TCA Cycle (Citric acid cycle or Krebs cycle)

The TCA cycle, also known as the citric acid cycle or the Krebs cycle, is a series of biochemical reactions that take place in the mitochondria of eukaryotic cells and in the cytoplasm of prokaryotic cells. The TCA cycle is a key metabolic pathway that connects carbohydrate, fat, and protein metabolism. The main function of the TCA cycle is to oxidize acetyl-CoA, a two-carbon molecule derived from the breakdown of glucose, fatty acids, and amino acids, to carbon dioxide and water, while generating high-energy molecules such as NADH and FADH2. These molecules then donate their electrons to the electron transport chain, which drives the synthesis of ATP, the universal energy currency of the cell.

The TCA cycle was discovered by Hans Krebs in 1937, who received the Nobel Prize in Physiology or Medicine in 1953 for his work. The cycle is named after citric acid, the first molecule formed in the cycle by the condensation of acetyl-CoA and oxaloacetate. The cycle consists of eight enzymatic steps that regenerate oxaloacetate at the end, allowing the cycle to continue. The TCA cycle is also involved in various biosynthetic pathways, as some of its intermediates serve as precursors for other molecules such as amino acids, nucleotides, and heme.

The TCA cycle is a central metabolic pathway that plays a vital role in cellular respiration and energy production. It is also a source of diversity and complexity in metabolism, as it links different macromolecules and provides building blocks for various biosynthetic processes. Understanding the TCA cycle is essential for comprehending how cells utilize nutrients and generate energy under different physiological conditions.

The Krebs cycle, also known as the citric acid cycle or the tricarboxylic acid (TCA) cycle, is a series of biochemical reactions that take place in the cells of living organisms to generate energy from the breakdown of acetyl-CoA, a derivative of carbohydrates, fats, and proteins.

Eukaryotes

In eukaryotes, the Krebs cycle occurs in the mitochondrial matrix, which is the fluid-filled space inside the mitochondria. Mitochondria are organelles that are responsible for producing most of the energy (ATP) for the cell. They are often called the "powerhouses" of the cell.

The mitochondrial matrix contains all the enzymes and cofactors needed for the Krebs cycle, except for one: succinate dehydrogenase. This enzyme is embedded in the inner mitochondrial membrane, which is the innermost layer of the double membrane that surrounds the mitochondria. The inner mitochondrial membrane also contains the electron transport chain, which is a series of protein complexes that transfer electrons from NADH and FADH2 (the products of the Krebs cycle) to oxygen, generating ATP and water.

Prokaryotes

In prokaryotes, the Krebs cycle occurs in the plasma membrane, which is the outermost layer of the cell that separates it from the environment. The plasma membrane also contains the electron transport chain, which functions similarly to that in eukaryotes.

The plasma membrane of prokaryotes is usually composed of a phospholipid bilayer with embedded proteins. Some prokaryotes have additional layers outside the plasma membrane, such as a cell wall or a capsule. However, these layers do not affect the location of the Krebs cycle.

The plasma membrane contains all the enzymes and cofactors needed for the Krebs cycle. However, some prokaryotes may have variations in their Krebs cycle enzymes or pathways, depending on their metabolic needs and environmental conditions. For example, some bacteria can use alternative substrates or intermediates for the Krebs cycle, such as pyruvate or malate. Some bacteria can also bypass some steps of the Krebs cycle or use different enzymes for some reactions.

Glycolysis is the process of breaking down glucose, a six-carbon molecule, into two molecules of pyruvate, a three-carbon molecule. In the presence of oxygen, pyruvate can enter the mitochondria and undergo further oxidation in the citric acid cycle. However, before pyruvate can enter the cycle, it must be converted into acetyl CoA, a two-carbon molecule that can combine with oxaloacetate, a four-carbon molecule, to form citrate, a six-carbon molecule that initiates the cycle.

The conversion of pyruvate to acetyl CoA is catalyzed by a complex enzyme called pyruvate dehydrogenase. This enzyme requires five coenzymes: thiamine pyrophosphate (TPP), lipoic acid, coenzyme A (CoA), flavin adenine dinucleotide (FAD), and nicotinamide adenine dinucleotide (NAD+). The reaction involves three steps:

1. Decarboxylation: Pyruvate loses one carbon atom as carbon dioxide (CO2), forming a two-carbon molecule called hydroxyethyl-TPP.
2. Oxidation: Hydroxyethyl-TPP transfers its two-carbon group to lipoic acid, forming acetyl-lipoic acid. In this process, two electrons and one proton are released and transferred to NAD+, forming NADH and H+.
3. Transfer: Acetyl-lipoic acid transfers its acetyl group to CoA, forming acetyl CoA and regenerating lipoic acid.

The overall reaction can be summarized as:

Pyruvate + CoA + NAD+ → Acetyl CoA + CO2 + NADH + H+

This reaction is irreversible and highly exergonic, meaning that it releases a lot of free energy. The energy released is used to drive the synthesis of ATP in the electron transport chain.

The link between glycolysis and the citric acid cycle is important for several reasons:

• It allows the complete oxidation of glucose to CO2 and water, generating more ATP than glycolysis alone.
• It provides acetyl CoA for the citric acid cycle, which is the main source of reducing equivalents (NADH and FADH2) for the electron transport chain.
• It regulates the rate of the citric acid cycle by controlling the availability of acetyl CoA. The activity of pyruvate dehydrogenase is influenced by several factors, such as the concentration of NADH, acetyl CoA, ATP, and calcium ions.
• It connects the metabolic pathways of carbohydrates, fats, and proteins. Pyruvate can be derived from glycolysis of glucose or from other sources such as lactate, alanine, or glycerol. Acetyl CoA can be used for the synthesis of fatty acids or ketone bodies or for the degradation of certain amino acids.

