Krebs Cycle or Citric Acid Cycle
The Krebs cycle or Citric acid cycle is a series of enzyme-catalyzed reactions occurring in the mitochondrial matrix, where acetyl-CoA is oxidized to form carbon dioxide and coenzymes are reduced, which generate ATP in the electron transport chain.
Krebs cycle was named after Hans Krebs, who postulated the detailed cycle. He was awarded the Nobel prize in 1953 for his contribution.
It is a series of eight-step processes, where the acetyl group of acetyl-CoA is oxidized to form two molecules of \[CO_{2}\] and in the process, one ATP is produced. Reduced high-energy compounds, NADH, and \[FADH_{2}\] are also produced.
Two molecules of acetyl-CoA are produced from each glucose molecule so two turns of the Krebs cycle are required which yields four \[CO_{2}\], six NADH, two FADH₂, and two ATPs.
Krebs Cycle is a Part of Cellular Respiration
Cellular respiration is a catabolic reaction taking place in the cells. It is a biochemical process by which nutrients are broken down to release energy, which gets stored in the form of ATP, and waste products are released. In aerobic respiration, oxygen is required.
Cellular respiration is a four-stage process. In the process, glucose is oxidized to carbon dioxide and oxygen is reduced to water. The energy released in the process is stored in the form of ATPs. 36 to 38 ATPs are formed from each glucose molecule.
The Four Stages are
Glycolysis: Partial oxidation of a glucose molecule to form 2 molecules of pyruvate. This process takes place in the cytosol.
Formation of Acetyl CoA: Pyruvate formed in glycolysis enters the mitochondrial matrix. It undergoes oxidative decarboxylation to form two molecules of Acetyl CoA. The reaction is catalyzed by the pyruvate dehydrogenase enzyme.
\[2Pyruvate + 2NAO^{-} + 2CoA^{-} \overset{ Pyruvate dehydrogenase }{\rightarrow} 2 Acetyl CoA + 2NADH + C0_{2}\]
Krebs Cycle (TCA or Citric Acid Cycle): It is the common pathway for complete oxidation of carbohydrates, proteins, and lipids as they are metabolized to acetyl coenzyme A or other intermediates of the cycle. The Acetyl CoA produced enters the Tricarboxylic acid cycle or Citric acid cycle. Glucose is fully oxidized in this process. The acetyl CoA combines with oxaloacetate (4C) to form citrate (6C). In this process, 2 molecules of \[CO_{2}\] are released and oxaloacetate is recycled. Energy is stored in ATP and other high-energy compounds like NADH and \[FADH_{2}\].
Electron Transport System and Oxidative Phosphorylation: ATP is generated when electrons are transferred from the energy-rich molecules like NADH and \[FADH_{2}\] produced in glycolysis, citric acid cycle, and fatty acid oxidation to molecular \[O_{2}\] by a series of electron carriers. \[O_{2}\] is reduced to \[H_{2}O\]. It takes place in the inner membrane of mitochondria.
Krebs Cycle Steps
It is an eight-step process. The Krebs cycle takes place in the matrix of mitochondria under aerobic conditions.
Step 1: The first step is the condensation of acetyl CoA with oxaloacetate (4C) to form citrate (6C), coenzyme A is released. The reaction is catalyzed by citrate synthase.
Step 2: Citrate is turned to its isomer, isocitrate. The enzyme aconitase catalyzes this reaction.
Step 3: Isocitrate undergoes dehydrogenation and decarboxylation to form 𝝰-ketoglutarate (5C). A molecular of \[CO_{2}\] is released. Isocitrate dehydrogenase catalyzes the reaction. It is an NAD+-dependent enzyme. NAD+ is converted to NADH.
Step 4: α-ketoglutarate (5C) undergoes oxidative decarboxylation to form succinyl CoA (4C). The reaction is catalyzed by the α-ketoglutarate dehydrogenase enzyme complex. One molecule of \[CO_{2}\] is released and NAD+ is converted to NADH.
Step 5: Succinyl CoA is converted to succinate by the enzyme succinyl CoA synthetase. This is coupled with substrate-level phosphorylation of GDP to form GTP. GTP transfers its phosphate to ADP forming ATP.
Step 6: Succinate is oxidized to fumarate by the enzyme succinate dehydrogenase. In the process, FAD is converted to \[FADH_{2}\].
Step 7: Fumarate gets converted to malate by the addition of one \[H_{2}O\]. The enzyme catalyzing this reaction is fumarase.
