生物化学课件昌增益冯仁青chapter16citric acid cycle by compressed nhivnoiii

1、Participated by 5 cofactors (from 4 vitamines: pantothenate, niacin, thiamine, and riboflavin) and 3 enzymes.oxidative decarboxylation of pyruvatePyruvate dehydrogenase complexes from E. coli:the electron micrograph60 subunits in bacteria and more in mammals.Oxidative decarboxylation of pyruvate: th

2、e chemical stepsE1:decarboxylation & dehydrogenation;E2: acetyl transferring; E3: recovers lipoamide. Lipoyllysyl (attached to E2) swings from E1 to E2 to E3.Coenzyme A (CoA-SH): discovered in 1945 by Friz A. Lipmann, delivers activated acyl groups (with 2-24carbons) for degradation or biosynthe

3、sis.for his discovery of co-enzyme A and its importance for intermediary metabolism Hans Adolf Krebs Hans Adolf Krebs Friz A. Lipmann (1899-1986, Harvard Medical School)Oxaloacetate binding Oxaloacetate binding induces a large induces a large Conformational Change, Conformational Change, which creat

4、es a binding which creates a binding site for acetyl-CoAsite for acetyl-CoAOxaloacetateCarboxylmethyl-CoA(an analog)citrate synthaseOpen formClosed formAconitase is an iron-sulfur protein (having a 4Fe-4S center): Three of the four Fe atoms arecomplexed with inorganic sulfurand sulfhydryl groups, le

5、avingone Fe atom available to bind cistrate and isocitrate.Aconitase TPP lipoate FAD(E1, E2, E3)Step 4: Oxidative decarboxylation of a-ketoglutarate. the carbon released as CO2 is again notfrom the acetyl;The a a-ketoglutarate dehydrogenase complex closely resembles the pyruvate dehyrogenase complex

6、 in structure and function (E1 as well as E2 are similar, E3 being identical).step 5: Substrate-level phosphorylation to form GTP/ATP. Via a phosphoenzymeintermediateDimeric succinyl-CoA synthetase (from E. coli )Phospho-His246Coenzyme AStabilizing electric dipoles of two power helices(embedded in t

7、he inner membrane of mitochondria)step 6: Oxidation of succinate. Succinate dehydrogenase: a flavoprotein with a covalently bound FAD and three kinds of iron-sulfur centers. Malonate (丙二酸) is a strong competitive inhibitor.(a stereospecific enzyme, only act on the trans and L isomers)step 7: hydrati

8、on of fumarate.Fumarase does not act on maleate and D-malate.step 8: oxidation of L-malate to regenerate oxaloacetate.Pyruvate dehydrogenase complex commits the carbon of glucose toeither oxidation to CO2 or incorporation into lipidsA “no way back” committing stepThe hub of intermediary metabolismTh

9、e citric acid cycle is amphibolicActive site 1Active site 2Biotinswitches betweentwo active sites.acetyl-CoA is a positive allosteric modulator of theenzyme.PyruvatecarboxylasecatalysisA transient carboxyphosphateis formedTwo acetyl-CoA molecules participatein each cycleOne net succinateis producedThe glyoxylateCycleShares 3 steps with citric acid cyclebut bypasses the two decarboxylation steps using isocitrate lyase and malate synthase.MalatesynthaseisocitratelyaseFrom fatty acids to glucose in germinating seeds: via three compartmentsThe