Krebs cycle was named after Hans Krebs, who postulated the detailed cycle.
The Krebs cycle or Citric acid cycle is a series of enzyme catalysed reactions occurring in the mitochondrial matrix, where acetyl-CoA is oxidised 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 oxidised to form two molecules of CO₂ and in the process, one ATP is produced. Reduced high energy compounds, NADH and FADH₂ 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 CO₂, 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 oxidised 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:
Krebs Cycle Steps It is an eight-step process. Krebs cycle takes place in the matrix of mitochondria under aerobic condition. Step 1: First step is the condensation of acetyl CoA with oxaloacetate (4C) to form citrate (6C), coenzyme A is released. The reaction is catalysed by citrate synthase. Step 2: Citrate is turned to its isomer, isocitrate. The enzyme aconitase catalyses this reaction. Step 3: Isocitrate undergoes dehydrogenation and decarboxylation to form 𝝰-ketoglutarate (5C). A molecular of CO₂ is released. Isocitrate dehydrogenase catalyses 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 α-ketoglutarate dehydrogenase enzyme complex. One molecule of CO₂ 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 oxidised to fumarate by the enzyme succinate dehydrogenase. In the process, FAD is converted to FADH₂. Step 7: Fumarate gets converted to malate by addition of one H₂O. The enzyme catalysing 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 catalyses the reaction. [Image will be Uploaded Soon] 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:
Note 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, Krebs cycle yields 4 CO₂, 6NADH, 2 FADH₂ 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
Q1. What is the Krebs Cycle? Ans. Also known as the citric acidity cycle, Kreb’s cycle is a chain of reactions occurring in the mitochondria, through which almost all living cells produce energy in aerobic respiration. It consumes oxygen to give out water and carbon dioxide are products. Here , ADP is converted into ATP. This cycle renders electrons and hydrogen required for the electron chain transport. Q2. How Many ATP are Produced in Krebs Cycle? Ans. 2 ATPs are produced in one Krebs Cycle.For complete oxidation of a glucose molecule, Krebs cycle yields 4 CO₂, 6NADH, 2 FADH₂ and 2 ATPs. Q3. Where Does Krebs Cycle Occur? Ans. Mitochondrial matrix. In all eukaryotes, mitochondria is the site where Krebs cycle takes place. The cycle takes place in a mitochondrial matrix producing a chemical energy in the form of NADH, ATP, FADH₂. These are produced as a result of oxidation of the end product of glycolysis – pyruvate. Q4. How the Krebs Cycle Works? Ans. It is a Eight Step Process
Q5. Why is Krebs Cycle Called an Amphibolic Pathway? Ans. It is called amphibolic as in the Krebs cycle both catabolism and anabolism takes place. Amphibolic pathway indicates the one involving both catabolic and anabolic procedures.
2Pyruvate + 2NAD⁺ + 2CoA → Pyruvate dehydrogenase → Pyruvate dehydrogenase 2AcetylCoA + 2NADH + CO 2 2
2 Acytyl CoA + 6 NAD⁺ + 2 FAD + 2ADP +2Pᵢ + 2H₂O → 4CO₂ + 6 NADH + 2FADH₂ + 2ATP + CoA