The Citric Acid Cycle

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Learn all about the citric acid cycle in just a few minutes! Jessica Pamment, professional lecturer at DePaul University, introduces the series of steps used to catalyze products following the oxidation of pyruvate.

The citric acid cycle consists of a series of steps used to catalyze products following the oxidation of pyruvate.

Located within the matrix of the mitochondria, the citric acid cycle takes place. This is a series of chemical reactions used by all aerobic organisms to release ATP. Also called the Krebs cycle, this is an aerobic process even though oxygen is not required within the cell itself. This is because oxygen is needed later, during the electron transport chain, which creates the ATP and NADH used to drive this cycle.

The acetyl CoA made during the oxidation of pyruvate is used as a reactant to drive the citric acid cycle. The citric acid cycle occurs as a closed loop. Thus, the last product made from this process can be used as a starting reactant in the first step of the cycle. Two carbon atoms from the original glucose molecule have been converted into carbon dioxide; the remaining four carbon atoms will also be converted to carbon dioxide during the citric acid cycle. Thus, for every glucose molecule, a total of six carbon dioxide molecules are produced. These six carbon dioxide molecules are cellular waste that must be removed from the cell.

Steps of the Citric Acid Cycle

The citric acid cycle starts after the oxidation of pyruvate. It consists of eight sequential steps that use the products of one reaction as the reactants of the next. The end result is the production of NADH and the reformation of oxaloacetate, which is then recycled to start the cycle over again.

The citric acid cycle involves the following eight steps to catalyze products following the oxidation of pyruvate.

Step 1. Citrate synthase transfers the 2-carbonyl acetyl group from acetyl CoA to a 4-carbon molecule called oxaloacetate. This results in the production of a 6-carbon citrate molecule. This step involves a condensation reaction and is irreversible because a lot of energy is released during this process. ATP controls the rate of this reaction as a type of negative feedback. Thus, if ATP levels are high, the rate of reaction will slow down. If ATP levels are low, the rate of the reaction will increase.

Step 2. Aconitase, an enzyme, catalyzes the conversion of citrate into its isomer, isocitrate. This happens when one water molecule is removed and another water molecule is added.

Step 3. Isocitrate dehydrogenase, another enzyme, oxidizes isocitrate with the help of NAD⁺. NADH is produced and one molecule of carbon dioxide is released. This results in the production of a 5-carbon molecule called alpha-ketoglutarate.

Step 4. A dehydrogenase enzyme oxidizes alpha-ketoglutarate with the help of NAD⁺. This produces NADH and causes another carbon dioxide molecule to be released. Now the remaining 4-carbon molecule is added to CoA to make succinyl-CoA.

Step 5. Succinyl-CoA synthetase catalyzes the removal of CoA. This results in the conversion of succinyl-CoA to succinate. Energy is released during this process, which forms ATP from ADP.

Step 6. Succinate dehydrogenase oxidizes succinate when two hydrogen atoms are transferred to FAD. This results in the production of FADH₂ and the formation of fumarate.

Step 7. Fumarase is an enzyme used to convert fumarate to malate by adding one water molecule to the compound.

Step 8. Malate dehydrogenase oxidizes malate with the help of NAD⁺. NADH is produced, and the original oxaloacetate molecule first used in step 1 is regenerated. Now the cycle can start all over again.

The citric acid cycle is a closed-loop metabolic pathway that involves a series of eight steps to produce ATP and other molecules. The last molecule produced in step 8 regenerates the starting reactant in step 1 so the cycle can start all over again.

Citric Acid Cycle Net Reaction

The citric acid cycle uses multiple reactions for the release of energy.

The citric acid cycle uses multiple reactions for the release of energy. This energy is used to produce three molecules of NADH, one molecule of FADH₂, and one molecule of ATP from each acetyl group (the acyl with chemical formula CH₃CO; a methyl group that is single-bonded to a carbonyl) that is oxidized. In this reaction, four carbon atoms from the original glucose molecule are also used to produce carbon dioxide during this pathway. The molecules NADH and FADH₂ will be used in the final part of aerobic respiration to make ATP molecules.

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