Pyruvate Oxidation and Krebs Cycle Jennifer Agnes Hilary Lucy

Quick Review -In glycolysis, the first stage of cellular respiration, glucose, a 6-C chain molecule was broken down into 2 pyruvate molecules in a series of 10 steps. Net payoff: 2 ATP (2 used, 4 produced) 2 NADH -The pyruvate then moves from the cytoplasm into the mitochondrial matrix.

Glycolysis

REMEMBER: After Glycolysis which takes place in the cytosol, all the steps take place in the mitochondria

Depending on the presence of oxygen, the pyruvate will either enter a fermentation process (lactic acid or alcohol) or proceed towards the Krebs Cycle.

-> 2 acetyl-CoA + 2 NADH + 2H2+ 2 CO2 Pyruvate Oxidation overall purpose: convert Pyruvate into Acetyl-CoA 2 pyruvate + 2 NAD + 2 CoA  -> 2 acetyl-CoA + 2 NADH + 2H2+ 2 CO2 acetyl CoA - central molecule in energy metabolism proteins, lipids, can also be broken down into acetyl CoA can produce fat or ATP, depending on ATP levels in the cell

-> 2 acetyl-CoA + 2 NADH + 2H2+ 2 CO2 Pyruvate Oxidation net production: 2 NADH - electron transport chain 2 acetyl-CoA - enter Krebs Cycle 2H+ - dissolves in matrix 2 CO2 - diffuses out of cell   2 pyruvate + 2 NAD + 2 CoA  -> 2 acetyl-CoA + 2 NADH + 2H2+ 2 CO2

- transition step between glycolysis and Kreb's Cycle Pyruvate Oxidation - transition step between glycolysis and Kreb's Cycle

1. carboxyl functional group removed as CO2 - decarboxylation catalyzed by enzyme pyruvate decarboxylase 2. redox reaction that converts NAD+ -> NADH and H+ pyruvate is oxidized, NAD+ is reduced   3. CoA is attached to acetate compound to form Acetyl CoA -> unstable carbon-sulfur bond in preparation for the Krebs Cycle

Krebs Cycle metabolic pathway amphibolic (catabolism and anabolism) cyclic -> oxacetate reused  produces precursors to ATP   net production:  3 NADH x 2 = 6 NADH 1 FADH2 x 2 = 2 FADH2 - later converted to ATP in the electron transport chain

Krebs Cycle Equation Oxaloacetate + Acetyl-CoA+ ADP + Pi +3NAD+ + FAD   -->    CoA+ATP+3NADH +3H++FADH2+ 2CO2+ Oxaloacetate

Krebs Cycle: The Simple Version

Also known as the citric or tricarboxylic acid cycle (TCA) because citric acid has THREE CARBOXYL groups

Hans Krebs (1900-1981) German-born British biochemist who received a Nobel Prize in Physiology for the discovery of the Krebs Cycle.       -Krebs discovered that citrate formed when pyruvate and oxaloacetate       combined, thus explaining the reason why the reactions form a cycle. Krebs also discovered the urea cycle with another biochemist, Kurt Henseleit. In this cycle, ammonia is converted to urea in the tissues of mammals; being far less toxic than ammonia, urea is excreted in the urine of most mammals.

Step1: Acetyl+Oxaloacetate= Citrate CoA-SH freed http://rpi.edu/dept/bcbp/molbiochem/MBWeb/mb1/krebscyc/krebscyc.htm Interactive Krebs Cycle (with enzyme names) Step1: Acetyl+Oxaloacetate= Citrate CoA-SH freed Enzyme:  Citrate Synthase Step 2: Citrate rearranged to isocitrate Enzyme:Aconitase Step 3: Oxidative decarboxylation: Loss of 2 H atoms to NAD+ (forms NADH + H+), removal of C as CO2,  isocitrate becomes a-ketoglutarate Enzyme:  Isocitrate DH   Step 4: Succinyl-CoA produced. Oxidative decarboxylation occurs again Enzyme:a-ketoglutarate DH (multi-enzyme complex)

