HMB 205 final notes - Lipid Synthesis A variety of lipids are found in microorganisms particularly in cell membranes Most contain fatty acids or their

HMB 205 final notes - Lipid Synthesis A variety of lipids...

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Lipid Synthesis A variety of lipids are found in microorganisms, particularly in cell membranes. Most contain fatty acids or their derivatives. Fatty acids are monocarboxylic acids with long alkyl chains that usually have an even number of carbons (the average length is 18 carbons). Some may be unsaturated—that is, have one or more double bonds. Most microbial fatty acids are straight chained, but some are branched. Gram-negative bacteria often have cyclopropane fatty acids (fatty acids with one or more cyclopropane rings in their chains). Fatty acid synthesis is catalyzed by the fatty acid synthetase complex with acetyl-CoA and malonyl-CoA as the substrates and NADPH as the reductant. Malonyl-CoA arises from the ATPdriven carboxylation of acetyl- CoA (as shown on the figure below). Synthesis takes place after acetate and malonate have been transferred from coenzyme A to the sulfhydryl group of the acyl carrier protein (ACP), a small protein that carries the growing fatty acid chain during synthesis. The synthetase adds two carbons at a time to the carboxyl end of the growing fatty acid chain in a two-stage process. First, malonyl-ACP reacts with the fatty acyl-ACP to yield CO2 and a fatty acyl-ACP two carbons longer. The loss of CO2 drives this reaction to completion. Notice that ATP is used to add CO2 to acetyl-CoA, forming malonyl-CoA. The same CO2 is lost when malonyl-ACP donates carbons to the chain. Thus carbon dioxide is essential to fatty acid synthesis but it is not permanently incorporated. Indeed, some
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microorganisms require CO2 for good growth, but they can do without it in the presence of a fatty acid like oleic acid (an 18-carbon unsaturated fatty acid). In the second stage of synthesis, the β-keto group arising from the initial condensation reaction is removed in a three step process involving two reductions and a dehydration. The fatty acid is then ready for the addition of two more carbon atoms. Unsaturated fatty acids are synthesized in two ways. Eukaryotes and aerobic bacteria like Bacillus megaterium employ an aerobic pathway using both NADPH and O2. A double bond is formed between carbons nine and ten, and O2 is reduced to water with electrons supplied by both the fatty acid and NADPH. Anaerobic bacteria and some aerobes create double bonds during fatty acid synthesis by dehydrating hydroxy fatty acids. Oxygen is not required for double bond synthesis by this pathway. The anaerobic pathway is present in a number of common gram-negative bacteria (e.g., Escherichia coli and Salmonella typhimurium ), gram-positive bacteria (e.g., Lactobacillus plantarum and Clostridium pasteurianum ), and cyanobacteria. Eukaryotic microorganisms frequently store carbon and energy as triacylglycerol, glycerol esterified to three fatty acids. Glycerol arises from the reduction of the glycolytic intermediate dihydroxyacetone phosphate to glycerol 3-phosphate, which is then esterified with two fatty acids to give phosphatidic acid ( below ).
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