fatty acids enter the krebs cycle in the cytosol.
fatty acids enter the krebs cycle in the cytosol.
where do fatty acids enter the krebs cycle?
in the cytosol
acetyl-coa is formed into malonyl-coa by acetyl-coa carboxylase, at which point malonyl-coa is destined to feed into the fatty acid synthesis pathway. acetyl-coa carboxylase is the point of regulation in saturated straight-chain fatty acid synthesis, and is subject to both phosphorylation and allosteric regulation. regulation by phosphorylation occurs mostly in mammals, while allosteric regulation occurs in most organisms. allosteric control occurs as feedback inhibition by palmitoyl-coa and activation by citrate. when there are high levels of palmitoyl-coa, the final product of saturated fatty acid synthesis, it allosterically inactivates acetyl-coa carboxylase to prevent a build-up of fatty acids in cells. citrate acts to activate acetyl-coa carboxylase under high levels, because high levels indicate that there is enough acetyl-coa to feed into the krebs cycle and conserve energy.
fatty acid synthesis
the citric acid cycle (also known as the tricarboxylic acid cycle or the krebs cycle) is used by aerobic organisms to produce energy via the oxidation of acetyl-coa, which is derived from pyruvate (a product of glycolysis). the citric acid cycle accepts acetyl-coa and metabolizes it to form carbon dioxide. a related cycle, called the glyoxylate cycle, is found in many bacteria and plants. in plants, the glyoxylate cycle takes place in glyoxysomes. in this cycle, isocitrate lyase and malate synthase skip over the decarboxylation steps of the citric acid cycle. in other words, malate synthase works together with isocitrate lyase in the glyoxylate cycle to bypass two oxidative steps of krebs cycle and permit carbon incorporation from acetate or fatty acids in many microorganisms. together, these two enzymes serve to produce succinate (which exits the cycle to be used for synthesis of sugars) and malate (which remains in the glyoxylate cycle). during this process, acetyl-coa and water are used as substrates. as a result, the cell does not lose 2 molecules of carbon dioxide as it does in the krebs cycle. the glyoxylate cycle, facilitated by malate synthase and isocitrate lyase, allows plants and bacteria to subsist on acetyl-coa or other two carbon compounds. for example, "euglena gracilis", a single-celled eukaryotic alga, consumes ethanol to form acetyl-coa and subsequently, carbohydrates. within germinating plants, the glyoxylate cycle allows the conversion of reserve lipids into carbohydrates within glyoxysomes.
malate synthase