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Unsaturated fatty acids are those that contain one or more double bonds in their alkyl-chain. Polyunsaturated fats with two double bonds usually have one between carbons 9 and 10 in the alkyl chain and another three carbons away (carbons 12-13). The presence of a double bond in the fatty acid chain changes the shape of the side group, and alters membrane fluidity when present in phospholipids. Polyunsaturated fatty acids in the diet such as omega-3 fatty acids from fish oil may have a positive influence on health, reducing cardiovascular disease and cancer. The double bonds in polyunsaturated fats cannot proceed directly through beta-oxidation to generate energy. Beta-oxidation will proceed normally through the first few rounds with a polyunsaturated fat, up until the point where the beta-oxidation enzymes encounter the first double bond. The enzymes of beta-oxidation are unable to act on substrates containing a double bond between the beta and gamma carbons. To solve this problem, the enzyme enoyl-CoA isomerase moves the first double bond to a position that the beta oxidation enzymes can act on, one carbon closer to the end of the alkyl chain. Beta-oxidation can then proceed for one cycle and part of the next, until it encounters the next double bond, which usually starts at an even-numbered carbon. A reductase then reduces the fatty acid, leaving a trans-3 double bond in mammals. The enzyme 3,2 enoyl-CoA isomerase moves this double bond closer to the end of the fatty acid chain, where the fatty acid can serve as a substrate for the normal beta-oxidation enzymes to continue beta-oxidation to completion. (This definition may be outdated - see the DesignNote.)
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