustion of acetyl-CoA, whereupon ATP is generated through oxidative phosphorylation. Several intermediates in the TCA cycle, however, can serve as substrates for other biosynthetic pathways, and therefore constitute important "leaks" in the cycle. Citrate, for example, can be used for fatty acid and/or steroid biosynthesis in the cytoplasm, and oxaloacetate (OAA) for glucose biosynthesis. Therefore, it is necessary to replenish these intermediates when they leak away into other pathways so that the TCA cycle can continue to operate. Reactions that fulfill this purpose are known as anaplerotic (i.e., to "fill up") reactions. A major anaplerotic reaction in muscle, for example, is the carboxylation of pytuvate to OAA.

TCA Cycle Intermediates are Converted to Other Essential Compounds: Citrate is permeable to mitochondrial membranes, and hence, may leave mitochondria to enter the cytoplasm. There, under the influence of citrate lyase (also called citrate cleavage enzyme), citrate may be split into OAA and acetyl-CoA, with carbon atoms from the latter being used for cytoplasmic lipid biosynthesis. This process is used by liver cells in converting carbohydrate to fat. Since the conversion of pyruvate to acetyl-CoA occurs in mitochondria, and since acetyl-CoA is largely impermeable to mitochondrial membranes, citrate serves as a carrier of carbon atoms between the standard pathway of carbohydrate oxidation, and the pathway of lipid (particularly fatty acid) biosynthesis.

Within mitochondria of liver cells, if there is insufficient OAA available to couple with acetyl-CoA in the formation of citrate, a separate pathway of ketone body formation from acetyl-CoA (through HMG-CoA) may be activated.

Virtually all organisms are capable of synthesizing amino acids, although there may be differences in the types and number of amino acids synthesized by any one organism. For example, most plants and bacteria can synthesize all 20 amino acids required for protein biosynthesis, but animal cells are capable of synthesizing only certain ones, relying on dietary intake to account for the remainder. Whatever the organism, some intermediates of the TCA cycle serve as important precursors for amino acid formation. Two particularly important compounds are OAA and a-ketoglutarate (a-KG=), while 3-phosphoglycerate and pyruvate from the Embden-Meyerhoff pathway also serve as amphibolic intermediates that can give rise to amino acids. Fumarate, succinylCoA, acetyl-CoA, and acetoacetyl-CoA are used in plant and bacterial amino acid biosynthesis, but not in animal. Generally speaking, the anabolic utilization of acetyl-CoA for biosynthetic reactions does not create as much of an operational strain on the TCA cycle as does the removal of one or more of the internal intermediates.

Nearly all mammalian organisms are capable of synthesizing pyrimidine and purine nucleotides in the same manner, from elementary substances such as CO2, NH3, aspartate (Asp), glycine (Gly), glutamine (Gln), tetrahydrofolate (H4-folate), and ribose. Two separate pathways are involved, one for pyrimidines, and another for purines. The first nucleotide product of the pyrimidine pathway is uridine-5'-monophosphate (UMP), which then becomes the parent compound from which other pyrimidine nucleotides are produced. Aspartate gives rise to carbon atoms 4, 5, and 6 of UMP, as well as to nitrogen atom number 1. Therefore, as the cellular requirement for pyrimidine biosynthesis increases, carbon atoms from OAA have a tendency to leak away from the TCA cycle in the form of Asp. Aspartate and Gln also participate in purine biosynthesis; however, each donates nitrogen atoms only. Therefore, their participation in purine biosynthesis does not remove carbon atoms from the TCA cycle. Purine carbon atoms are derived from Gly, CO2, N10-formyl-H4 folate, and N5 ,N10-methenyl-H4 folate.

Succinyl-CoA also leaks away from the TCA cycle into porphyrin metabolism. Porphyrins are cyclic compounds that have a high affinity for binding metal ions, usually ferrous (Fe++) iron, and they give rise to heme. Although heme is produced in virtually all mammalian tissues, its synthesis in bone marrow and liver is most pronounced because of requirements for incorporation into hemoglobin and the cytochromes, respectively. Therefore, porphyrin biosynthesis in these tissues represents another formidable leak in the TCA cycle.

Connections between the TCA cycle to the biosynthesis of glucose are made through OAA, malate, Asp, and phosphoenolpyruvate (PEP). Oxaloacetate itself cannot cross mitochondrial membranes, but, via reversible conversions to either malate or Asp, cytoplasmic and mitochondrial OAA/Asp and OAA/malate end up functioning as single pools that serve both cell compartments. As a gluconeogenic precursor, OAA may be formed from amino acids, lactate, or propionate via either the pyruvate carboxylase or malate dehydrogenase steps, or by deamination of Asp. Phosphoenolpyruvate carboxykinase then converts OAA to PEP on its way to forming glucose. If, however, OAA is completely recycled through the TCA cycle, it does not become a drain on the overall OAA/Asp pool.

