Chapter 16The citric acid cycleYou will be tested on the entire chapterCitric acid cycle also known as the tricarboxylic acid (TCA) cycle or the Krebs cycleCatabolism of proteins, fats, & carbohydrates in the 3 stages of cellular respirationAfter GlycolysisWhat next ……Glucose + 2 ADP + 2 Pi + 2 NAD+Glycolysis2 Pyruvate + 2 ATP + 2 NADH + 2 H++ 2 H2O , Cell needs to regenerate the NAD+that was utilized in glycolysis, NAD+regeneration can be accomplished by aerobic or anaerobic metabolismThree possible fates of pyruvateAerobic MetabolismPyruvateAnaerobic metabolismAcetaldehydeEthanolAlcoholic fermentationLactate dehydrogenaseTCA cycleElectron transportNADH + H+NAD+Alcohol dehydrogenaseNADH + H+NAD+Acetyl CoAL-lactateHomolacticfermentationAdapted from Chemistry 153A Course Compendium by M. A. BatesAerobic MetabolismPyruvate dehydrogenase complexA cluster of 3 enzymes located in the - mitochondria of eukaryotic cells- cytosol of prokaryotesA classic well studied example of a multienzyme complexNon-covalent stable aggregation of enzymesOverall reaction catalyzed by the pyruvate dehydrogenase complexOxidative decarboxylationIrreversible reactionRequires sequential action of - 3 different enzymes & 5 different coenzymesDifferent Coenzymes or Prosthetic Groups Required for Pyruvate Dehydrogenase Complex are1. Thiamine pyrophosphate (TPP)2. Flavin adenine dinucleotide (FAD)3. Coenzyme A (CoA)4. Nicotinamide adenine dinucleotide (NAD+)5. LipoateFour different vitamins required in human nutrition are vital components of this systemThiamine (in TPP)Riboflavin (in FAD) Niacin (in NAD) Pantothenate (in CoA)1. Thiamine pyrophosphate (TPP)2. Flavin adenine dinucleotide (FAD)3. Coenzyme A (CoA)Hydroxyl group of pantothenic acid is joined to ADP moeityCritical to its role as an acyl carrier(vitamin B5)Modified ADPCarboxylic group attached to β-mercaptoethylamine4. Nicotinamide adenine dinucleotide (NAD+)5. LipoateLipoate has 2 thiol groupsLipoyllysyl moiety is the prosthetic group of dihydrolipoyl transacetylase(E2)Pyruvate Dehydrogenase Complex¾ Consists of 3 distinct enzymes1) Pyruvate dehydrogenase (E1)2) Dihydrolipoyl transacetylase (E2)3) Dihydrolipoyl dehydrogenase (E3)¾ Form non-covalent stable aggregation of enzymes¾ Each present in multiple copies¾ The # of copies of each enzyme & the actual size of the complex varies among organismsPyruvate Dehydrogenase Complex from E. coli¾ E2forms the core of the cluster to which other enzymes are attachedE224 copies Contains bound lipoateE124 copies Contains bound TPPE312 copies Contains bound FADReaction sequence of the pyruvate dehydrogenase complex1) Pyruvate + TPP-E1CO2+ Hydroxyethyl-TPP-E12) Hydroxyethyl-TPP-E1+ Lipoamide-E2TPP-E1+ Acetyl-dihydrolipoamide-E23) Acetyl-dihydrolipoamide-E2+ CoA Acetyl-CoA + Dihydrolipoamide-E24) Dihydrolipoamide-E2+ FAD-E3[cysS-Scys] Lipoamide-E2+ FADH2-E3-[cysS-cysS]5) FADH2-E3-[cysS-cysS] + NAD+FAD-E3-S-S + NADH + H+Overall ReactionPyruvate + NAD++ CoAAcetyl-CoA + NADH + H++ CO2Enzyme Reaction CatalyzedE1 Reaction 1 & 2E2 Reaction 3E3 Reaction 4 & 5Aerobic oxidation of glucose that includes the oxidation of pyruvate provides energy in the brain Thiamine (vitamin B1) deficiency- Unable to oxidize pyruvate normally- Beriberi, a disease that is characterized by loss of neural function is due to thiamine deficiencyHabitual consumption of large amounts of alcohol can lead to thiamine deficiencyBeriberi - is a disease caused by a deficiency of thiamine (vitamin B1) - that affects the muscles, heart, nerves, and digestive system- literally means "I can't, I can't" in Singhalese, which reflects the crippling effect it has on its victims- is common in parts of southeast Asia, where white rice is the main foodhttp://www.nlm.nih.gov/medlineplus/all_healthtopics.html#PReactions of the citric acid cycle466445Reactions of the citric acid cycleSteps 1, 3, & 4 irreversibleEnergy is conserved by electron transfer to FAD or NAD+, forming FADH2or NADH + H+4 out of 8 steps are oxidationsAll of these reactions take place in mitochondriaCitric acid cyclePractice sheet # 1Citric acid cyclePractice sheet # 21) Formation of citrateCondensation of acetyl CoA with oxaloacetate12intermediateS-CoACitrate synthaseOxaloacetateA stable analog of acetyl-CoA boundOpen form of enzymeClosed form with oxaloacetate2) Formation of Isocitrate via cis-AconitateThe equilibrium mixture at pH 7.4 & 25 oC contains less than 10% isocitrateIn the cell the reaction is pulled to the right because isocitrate isconsumed rapidly, lowering its steady state concentrationIron-sulfur center in aconitaseBasic residue(keeps the citrate in active site)1234Aconitase SFeFeS4Fe-4S
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