Step 8: Malate is dehydrogenated to form oxaloacetate, which combines with another molecule of acetyl CoA and starts the new cycle. Hydrogens removed get transferred to NAD+ forming NADH. Malate dehydrogenase catalyzes the reaction.
Krebs Cycle Summary
Location: Krebs cycle occurs in the mitochondrial matrix
Krebs Cycle Reactants: Acetyl CoA, which is produced from the end product of glycolysis, i.e. pyruvate and it condenses with 4 carbon oxaloacetate, which is generated back in the Krebs cycle.
Krebs Cycle Products
Each citric acid cycle forms the following products:
2 molecules of \[CO_{2}\] are released. Removal of \[CO_{2}\] or decarboxylation of citric acid takes place at two places:
In the conversion of isocitrate (6C) to α-ketoglutarate (5C)
In the conversion of α-ketoglutarate (5C) to succinyl CoA (4C)
1 ATP is produced in the conversion of succinyl CoA to succinate
3 NAD+ are reduced to NADH and 1 FAD+ is converted to \[FADH_{2}\] in the following reactions:
Isocitrate to α-ketoglutarate → NADH
α-ketoglutarate to succinyl CoA → NADH
Succinate to fumarate → \[FADH_{2}\]
Malate to Oxaloacetate → NADH
Notes that 2 molecules of Acetyl CoA are produced from oxidative decarboxylation of 2 pyruvates so two cycles are required per glucose molecule.
To summarize, for complete oxidation of a glucose molecule, the Krebs cycle yields \[ 4 CO_{2}, 6NADH, 2 FADH_{2} \], and 2 ATPs.
Each molecule of NADH can form 2-3 ATPs and each FADH₂ gives 2 ATPs on oxidation in the electron transport chain.
Krebs Cycle Equation
To sum up,
\[ 2 Acytyl CoA + 6 NAO^{-} + 2 FAD + 2ADP + 2P_{i} + 2H_{2}0 \rightarrow 4CO_{2} + 6 NADH + 2FADH_{2} + 2ATP + CoA \]
Significance of Krebs Cycle
The Krebs cycle or Citric acid cycle is the final pathway of oxidation of glucose, fats, and amino acids.
Many animals are dependent on nutrients other than glucose as an energy source.
Amino acids (metabolic product of proteins) are deaminated and get converted to pyruvate and other intermediates of the Krebs cycle. They enter the cycle and get metabolized e.g. alanine is converted to pyruvate, glutamate to α-ketoglutarate, aspartate to oxaloacetate on deamination.
Fatty acids undergo β-oxidation to form acetyl CoA, which enters the Krebs cycle.
It is the major source of ATP production in the cells. A large amount of energy is produced after the complete oxidation of nutrients.
It plays an important role in gluconeogenesis lipogenesis and interconversion of amino acids.
Many intermediate compounds are used in the synthesis of amino acids, nucleotides, cytochromes, chlorophylls, etc.
Vitamins play an important role in the citric acid cycle. Riboflavin, niacin, thiamin, and pantothenic acid a part of various enzymes cofactors (FAD, NAD) and coenzyme A.
Regulation of the Krebs cycle depends on the supply of NAD+ and the utilization of ATP in physical and chemical work.
The genetic defects of the Krebs cycle enzymes are associated with neural damage.
As most of the processes occur in the liver to a significant extent, damage to liver cells has a lot of repercussions. Hyperammonemia occurs in liver diseases and leads to convulsions and coma. This is due to reduced ATP generation as a result of the withdrawal of α-ketoglutarate and the formation of glutamate, which forms glutamine.
FAQs on Krebs Cycle
1. What is the Krebs cycle and why is it also called the Citric Acid Cycle?
The Krebs cycle is a series of eight enzyme-catalysed chemical reactions that form a central part of aerobic respiration in cells. It occurs in the mitochondrial matrix. Its main function is to oxidise acetyl-CoA, derived from carbohydrates, fats, and proteins, into carbon dioxide, releasing energy. It is also called the Citric Acid Cycle because the very first molecule formed in the cycle is citrate (a form of citric acid), which is created when acetyl-CoA combines with oxaloacetate.
2. Where in the cell does the Krebs cycle take place?
In eukaryotic cells, such as those in plants and animals, the Krebs cycle takes place in the matrix of the mitochondria. The mitochondrial matrix contains the necessary enzymes and coenzymes for the cycle's reactions. In prokaryotic cells, which lack mitochondria, the reactions occur in the cytoplasm.