Replenishment of TCA Cycle Intermediates: Because of all the anabolic involvements for which TCA cycle intermediates leak away, cycle activity would become deficient (so that not enough acetyl-CoA could be oxidized) if there were no processes whereby these intermediates are replenished. One of the functions of glucose is to replenish the TCA machinery, so that acetyl-CoA (derived from fat oxidation) can be completely oxidized. Pyruvate, formed from glucose via the Embden-Meyerhoff pathway, can be carboxylated by pyruvate carboxylase to OAA when there is a need to couple this intermediate with rising titers of acetyl-CoA in the formation of citrate. The saying goes that "fats are burned in a carbohydrate flame". This is certainly true of aerobic muscle fibers, which during sustained exercise may be oxidizing glucose (for OAA formation), and fatty acids (for acetyl-CoA formation) in the ratio of about 3:6.

Proteins yield amino acids that can also be converted to TCA cycle intermediates through transamination and transdeamination reactions, and thus, serve a similar purpose as that of glucose oxidation. Generally, all nonessential amino acids are in equilibrium with their respective a-ketoacids via transamination (e.g., Ala/pyruvate, Asp/OAA, and Glu/a-KG=). For the net conversion of amino acids to TCA cycle intermediates, however, transamination is not enough, since one amino acid is gained for each one lost. For the net conversion to a TCA cycle intermediate, an amino acid must not merely exchange, but rather get rid of the amino group, and this is usually accomplished through Glu formation. Glutamate can then be oxidatively deaminated by glutamate dehydrogenase; the NH3 thus formed can then be converted to urea in the liver. By these combined mechanisms, all nonessential, and some essential amino acids can be converted to their corresponding a-ketoacids. Most of these ketoacids are in equilibrium with TCA cycle intermediates or with pyruvate, and are therefore glucogenic. Some amino acids yield acetyl-CoA and/or acetoacetyl-CoA, and, thus, are ketogenic. In general, dietary proteins contain a greater proportion of glucogenic than ketogenic amino acids.

Propionate is also a source for replenishment of TCA cycle intermediates through succinyl-CoA formation. Though not of importance in the diets of dogs and cats, it is of the utmost importance for ruminant animals who rely on gluconeogenesis from propionate, amino acids, glycerol, and lactate for their glucose supply. Propionate conversion to succinyl-CoA requires the B vitamins pantothenate (a source of coenzyme A.SH), biotin (a CO2 shuttler in combination with Mn++), and cobalamin (B12; a cobalt-containing vitamin which rearranges methylmalonyl-CoA to succinyl-CoA). The B-complex vitamins are usually formed in adequate amounts by bacterial flora present in the digestive tracts of animals, or they are received through the diet.

In summary, it is well established that some intermediates in the TCA cycle are also members of other metabolic pathways. For example, in the liver OAA can be converted (via PEP) to glucose, but this can only occur if carbon atoms are fed into the cycle in addition to those from acetyl-CoA (e.g., as a-KG= formed from the deamination of Glu, or as succinyl-CoA formed from propionate). In other words, some of the reactions of the TCA cycle, but not the cycle itself, can be used biosynthetically. This "multiple use" of TCA cycle reactions in some tissues will influence the way in which flux of intermediates through the cycle is regulated.

In some respects the TCA cycle can be considered to consist of two subsequent steps: 1) the span from acetyl-CoA and OAA to a-KG=, and 2) the span from a-KG= or succinyl-CoA to malate and/or OAA. In some tissues and under certain metabolic conditions this division of the cycle into two separate steps allows us to better understand how it is possible for intermediates to feed into the cycle at, or after, the level of a-KG=, and be withdrawn from the cycle at the level of malate or OAA. This is accomplished without necessarily interfering with the normal operation of energy generation via the conventional cycle. This division is particularly relevant when amino acid metabolism is being considered. For example, when hepatocytes are conducting gluconeogenesis during sustained aerobic exercise, malate and OAA are being siphoned away from the cycle to generate PEP in the cytoplasm. Conversely, in exercising muscle cells, the primary substrate entry point is at the level of acetyl-CoA (from fatty acid oxidation), and OAA (from glucose oxidation). However, in both tissue types net flux through the TCA cycle remains the same.

Although the TCA cycle functions within mitochondria, several reactions may also occur in the cytoplasm. For example, extramitochondrial malate dehydrogenase plays a role in the malate shuttle for the reoxidation of cytosolic NADH to NAD+, and extramitochondrial isocitrate dehydrogenase is a substantial, if not main source of NADPH for fatty acid biosynthesis in adipocytes of ruminant animals. Other TCA cycle enzymes have also been found in the cytoplasm (e.g., aconitase), however their roles in this compartment are less well understood.

