Krebs cycle

1. Krebs cycle

2. Glycolysis • Glucose converted to pyruvate. • First half uses 2 ATP • Forms 2 separate G3P (glyceraldehyde 3-phosphate)

3. Glycolysis • Second half generates 4 ATP, 2 NADH & 2 pyruvate • Net results are 2 ATP, 2 NADH and 2 pyruvate • Takes place in the cytoplasm

5. Krebs Cycle (Citric acid cycle) • Series of 8 sequential reactions • Matrix of the mitorchondria • Synthesis of ATP • Generation of 10 energetic electrons • 3 CO2 molecules

6. Krebs Cycle

7. Reaction 1: Condensation • 2-carbon acetyl group from acetyl-CoA • Joins with oxaloacetate a four-carbon molecule • Forms a six-carbon molecule, citrate.

8. Reaction 2: Isomerization • Hydroxyl (-OH) group of citrate is repositioned • A water molecule is removed from one carbon • Water is added to another carbon on the same citrate molecule. • As a result, an –H group & an –OH group change positions. • Product is isocitrate-an isomer of citrate

9. Reaction 3: The First Oxidation • First energy yielding step of cycle • Isocitrate undergoes an oxidative decarboxylation reaction. • First: isocitrate is oxidized • Yielding a pair of electrons • Associated with a proton as a hydrogen atom • Reduces NAD+ to NADH.

10. Reaction 3: The First Oxidation • Second: oxidized intermediate is decarboxylated • Central carbon atom splits off to form CO2 • Yields a five-carbon molecule • α-ketoglutarate

11. Reaction 4: The Second Oxidation • α-ketoglutarate is decarboxylated • Looses a CO2 • CoEnzyme A is attached • Forms succinyl-CoA • Two electrons are extracted • Associated with a proton as a hydrogen atom • Reduce another molecule of NAD+ to NADH.

12. Reaction 5: Substrate-Level Phosphorylation • Linkage between the four-carbon succinyl group & CoA is a high-energy bond. • Bond is cleaved • Energy released drives phosphorylation of GDP, forming GTP. • GTP is readily converted into ATP, • Succinate 4-carbon fragment that remains

13. Reaction 6: Third Oxidation • Succinate is oxidized to fumarate • FAD+ is electron acceptor. • FAD+ remains in a part of the inner mitochondria membrane • FADH2 (reduced) is used in electron transport chain in the membrane

14. Reactions 7 & 8: Regeneration of Oxaloacetate. • A water molecule is added to fumarate, • Forms malate • Malate is then oxidized • Yields oxaloacetate a four-carbon molecule • Two electrons • Associated with a proton as a hydrogen • Reduce a molecule of NAD+ to NADH.

15. Reactions 7 & 8 : Regeneration of Oxaloacetate. • Oxaloacetate • Molecule that began the cycle • Combines with another two-carbon acetyl group from acetyl-CoA • Reinitiate the cycle.

16. Acetyl CoA CoA—SH 1 Fig. 9-12-1 Oxaloacetate Citrate Citric acid cycle

17. Acetyl CoA CoA—SH H2O 1 Fig. 9-12-2 Oxaloacetate 2 Citrate Isocitrate Citric acid cycle

18. Acetyl CoA CoA—SH 1 H2O Fig. 9-12-3 Oxaloacetate 2 Citrate Isocitrate NAD+ Citric acid cycle NADH 3 + H+ CO2 -Keto- glutarate

19. Acetyl CoA CoA—SH 1 H2O Fig. 9-12-4 Oxaloacetate 2 Citrate Isocitrate NAD+ Citric acid cycle NADH 3 + H+ CO2 CoA—SH -Keto- glutarate 4 CO2 NAD+ NADH Succinyl CoA + H+

20. Acetyl CoA CoA—SH 1 H2O Fig. 9-12-5 Oxaloacetate 2 Citrate Isocitrate NAD+ Citric acid cycle NADH 3 + H+ CO2 CoA—SH -Keto- glutarate 4 CoA—SH 5 CO2 NAD+ Succinate NADH P i Succinyl CoA + H+ GDP GTP ADP ATP

21. Acetyl CoA CoA—SH H2O 1 Fig. 9-12-6 Oxaloacetate 2 Citrate Isocitrate NAD+ Citric acid cycle NADH 3 + H+ CO2 Fumarate CoA—SH -Keto- glutarate 4 6 CoA—SH 5 FADH2 CO2 NAD+ FAD Succinate NADH P i + H+ Succinyl CoA GDP GTP ADP ATP

