The first step is a condensation step, combining the two-carbon acetyl group from acetyl CoA with a four-carbon oxaloacetate molecule to form a six-carbon molecule of citrate. CoA is bound to a sulfhydryl group -SH and diffuses away to eventually combine with another acetyl group.
This step is irreversible because it is highly exergonic. The rate of this reaction is controlled by negative feedback and the amount of ATP available.
If ATP levels increase, the rate of this reaction decreases. If ATP is in short supply, the rate increases. Step 2. Citrate loses one water molecule and gains another as citrate is converted into its isomer, isocitrate. Steps 3 and 4. CoA binds the succinyl group to form succinyl CoA. Step 5. A phosphate group is substituted for coenzyme A, and a high- energy bond is formed. This energy is used in substrate-level phosphorylation during the conversion of the succinyl group to succinate to form either guanine triphosphate GTP or ATP.
There are two forms of the enzyme, called isoenzymes, for this step, depending upon the type of animal tissue in which they are found. One form is found in tissues that use large amounts of ATP, such as heart and skeletal muscle. This form produces ATP. The second form of the enzyme is found in tissues that have a high number of anabolic pathways, such as liver. This form produces GTP. In particular, protein synthesis primarily uses GTP.
Step 6. Step six is a dehydration process that converts succinate into fumarate. Unlike NADH, this carrier remains attached to the enzyme and transfers the electrons to the electron transport chain directly. This process is made possible by the localization of the enzyme catalyzing this step inside the inner membrane of the mitochondrion. Step 7. Water is added to fumarate during step seven, and malate is produced. The last step in the citric acid cycle regenerates oxaloacetate by oxidizing malate.
Another molecule of NADH is produced. Two carbon atoms come into the citric acid cycle from each acetyl group, representing four out of the six carbons of one glucose molecule. Malate is oxidized forming oxaloacetate , the beginning substrate in the cycle. Two additional NADH molecules are also generated in the conversion of pyruvic acid to acetyl CoA prior to the start of the cycle.
The NADH and FADH2 molecules produced in the citric acid cycle are passed along to the final phase of cellular respiration called the electron transport chain. Actively scan device characteristics for identification.
Use precise geolocation data. Select personalised content. Create a personalised content profile. Measure ad performance. Select basic ads. Create a personalised ads profile. Select personalised ads. Apply market research to generate audience insights. Measure content performance. Develop and improve products. List of Partners vendors. Share Flipboard Email.
Regina Bailey. Biology Expert. Learning Objectives State two other names for the citric acid cycle. Briefly describethe function of the citric acid cycle during aerobic respiration and indicate the reactants and products.
Compare where the citric acid cycle occurs in prokaryotic cells and in eukaryotic cells. State the total number of ATP produced by substrate-level phosphorylation for each acetyl-CoA that enters the citric acid cycle. Summary Aerobic respiration involves four stages: glycolysis, a transition reaction that forms acetyl coenzyme A, the citric acid Krebs cycle, and an electron transport chain and chemiosmosis. The citric acid cycle, also known as the tricarboxylic acid cycle and the Krebs cycle, completes the oxidation of glucose by taking the pyruvates from glycolysis, by way of the transition reaction, and completely breaking them down into CO 2 molecules, H 2 O molecules, and generating additional ATP by oxidative phosphorylation.
The citric acid cycle provides a series of intermediate compounds that donate protons and electrons to the electron transport chain by way of the reduced coenzymes NADH and FADH 2.
0コメント