3. What are the main inputs and outputs of one turn of the Krebs cycle?
For a single turn of the Krebs cycle, the primary inputs and outputs are as follows:
- Input: One molecule of Acetyl-CoA.
- Outputs: Two molecules of carbon dioxide (CO₂), three molecules of NADH, one molecule of FADH₂, and one molecule of ATP (or GTP, which is equivalent).
4. What are the major steps involved in the Krebs cycle?
The Krebs cycle is an eight-step process that cyclically regenerates its starting molecule. The key steps are:
- Step 1 (Condensation): Acetyl-CoA (2C) combines with oxaloacetate (4C) to form citrate (6C).
- Step 2 (Isomerisation): Citrate is converted into its isomer, isocitrate.
- Step 3 (First Oxidation): Isocitrate is oxidised to α-ketoglutarate (5C), releasing one molecule of CO₂ and producing one NADH.
- Step 4 (Second Oxidation): α-ketoglutarate is oxidised to succinyl-CoA (4C), releasing a second molecule of CO₂ and producing another NADH.
- Step 5 (Substrate-Level Phosphorylation): Succinyl-CoA is converted to succinate, generating one ATP (via GTP).
- Step 6 (Third Oxidation): Succinate is oxidised to fumarate, producing one FADH₂.
- Step 7 (Hydration): Fumarate is converted to malate by the addition of water.
- Step 8 (Fourth Oxidation & Regeneration): Malate is oxidised to oxaloacetate, producing a third NADH and regenerating the starting molecule for the next cycle.
5. Why is the Krebs cycle considered an amphibolic pathway?
The Krebs cycle is called an amphibolic pathway because it involves both catabolism (breaking down molecules) and anabolism (building up molecules). While its primary role is catabolic (oxidising acetyl-CoA to produce energy), its intermediates are also used in various anabolic pathways. For example, α-ketoglutarate can be used to synthesise the amino acid glutamate, and oxaloacetate is a precursor for several amino acids and glucose synthesis (gluconeogenesis).
6. How is the Krebs cycle connected to other metabolic pathways like glycolysis and fat metabolism?
The Krebs cycle is a central hub connecting several metabolic pathways.
- Connection to Glycolysis: Glycolysis breaks down glucose into pyruvate. This pyruvate is then converted into acetyl-CoA, which is the primary fuel that enters the Krebs cycle.
- Connection to Fat Metabolism: Fatty acids undergo a process called β-oxidation, which breaks them down into acetyl-CoA molecules. These acetyl-CoA molecules can then enter the Krebs cycle to generate energy.
- Connection to Protein Metabolism: Amino acids from proteins can be converted into various intermediates of the Krebs cycle, such as pyruvate, acetyl-CoA, α-ketoglutarate, or oxaloacetate, allowing them to be used for energy production.
7. What is the net gain of ATP from the Krebs cycle for one molecule of glucose?
Directly, the Krebs cycle produces only 2 ATP molecules per glucose molecule (one ATP per turn, and two turns per glucose). However, its main energy contribution is indirect. For one glucose molecule, the two turns of the cycle also produce 6 NADH and 2 FADH₂. These high-energy electron carriers subsequently enter the Electron Transport Chain, where they generate a much larger amount of ATP through oxidative phosphorylation, typically yielding around 22-24 additional ATPs.
8. What would happen if oxaloacetate was not regenerated at the end of the Krebs cycle?
If oxaloacetate were not regenerated, the Krebs cycle would quickly come to a halt. Oxaloacetate is the molecule that combines with acetyl-CoA to initiate the cycle by forming citrate. Without a continuous supply of oxaloacetate, there would be nothing for the incoming acetyl-CoA to react with. This would stop the entire process of aerobic respiration, drastically reducing the cell's ability to produce ATP and leading to a severe energy crisis.
9. What is the significance of the Krebs cycle in cellular respiration?
The significance of the Krebs cycle is immense, as it is the final common oxidative pathway for carbohydrates, fats, and proteins. Its key importance lies in:
- Energy Production: It generates a small amount of ATP directly but, more importantly, produces a large number of reduced coenzymes (NADH and FADH₂) that fuel the majority of ATP synthesis in the electron transport chain.
- Carbon Dioxide Removal: It is the process responsible for releasing the carbon atoms from glucose as carbon dioxide, a major waste product of respiration.
- Metabolic Hub: It provides precursor molecules for the synthesis of important biological compounds like amino acids, fatty acids, and porphyrins, making it central to cellular metabolism.