SUMMARY

The TCA cycle is an important metabolic pathway that has both oxidative and biosynthetic functions. It generates ATP through oxidative phosphorylation and produces intermediates that can be used for other biosynthetic pathways. These intermediates, such as citrate and oxaloacetate, can be used for fatty acid and glucose biosynthesis, respectively. To ensure the TCA cycle continues to operate, anaplerotic reactions replenish these intermediates. The TCA cycle intermediates also play a role in amino acid and nucleotide biosynthesis. Additionally, some intermediates, like succinyl-CoA, leak away from the TCA cycle into other metabolic pathways, such as porphyrin metabolism. The TCA cycle is connected to glucose biosynthesis through intermediates like OAA and malate. Glucose and amino acids can replenish TCA cycle intermediates. The TCA cycle can be divided into two steps, and certain reactions can occur in the cytoplasm. Overall, the TCA cycle is a complex pathway with multiple functions and connections to other metabolic processes.

OUTLINE

The TCA cycle is an amphibolic pathway with both oxidative and biosynthetic functions.
The oxidative functions involve the complete combustion of acetyl-CoA, generating ATP through oxidative phosphorylation.
Intermediates in the TCA cycle can be used as substrates for other biosynthetic pathways, creating "leaks" in the cycle.
Citrate can be used for fatty acid and steroid biosynthesis in the cytoplasm.
Oxaloacetate (OAA) can be used for glucose biosynthesis.
Anaplerotic reactions replenish the leaked intermediates to ensure the TCA cycle can continue operating.
Pyruvate carboxylation to OAA is a major anaplerotic reaction in muscle.
Citrate can leave mitochondria and be split into OAA and acetyl-CoA in the cytoplasm for lipid biosynthesis.
Ketone body formation from acetyl-CoA may be activated if there is insufficient OAA available in liver cells.
TCA cycle intermediates, such as OAA and α-ketoglutarate, are important precursors for amino acid formation.
Fumarate, succinyl-CoA, acetyl-CoA, and acetoacetyl-CoA are used in plant and bacterial amino acid biosynthesis, but not in animals.
TCA cycle intermediates also play a role in nucleotide biosynthesis, with OAA and malate being involved in glucose biosynthesis.
Succinyl-CoA leaks into porphyrin metabolism, which is important for heme synthesis in bone marrow and liver.
Connections between the TCA cycle and glucose biosynthesis are made through OAA, malate, aspartate, and phosphoenolpyruvate (PEP).
Glucose and proteins can replenish TCA cycle intermediates through various reactions.
Propionate is a source for replenishing TCA cycle intermediates through succinyl-CoA formation.
The TCA cycle can be considered to consist of two subsequent steps: acetyl-CoA and OAA to α-ketoglutarate, and α-ketoglutarate or succinyl-CoA to malate and/or OAA.
Some reactions of the TCA cycle can occur in the cytoplasm, such as the malate shuttle and NADPH generation for fatty acid biosynthesis.

QUESTIONS

Q: What is the TCA cycle?

A: The TCA cycle is an amphibolic pathway that involves both oxidative and biosynthetic functions.

Q: What are the oxidative functions of the TCA cycle?

A: The oxidative functions of the TCA cycle involve the complete combustion of acetyl-CoA, which generates ATP through oxidative phosphorylation.

Q: What are the biosynthetic functions of the TCA cycle?

A: Several intermediates in the TCA cycle can serve as substrates for other biosynthetic pathways, such as fatty acid and steroid biosynthesis.

Q: What are anaplerotic reactions?

A: Anaplerotic reactions are reactions that replenish the intermediates of the TCA cycle when they leak away into other pathways.

Q: What is a major anaplerotic reaction in muscle?

A: The carboxylation of pyruvate to oxaloacetate is a major anaplerotic reaction in muscle.

Q: How are TCA cycle intermediates converted to other essential compounds?

A: Citrate can be split into oxaloacetate and acetyl-CoA, with carbon atoms from the latter being used for cytoplasmic lipid biosynthesis.

Q: What is the role of citrate in lipid biosynthesis?

A: Citrate serves as a carrier of carbon atoms between the standard pathway of carbohydrate oxidation and the pathway of lipid biosynthesis.

Q: What happens if there is insufficient oxaloacetate available in liver cells?

A: A separate pathway of ketone body formation from acetyl-CoA may be activated.

Q: What are the important precursors for amino acid formation?

A: Oxaloacetate and alpha-ketoglutarate are important precursors for amino acid formation.

Q: What are the two separate pathways involved in nucleotide biosynthesis?

A: There are separate pathways for pyrimidine and purine nucleotide biosynthesis.

