清華大學(xué)生物化學(xué)課件--脂肪分解代謝-文庫吧資料
2024-08-18 09:33本頁面
【正文】 lso bound to the inner membrane of the mitochondria. ? AcylCoA dehydrogenase exists in three isozymes acting on the acylCoAs of long (1218C), medium (414C) and short (48C) chains. ? The hydration step, stereospecifically catalyzed by enoylCoA hydratase, converts the trans?2enoylCoA to LbhydroxylacylCoA (the cis isomer can also be converted, but to the D isomer). ? The 2nd dehydrogenation is catalyzed by LbhydroxylacylCoA dehydrogenase, converting LbhydroxylacylCoA to bketoacylCoA (not act on the D isomer), with electrons collected by NAD+. ? The acylCoA acetyltransferase (or monly called thiolase) catalyzes the nucleophilic attack of CoA to the carbonyl carbon, cleaving bketoacylCoA between the a and b carbon (thiolysis), generating two acylCoA molecules with one entering the citric acid cycle and the other reentering the b oxidation pathway. ? The first three reactions of the b oxidation used the same reaction strategy as the last three reactions in the citric acid cycle, all involving the oxidation of a highly reduced carbon (from CH2 to CHO). A conserved reaction sequence to induce a carbonyl function on the carbon b to a carboxyl group Stages of fatty acid oxidation PalmitolCoA + 7CoA + 7FAD + 7NAD+ + 7H2O 8 acetylCoA + 7FADH2 + 7NADH + 7H+ The plete oxidization of each 16carbon palmitate (to H2O and CO2) yields ~106 ATP (~32 ATP per glucose, both having about 60% of actual energy recovery). PalmitoylCoA+23O2+108Pi+108ADP CoA+108ATP+16CO2+23H2O ***** Hibernating grizzly bears use body fat as their sole fuel 9. Oxidation of unsaturated fatty acids requires one or two auxiliary enzymes, an isomerase and a reductase ? The isomerase converts a cis?3 double bond to a trans?2 double bond. ? The reductase (2,4dienoylCoA reductase) converts a trans?2, cis?4 structure to a trans?3 structure, which will be further converted to a trans?2 structure by the isomerase. ? NADPH is needed for the reduction (from two double bonds to one). Oxidation of a monounsaturated fatty acid: the enoylCoA isomerase helps to reposition the double bond Both an isomerase and a reductase are needed for oxidizing a polyunsaturated fatty acid. 10. PropionylCoA generated from oddnumber fatty acids (and three amino acids) is converted to succinylCoA ? Oddnumber fatty acids, monly found in the lipids of some plants and marine anisms, will also be oxidized via the b oxidation pathway, but generating a propionylCoA at the last step of thiolysis. ? PropionylCoA is converted to succinylCoA via an unusual enzymatic pathway of three reactions. PropionylCoA can be converted to succinylCoA via a carboxylation, an epimerization and an intramolecular group shifting ? PropionylCoA is first carboxylated at the a
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