22. Acetyl CoA CoA—SH H2O 1 Fig. 9-12-7 Oxaloacetate 2 Malate Citrate Isocitrate NAD+ Citric acid cycle NADH 3 + H+ 7 H2O CO2 Fumarate CoA—SH -Keto- glutarate 4 6 CoA—SH 5 FADH2 CO2 NAD+ FAD Succinate NADH P P i + H+ Succinyl CoA GDP GTP ADP ATP

23. Acetyl CoA CoA—SH NADH H2O 1 +H+ Fig. 9-12-8 NAD+ Oxaloacetate 8 2 Malate Citrate Isocitrate NAD+ Citric acid cycle NADH 3 + H+ 7 H2O CO2 Fumarate CoA—SH -Keto- glutarate 4 6 CoA—SH 5 FADH2 CO2 NAD+ FAD Succinate NADH P i + H+ Succinyl CoA GDP GTP ADP ATP

24. Krebs Cycle • 2 pyruvate from glycolysis • 6 CO2 molecules • 2 ATP molecules • 10 electron carriers • 8 NADH molecules • 2 FADH2

25. Glycolysis & the Krebs cycle • produced a large amount of electron carriers. • These carriers enter the electron transport chain • Help produce ATP

26. Electron transport chain • Energy captured by NADH is not harvested all at once. • Transferred directly to oxygen • 2 electrons carried by NADH are passed along the electron transport chain if oxygen is present.

27. Oxidative phosphorylation Formation of ATP • 1. Electron transport chain • Series of molecules embedded in the inner membranes of mitochondria. • Electrons are delivered at the top of the chain • Oxygen captures them at the bottom

28. Electron transport chain • Large protein complexes • Smaller mobile proteins • Smaller lipid molecule called ubiquinone (Q)

29. NADH 50 e– 2 NAD+ FADH2 e– 2 FAD Multiprotein complexes  FAD Fig. 9-13 40 FMN  Fe•S Fe•S Q  Cyt b Fe•S 30 Cyt c1 IV Free energy (G) relative to O2 (kcal/mol) Cyt c Cyt a Cyt a3 20 e– 2 10 (from NADH or FADH2) O2 2 H+ + 1/2 0 H2O

30. Electrons move towards a more electronegative carrier • Electrons move down an electron gradient • This flow of electron creates the active transport of protons out into the matrix

31. 2. Chemiosmosis • Protons diffuse back into the matrix through a proton channel • It is coupled to ATP synthesis

33. Electron transport chain • Carbon monoxide & cyanide affect the electron transport in the mitochondria • Shuts down the production of ATP • Cell dies as does the organism

35. Fermentation • Anaerobic conditions • H atoms (NADH) are donated to organic compounds • Regenerates NAD+

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38. Proteins and fats • Other organic molecules are an important source of energy.

39. Proteins • First are broken down to amino acids • Each amino acid undergoes a process called deamination. • Removal of the nitrogen containing side group • After a series of reactions the carbon groups enter the either glycolysis or the krebs cycle

40. Fats • Fats are broken down to FA & glycerol • Each FA undergoes β oxidation • Conversion of the FA to several acetyl groups • These groups combine with coenzyme A to make acetyl-CoA

41. Regulation • Control of the glucose catabolism • Occurs at 3 key points • 1. Control point in glycolysis • Enzyme phosphofructokinase • Catalyzes the conversion of fructose 6-phosphate to fructose 1,6 bisphosphate.

42. Regulation • High levels of ATP inhibit phosphofructokinase • ADP & AMP activate the enzyme • Low levels of citrate also activate the enzyme

43. Regulation • 2. Pyruvate dehydrogenase • Enzyme that removes CO2 from pyruvate. • High levels of NADH will inhibit its action

44. Regulation • 3. High levels of ATP inhibit the enzyme citrate synthetase • Enzyme that starts the Krebs cycle • Combines Acetyl-CoA with oxaloacetate to make citrate

45. Evolution • Krebs cycle & ETC function only in aerobic conditions • Glycolysis occurs in both • Early bacteria used only glycolysis to make ATP before O2 • All kingdoms of life use glycolysis • Occurs outside the mitochondria • Indicates mitochondria developed later.