Krebs cycle or citric acid cycle

Transcription

1 Krebs cycle or citric acid cycle 1

2 Cycle location Krebs cycle : substrate balance Krebs cycle is an aerobic metabolism allowing conversion of pyruvate (from glycolysis) or 3SCoA (from Lynen) into 2.pe Krebs occurs in the mitochondria Metabolic previsions Pyruvate Acetyl-CoA xidation 5 4 Squelette breakdow 2 1 Hydrolyse 0 1 2

3 H 2 Krebs cycle HSCoA 3 SCoA HSCoA Isocitrate NADH 3 NAD + H PDH C CS HC HC AA Aconitase Pyruvate 2 NAD + NADH Citrate MDH IDH NAD + 2 NADH H Malate C KG Fumarase FADH 2 FAD ATP ADP Fumarate Succinate Ligase KGDH 2 SCoA C= NAD + NADH SuccinylCoA Cycle analysis This cycle allows catabolism of pyruvate or acetyl CoA, it forms 3 2 from pyruvate or 2 2 from acetyl CoA The reactions that liberate 2 is a skeleton breakdowns called decarboxylation ne important part of the cycle is directed to generat substrates required for decarboxylation. It is very important to be able to identify these substrates, only some strutures can be decarboxylated: ketoacides b ketoacides b Hydroxyacides Note : Also the amino acids can also be decarboxylated but this occurs only when specific amines are synthetized in the anabolism. ther decarboxylations occurs in aromatic compounds. Attention : Decarboxylation is a subtype of skeleton breakdon reaction. 3

4 Keto acids decarboxylation xidative decarboxylation (enzyme: ketoacide dehydrogenase) In general, ketoacids are converted to acyl-coa by an oxidative decarboxylation involving 3 coupled reactions : «oxidation + skeleton breakdown, condensation + condensation». C C 0 HSCoA NAD + NADH (TPP, FAD, LipSS) C C SCoA + 2 This complex reaction involves one enzyme and 5 coenzymes: Coenzyme A, NAD +, FAD, thiamine pyrophosphate (TPP), and lipoic acid (LipSS) (see diaporama 9). Non oxidative decarboxylation (enzyme: ketoacide decarboxylase) C C 0 TPP C C H + 2 This reaction is exceptional. The reaction is a skeleton breakedown non red-ox. This reaction involves one enzyme and one coenzyme: thiamine pyrophosphate (TPP) Decarboxylations are all irreversible. Decarboxylation reactions in Krebs b Ketoacid decarboxylation (enzyme: b keto acide decarboxylase) C C This is a skeleton non oxidative breakedown NB : In vitro, this reaction takes some hours. However, decarboxylasee strongly accelerate the reaction. C C H + 2 b Hydroxyacid oxidative decarboxylation (enzyme: b hydroxyacide DH) b Hydroxyacids are oxidated in b ketoacides that are then decarboxylated, this two reactions are assured by one enzyme (b ketoacide is not liberated). H C C H NAD + NADH C C NB : The involved reactions are: skeleton breakdown and oxidation C C H + 2 Note : b alcohol MUST BE primary or secondary. 4

5 Krebs cycle analysis Step 1 : Pyruvate Acetyl-CoA + 2 HSCoA H 3 C C H 3 C C SCoA + 2 NAD Pyruvate NADH (TPP, FAD, LipSS) Acétyl-CoA (1) xidation + condensation + eskeleton breakdown HSCoA, NAD + NADH, FAD, TPP, LipSS Pyruvate DH Irreversible Comments : 1. It is an oxidative decarboxylation of an ketoacid. 5

6 Step 2 : Acetyl-CoA + AA citrate 3 SCoA C xaloacétate (AA) HSCoA HC Citrate Skeleton synthesis + hydrolysis (1) Aucun Citrate synthase (CS) Irreversible (2) Comments : (1) This synthesis is an aldolisation. (2) Aldolisation is reversible but irreversible when coupled. Step 3 : Citrate Isocitrate H 2 2 HC Citrate H H C HC Cis aconitate Isocitrate Dehydration + hydration Aucun Isomerase (aconitase) Reversible (2) (1) Comments : (1) Isomerisation. (2) Evident!. 6

7 H HC 2 NAD + NADH Isocitrate Step 4 : Isocitrate KG cétoglutarate ( KG) (1) xidation + skeleton breakdown NAD + NADH DH Irreversible Comments : (1) Isocitrate is a b hydroxyacide. Step 5 : KG Succinyl-CoA HSCoA 2 SCoA NAD + NADH (TPP, LipSS, FAD) KG Succinyl-CoA (1) xidation + skeleton breakdown + Condensation HSCoA, NAD + NADH, TPP, LipSS, FAD DH Irreversible Commentaires : (1) KG is an ketoacide. 7

8 Step 6 : Succinyl-CoA Succinate SCoA HSCoA ADP +P ATP 2 Hydrolysis condensation (DT) ADP ATP Ligase Reversible Comments : NB Energy of oxidation is recovered in ATP Steps 7 a 10 : Succinate AA (AA regeneration) Succinate C AA - - need to be oxidized into ketone (--). As in Lynen, we see the sequence: FAD FADH 2 Succinate Fumarate xydation DH H 2 Hydratation Désydratase NAD + NADH H C AA xydation DH results in mobile hydrogen : non activator co- Comments : Two enzyme needed 8

9 Substrate and energetic balance From pyruvate Pyruvate + 4 NAD + + FAD + ADP + P -----> NADH + FADH 2 + ATP From acetyl-coa AcetylCoA+ 3 NAD + + FAD + ADP + P -----> NADH + FADH 2 + ATP ATP (considering respiratory chain) From pyruvate From acetyl-coa Regenerated Coenzymes ATP formed Regenerated Coenzymes NADH FADH ATP Total ATP formed Amphibolic character of CTA (Krebs) cycle Krebs cycle intermediates are also precursors in anabolic reactions. They are exported to the cytosol in where anabolism takes place. Example : Glutamate anabolism Cytosol Mitochondrie Glycolyse Cycle de Krebs Glucose Pyruvate Pyruvate NADP + NADPH Acétyl-CoA AA KG Glu KG Glutamate DH Since Krebs is stopped after KG synthesis, AA is not regenerated. AA is then produced from glycolysis. This AA synthesis is called «anaplerotic reaction» 9

10 In animal cells Anaplerotic reactions xaloacetate is synthetized from pyruvate by a carboxylation catalyzed by a pyruvate carboxylase and biotin as activator coenzyme. Pyruvate carboxylase (ligase) ATP ADP + P 3 2 ' DPG1 Generality : Carboxylation reactions requires always ATP and biotin. Cas des cellules végétales xaloacetate is synthetized from PEP by a carboxylase that hydrolyze phosphate group. Biotin is not required. C P 2 P PEP carboxylase ' 10

11 Diapositive 19 DPG1 D PG; 16/11/2016