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. Two carbon dioxide molecules are released on each turn of the cycle; however, these do not necessarily contain the most recently-added carbon atoms.
The two acetyl carbon atoms will eventually be released on later turns of the cycle; thus, all six carbon atoms from the original glucose molecule are eventually incorporated into carbon dioxide. These carriers will connect with the last portion of aerobic respiration to produce ATP molecules. Several of the intermediate compounds in the citric acid cycle can be used in synthesizing non-essential amino acids; therefore, the cycle is amphibolic both catabolic and anabolic.
This is consistent with its spread early in the evolution of our oxygen environment. Because of the central role of Krebs cycle intermediates in other biochemical pathways, parts of the pathway may even have pre- dated the complete respiratory pathway.
The Krebs cycle takes place in mitochondria of eukaryotic cells. After the oxidation of pyruvate, the Ac-S-CoA enters the Krebs cycle, condensing with oxaloacetate in the cycle to form citrate.
There are four redox reactions in the Krebs cycle. Also, follow the carbons in pyruvate into CO2. The Krebs Cycle as it occurs in animals is summarized below. Note that in bacteria, ATP is made directly at this step. Both of these electron carriers carry a pair of electrons. If you include the electrons on each of the NADH molecules made in glycolysis, how many electrons have been removed from glucose during its complete oxidation?
Remember that glycolysis produces two pyruvates per glucose, and thus two molecules of Ac-S-CoA. The next reaction of the citric acid cycle is catalyzed by the enzyme acontinase. In this reaction, a water molecule is removed from the citric acid and then put back on in another location. This transformation yields the molecule isocitrate.
Two events occur in reaction 3 of the citric acid cycle. In reaction 4 of the citric acid cycle, alpha-ketoglutarate loses a carbon dioxide molecule and coenzyme A is added in its place. The enzyme that catalyzes this reaction is alpha-ketoglutarate dehydrogenase. The mechanism of this conversion is very similar to what occurs in the first few steps of pyruvate metabolism. The resulting molecule is called succinyl-CoA.
The enzyme succinyl-CoA synthetase catalyzes the fifth reaction of the citric acid cycle.
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