Tricarboxylic Acid Cycle. TCA Cycle; Krebs Cycle; Citric Acid Cycle

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Harvey Perkins
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1 Tricarboxylic Acid ycle TA ycle; Krebs ycle; itric Acid ycle

2 The Bridging Step: Pyruvate D hase O H pyruvate O O - NAD + oash O 2 NADH O H S - oa acetyl oa

3 Pyruvate D hase omplex Multienzyme complex (E. coli enzyme has 60 subunits) Three activities: pyruvate d hase (E1); dihydrolipoyl transacetylase (E2); dihydrolipoyl d hase (E3) Prime example of metabolite channeling (substrates acted upon immediately on enzyme surface - no diffusion into cytosol)

4 1) Pyruvate d hase : loss of O 2 from pyruvate with transfer of the remaining two-carbon unit as a hydroxyethyl group (thiamine pyrophosphate (TPP) used as a cofactor) 2) Dihydrolipoyl transacetylase : hydroxyethyl group transferred to lipoic acid and oxizided to a carboxylic acid (lipoic acid cofactor converted to acetyl dihydrolipoamide); acetyl group transferred to oa [arsenic binds lipoamide] 3. Dihydrolipoyl d hase : lipoic acid regenerated using NAD + and FAD; NADH is produced Thiamine (TPP); Riboflavin (FAD); Niacin (NAD + ); Pantothenate (oa); Lipoic Acid

5 OH H 3 --H TPP E1 S S E2 E3 FAD

6 TPP E1 H 3 -- O S S E2 E3 FAD Acetyl oa TPP HS HS E1 E2 E3 FAD

7 TPP S S E1 E2 E3 FADH 2 NAD+ NADH TPP S S E1 E2 E3 FAD

8 Regulation of Pyruvate D hase Acetyl oa and NADH allosterically inhibit (product inhibition) Mammalian pyr. d hase is phosphorylated and inactivated by a pyr. d hase kinase. This kinase itself is activated allosterically by NADH and acetyl oa. This effect is reversed by pyr. d hase phosphatase, which removed the phosphate and reactivates the enzyme.

9 AMP activates and GTP inhibits pyruvate dehydrogenase. This commits pyruvate to energy production.

10 SH H 2 β-mercaptoethylamine H 2 NH O OH H 3 O O -H 2 -H 2 -NH--H--H 2 O-P-O-P-O - O H 3 O- O- H 2 A Pantothenic acid oenzyme A O 2- PO 3 OH 3-5 -ADP

11 1. itrate Synthase O H 2 oxaloacetate (OAA) acetyl oa oash H 2 HO H 2 citrate

12 Regulation of itrate Sythase: Reaction has a large negative DG = kj/mol NADH and Succinyl oa are allosteric inhibitors

13 2. Aconitase H 2 HO H 2 citrate H 2 H H OH isocitrate

14 3. Isocitrate D hase H 2 H H OH isocitrate NAD + NADH O 2 H 2 H 2 O α-ketoglutarate (αkg)

15 Regulation of Isocitrate D hase Mammalian enzyme: NADH and ATP are allosteric inhibitors; ADP and NAD+ are allosteric activators E. oli enzyme: phosphorylation of the enzyme by a specific protein kinase abolishes activity; removal of the phosphate by a phosphatase restores activity

16 4. a-ketoglutarate D hase NADH oash NAD + H 2 H 2 O O 2 α-ketoglutarate (αkg) H 2 H 2 SoA O Succinyl-oA Reaction mechanism identical to that of pyruvate d hase: same cofactors utilized (succinyl group transferred)

17 5. Succinyl oa Synthetase H 2 H 2 SoA O Succinyl-oA GDP, Pi GTP oash H 2 H 2 H 2 O O - Succinate

18 6. Succinate D hase H 2 H 2 H 2 Succinate FAD FADH 2 H - OO OO- - H Fumarate

19 Regulation of Succinate D hase Enzyme is a large multisubunit enzyme with muliple cofactors like pyruvate d hase. Enzyme transfers electrons from the substrate succinate to ubiquinone (Q) Malonate (analogue of succinate) is a competitive inhibitor and blocks the cycle at this step; αkg, citrate, succinate accumulate in its presence

20 7. Fumarase H - OO OO- - H Fumarate H 2 O HO H H 2 - OO Malate

21 8. Malate D hase HO H NAD+ - OO H 2 NADH O Malate - OO H 2 OAA

22 Overall Equation for TA: Acetyl oa + 3NAD + + Q(FAD) + GDP + Pi + 2H 2 O oash + 3NADH + QH 2 (FADH 2 ) + GTP + 2O 2 + 2H + *No net degradation of intermediates in TA ycle - they are reformed with each full turn of the cycle.

23 *NADH and QH 2 are oxidized by the respiratory electron transport chain. 3ATP per NADH and 2ATP per QH 2. Reaction Energy Yielding Product ATP s Isocitrate D hase NADH 3 α Kg D hase NADH 3 Succinyl oa Synthetase GTP (ATP) 1 Succinate D hase QH 2 2 Malate D hase NADH 3 One Round of TA 12

24 Amount of ATP formed per 1 Glucose: ATP s Glycolysis 8 Pyruvate D hase 6 TA 24 38

25 The Glyoxylate ycle A shunt within the TA cycle Biosynthetic route that leads to formation of glucose from acetyl oa Occurs in plants, bacteria and yeast

26 Isocitrate is cleaved by isocitrate lyase to form succinate and glyoxylate: H 2 H H OH isocitrate H 2 H 2 Succinate O H Glyoxylate

27 Glyoxylate condenses with acetyl oa to form malate: O H H + 3 =O S-oA Glyoxylate Acetyl oa Malate Synthase HO H H 2 Malate NO ARBON ATOMS LOST AS O 2! THUS A NET SYNTHESIS OF MALATE IS AHEIVED.

28 Acetyl oa Glucose Malate Fumarate OAA Acetyl oa oash Glyoxylate itrate Isocitrate Succinate αkg Succinyl oa

29 Glyoxylate ycle requires transfer of metabolites between the mitochondrion, cytosol and a special organelle, the glyoxysome. Glyoxysome: Isocitrate cleaved to succinate and glyoxylate. Glyoxylate condenses with acetyl oa to form malate. Succinate goes to mitochondrion; malate to cytosol. Mitochondrion: Succinate enters the TA cycle. ytosol: Malate converted to OAA; OAA to glucose by the gluconeogenesis pathway.

CITRIC ACID CYCLE ERT106 BIOCHEMISTRY SEM /19 BY: MOHAMAD FAHRURRAZI TOMPANG

CITRIC ACID CYCLE ERT106 BIOCHEMISTRY SEM 1 2018/19 BY: MOHAMAD FAHRURRAZI TOMPANG Chapter Outline (19-1) The central role of the citric acid cycle in metabolism (19-2) The overall pathway of the citric