Q: How are TCA cycle intermediates connected to the biosynthesis of glucose?

A: Oxaloacetate, malate, aspartate, and phosphoenolpyruvate are connected to the biosynthesis of glucose.

Q: How are TCA cycle intermediates replenished?

A: Glucose and proteins can replenish TCA cycle intermediates through various metabolic reactions.

Q: What is the role of glucose in replenishing TCA cycle intermediates?

A: Glucose provides carbon atoms for acetyl-CoA production in the TCA cycle. It is converted to pyruvate through glycolysis, then pyruvate is converted to acetyl-CoA. Acetyl-CoA enters the TCA cycle to generate energy by oxidizing carbon compounds. Glucose fuels the TCA cycle and replenishes its intermediates.

Mind Map

The TCA cycle is an amphibolic pathway with both oxidative and biosynthetic functions.
Intermediates in the TCA cycle can leak into other pathways, requiring replenishment for the cycle to continue.

Main Branches:

1. Oxidative Functions of the TCA Cycle
2. Biosynthetic Functions of the TCA Cycle
3. Anaplerotic Reactions
4. Conversion of TCA Cycle Intermediates to Other Compounds
5. TCA Cycle Intermediates in Amino Acid Biosynthesis
6. TCA Cycle Intermediates in Nucleotide Biosynthesis
7. TCA Cycle Intermediates in Porphyrin Biosynthesis
8. Connections between the TCA Cycle and Glucose Biosynthesis
9. Replenishment of TCA Cycle Intermediates

Multiple Uses of TCA Cycle Reactions
Division of the TCA Cycle into Two Steps
Cytoplasmic Reactions in the TCA Cycle

1. Oxidative Functions of the TCA Cycle

Complete combustion of acetyl-CoA
ATP generation through oxidative phosphorylation

2. Biosynthetic Functions of the TCA Cycle

Intermediates used for fatty acid and steroid biosynthesis
Reactions that fulfill the purpose of replenishing intermediates are known as anaplerotic reactions

3. Anaplerotic Reactions

Carboxylation of pyruvate to oxaloacetate (OAA) is a major anaplerotic reaction in muscle

4. Conversion of TCA Cycle Intermediates to Other Compounds

Citrate can be split into OAA and acetyl-CoA for cytoplasmic lipid biosynthesis
Ketone body formation from acetyl-CoA can be activated in liver cells with insufficient OAA

5. TCA Cycle Intermediates in Amino Acid Biosynthesis

OAA and alpha-ketoglutarate (alpha-KG) are important precursors for amino acid formation
Other intermediates like 3-phosphoglycerate and pyruvate can also give rise to amino acids

6. TCA Cycle Intermediates in Nucleotide Biosynthesis

OAA and alpha-KG are important precursors TCA cycle

Study Plan: CHAPTER 35: LEAKS IN THE TCA CYCLE

Day 1:

Read and understand the main concepts of the TCA cycle and its oxidative functions.
Focus on the role of intermediates in the TCA cycle and their importance in other biosynthetic pathways.
Take notes on the concept of "leaks" in the TCA cycle and the need for anaplerotic reactions.
Highlight the major anaplerotic reaction in muscle involving the carboxylation of pyruvate to OAA.

Day 2:

Review the conversion of TCA cycle intermediates to other essential compounds, such as citrate and acetyl-CoA.
Understand the role of citrate in fatty acid and steroid biosynthesis in the cytoplasm.
Study the permeability of citrate to mitochondrial membranes and its function as a carrier of carbon atoms.
Take notes on the separate pathway of ketone body formation from acetyl-CoA in the absence of sufficient OAA.

Day 3:

Focus on the synthesis of amino acids and the importance of TCA cycle intermediates as precursors.
Understand the role of OAA and alpha-ketoglutarate in amino acid formation.
Take note of other amphibolic intermediates, such as 3-phosphoglycerate and pyruvate, in amino acid biosynthesis.
Differentiate between the utilization of TCA cycle intermediates in plant/bacterial vs. animal amino acid biosynthesis.

Day 4:

Study the connection between the TCA cycle and the biosynthesis of glucose.
Focus on the role of OAA, malate, Asp, and PEP in linking the TCA cycle to glucose biosynthesis.
Understand the reversible conversions and transport mechanisms of OAA and malate between cell compartments.
Take note of the gluconeogenic precursors for OAA formation and the conversion of OAA to PEP.

Day 5:

Review the replenishment of TCA cycle intermediates and their importance in maintaining cycle activity.
Understand the role of glucose, pyruvate, and proteins in replenishing TCA cycle intermediates.
Study the processes of transamination and transdeamination in converting amino acids to TCA cycle intermediates.
Take note of the role of propionate in succinyl-CoA formation and its importance in ruminant animals.