Therefore, the link between glycolysis and the citric acid cycle is essential for cellular respiration and biosynthesis.

The cycle starts with the 4-carbon compound oxaloacetate, adds two carbons from acetyl-CoA, loses two carbons as CO2, and regenerates the 4-carbon compound oxaloacetate. Electrons are transferred by the cycle to NAD+ and FAD. As the electrons are subsequently passed to O2 by the electron transport chain, ATP is generated by the process of oxidative phosphorylation. ATP is also generated from GTP, produced in one reaction of the cycle by substrate-level phosphorylation. Oxidation of the carbons of acetyl-CoA to carbon dioxide requires capturing eight electrons from the molecule.

The following table summarizes the steps of the citric acid cycle, along with the enzymes involved and the products formed:

The following diagram illustrates the citric acid cycle and its connections to other metabolic pathways:

The citric acid cycle is a highly efficient pathway for the oxidation of acetyl CoA to carbon dioxide and water. In the process, it also generates high-energy molecules such as NADH, FADH2 and GTP that can be used to produce ATP, the universal energy currency of the cell.

Each molecule of acetyl CoA entering the citric acid cycle yields the following:

• Two CO2
• Three NADH
• One FADH2
• One GTP

The CO2 molecules are released as waste products of the cycle and do not contribute to the energy yield. The NADH and FADH2 molecules carry electrons that are transferred to the electron transport chain, where they are used to generate a proton gradient across the inner mitochondrial membrane. This gradient drives the synthesis of ATP by the enzyme ATP synthase. The GTP molecule is directly converted to ATP by a nucleoside diphosphate kinase.

The amount of ATP produced from each NADH and FADH2 depends on the efficiency of the electron transport chain and the coupling between electron transport and oxidative phosphorylation. In general, it is estimated that each NADH produces 2.5 ATP and each FADH2 produces 1.5 ATP. Therefore, the overall ATP yield from one acetyl CoA is 10 ATP (7.5 from NADH, 1.5 from FADH2, and 1 from GTP).

However, this calculation does not take into account the cost of transporting acetyl CoA into the mitochondrial matrix from the cytosol, where it is produced from pyruvate. In eukaryotes, this transport requires a carrier molecule called carnitine, which exchanges acetyl CoA for free CoA across the inner mitochondrial membrane. This exchange is coupled to the movement of one proton into the matrix, which reduces the proton gradient and thus the potential ATP yield by one unit. Therefore, the net ATP yield from one acetyl CoA is 9 ATP in eukaryotes.

In prokaryotes, where the citric acid cycle occurs in the plasma membrane, there is no need for carnitine-mediated transport of acetyl CoA, and thus no loss of ATP. Therefore, the net ATP yield from one acetyl CoA is 10 ATP in prokaryotes.

The citric acid cycle is thus a major source of energy production during aerobic respiration, as it accounts for most of the carbon dioxide and water formation and most of the NADH and FADH2 generation from glucose oxidation. The cycle also provides intermediates for various biosynthetic pathways, such as amino acid synthesis, fatty acid synthesis, and heme synthesis. The citric acid cycle is therefore considered to be the central hub of metabolism in living cells.

The citric acid cycle has several important functions in cellular metabolism:

• Oxidation of acetyl CoA to CO2. The citric acid cycle is the final common pathway for the oxidation of fuel molecules such as carbohydrates, fatty acids, and amino acids. The cycle converts acetyl CoA, derived from these molecules, into carbon dioxide and water, releasing energy in the process.
• Formation of NADH and FADH2 for entrance into the electron transport chain and subsequent ATP generation. The citric acid cycle captures the energy released from the oxidation of acetyl CoA in the form of reduced coenzymes NADH and FADH2. These coenzymes transfer their electrons to the electron transport chain, where they are used to generate a proton gradient across the inner mitochondrial membrane. This gradient drives the synthesis of ATP by the enzyme ATP synthase.
• Synthesis of several important molecules, including succinyl CoA (precursor molecule of heme), oxaloacetate (early intermediate molecule in gluconeogenesis and substrate for amino acid synthesis), α-ketoglutarate (substrate for amino acid synthesis), and citrate (substrate for fatty acid synthesis). The citric acid cycle also serves as a source of biosynthetic precursors for various pathways. For example, succinyl CoA is used to synthesize heme, a component of hemoglobin and cytochromes; oxaloacetate is used to produce glucose from non-carbohydrate sources (gluconeogenesis) and to synthesize certain amino acids (aspartate and asparagine); α-ketoglutarate is used to synthesize other amino acids (glutamate, glutamine, proline, and arginine); and citrate is used to produce fatty acids and cholesterol.
• It is responsible for the major share of energy release and supply during aerobic respiration. The citric acid cycle accounts for most of the energy production in aerobic organisms. The cycle generates about 10 ATP per acetyl CoA molecule, which is equivalent to about 32 ATP per glucose molecule (assuming that each glucose molecule produces two acetyl CoA molecules). This represents about 66% of the total ATP yield from glucose oxidation (the remaining 34% comes from glycolysis and the pyruvate dehydrogenase complex).
• Due to the many functions of the citric acid cycle, it is also considered to be the “central hub of metabolism”. This is because most of the absorbed nutrients, the fuel molecules are oxidized ultimately within the Kreb’s cycle and its intermediates are used for various biosynthetic pathways. The citric acid cycle connects with many other metabolic pathways and regulates their activity by providing or consuming intermediates. For example, the cycle can be inhibited by high levels of ATP or NADH, which signal that the cell has enough energy; or it can be stimulated by high levels of ADP or NAD+, which signal that the cell needs more energy. The cycle can also be modulated by hormones such as insulin and glucagon, which affect the availability of acetyl CoA from different sources.

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