Pyruvate is oxidized to form acetyl-CoA, which enters the citric acid cycle to produce ATP, electron carriers, and CO2 as a waste product.
The pyruvates that are formed by glycolysis are transported across the mitochondrial membrane in order to enter the second pathway of cellular respiration: the citric acid cycle. The citric acid cycle is a series of chemical reactions used by all aerobic organisms to generate ATP. Aerobic organisms require oxygen for metabolism, the biological processes that build or break molecules. The citric acid cycle is also known as the Krebs cycle or the tricarboxylic acid cycle. In eukaryotic cells, cells characterized by membrane-bound organelles, such as the nucleus, the citric acid cycle occurs inside the mitochondria, in the mitochondrial matrix. Mitochondria share many similarities with bacterial cells. Like bacteria, mitochondria multiply by dividing in half. They also have their own DNA, which is circular like bacterial DNA. In addition, mitochondria have two membranes: an outer membrane and an inner membrane, which is an indication that at one time, they may have been separate organisms taken into the cytoplasm of a larger cell. For these reasons, mitochondria are thought to have originated from free-living prokaryotes, unicellular organisms with no nucleus.
Before the pyruvates enter the citric acid cycle, they are first oxidized to form acetyl-CoA (acetyl coenzyme A). Carbon dioxide (CO2) is released. Then acetyl-CoA enters the citric acid cycle, during which electrons are removed from acetyl-CoA to produce the electron carriers NADH and FADH2, plus CO2 as a waste product, and a small amount of ATP. Electron carriers NADH and FADH2 are used in oxidative phosphorylation to generate large amounts of cellular energy.
The Oxidation of Pyruvate
Once formed, acetyl-CoA enters the citric acid cycle, which involves eight steps that use eight different enzymes to catalyze products following the oxidation of pyruvate.
Step 1. Citrate synthase, the first enzyme in a long chain of catalysts for the citric acid cycle, transfers the 2-carbonyl acetyl group from acetyl-CoA to a four-carbon molecule called oxaloacetate. This results in the production of a six-carbon citrate molecule. The step involves a condensation reaction, a chemical reaction that combines two molecules with the elimination of a water molecule. It 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 five-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 four-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 eventually forms ATP from ADP via a GTP intermediary.
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.