  Step 5: ATP formed via substrate level phosphorylation, Succinyl- CoA becomes succinate Enzyme:  Succinate thiokinase (Succinyl-CoA Synthetase) + Nucleoside diphosphate kinase Step 6: Succinate oxidized to fumarate (FADH2 produced) Enzyme:  Succinate DH Step 7: Fumarate hydrated to produce malate. Enzyme:  Fumarase Step 8: Malate is oxidized into oxaloacetate. Enzyme:  Malate DH

C atoms in glucose - what happens to them? Glucose - has a 6-carbon backbone  by the end of Krebs Cycle, all of them have been oxidized into CO2 - released from cell as waste

Summary of Krebs Cycle How many energy-producing molecules do we have per 1 glucose molecule? Intermediate step: Pyruvate oxidation 1 NADH x2= 2 NADH   Krebs Cycle:  3 NADHx2= 6 NADH 1 ATPx2= 2 ATP 1 FADH2x2= 2 FADH2 + 2 ATP, 2 NADH from glycolysis Total: 4 ATP, 10 NADH, 2 FADH2 --> forms 38 ATP in the electron transport chain

Helpful links Khan Academy video- extremely helpful and simple Salman, K. (Producer) (2009). Krebs / citric acid cycle [Web]. Retrieved from http://www.youtube.com/watch?v=juM2ROSLWfw Video that tracks the movement of the atoms during the various steps of the process. Very useful for precise information about how each molecule forms. Helpful for review Bielecki, A. (Producer) (2011). The citric acid cycle (krebs cycle) [Web]. Retrieved from http://youtu.be/7gR4s8ool1Y  Flash Video The citric acid cycle [Web]. (n/a). Retrieved from http://www.wiley.com/college/pratt/0471393878/student/animations/citric_acid_cycle/index.html Another instructional Video Krebs cycle [Web]. (n/a). Retrieved from http://youtu.be/O6bInBQXtmM  

Resources “Alpha Ketoglutarate Dehydrogenase - Links Krebs Cycle to Amino Acid production” (2002). Retrieved from http://everything2.com/title/Alpha+Ketoglutarate+Dehydrogenase+-+Links+Krebs+Cycle+to+Amino+Acid+production Dax, R., H.A., D.B. (2010). From Pyruvate to Acetyl-CoA: Oxidation. Retrieved from http://library.thinkquest.org/11226/main/c05txt.htm Diwan, J. J. (2007). Pyruvate Dehydrogenase and Krebs Cycle. Retrieved from http://www.rpi.edu/dept/bcbp/molbiochem/MBWeb/mb1/part2/krebs.htm Edwards, C. (2010). Energy Production Through the Krebs Cycle. Retrieved from http://drchadedwards.com/244/energy-production-through-the-krebs-cycle/ Freeman, W.H. (2002). The Citric Acid Cycle. Retrieved from http://www.ncbi.nlm.nih.gov/books/NBK21163/ Krebs cycle. (n.d.). Retrieved from http://cmapspublic2.ihmc.us/rid=1292768176697_195977725_68466/krebs_cycle.cmap  

  "Krebs Cycle." The Gale Encyclopedia of Science. Ed. K. Lee Lerner and Brenda Wilmoth Lerner. 3rd ed. Vol. 3. Detroit: Gale, 2004. 2249-2251. Gale Virtual Reference Library. Web. 29 Oct. 2011. McGraw Hill Co. (n.d.). Animation: How the krebs cycle works [Web]. Retrieved from http://highered.mcgraw-hill.com/sites/0072507470/student_view0/chapter25/animation__how_the_krebs_cycle_works__quiz_1_.html Noiva, Robert. "Krebs Cycle." Chemistry: Foundations and Applications. Ed. J. J. Lagowski. Vol. 3. New York: Macmillan Reference USA, 2004. 11-13. Gale Virtual Reference Library. Web. 29 Oct. 2011.   Rosenberg, H. I. (n.d.). Cellular respiration summary. Retrieved from http://people.ucalgary.ca/~rosenber/CellularRespirationSummary.html The citric acid cycle [Web]. (n/a). Retrieved from http://www.wiley.com/college/boyer/0470003790/animations/tca/tca.htm “The Tricarboxylic Cycle.” (2003). Retrieved from http://web.virginia.edu/Heidi/chapter20/chp20.htm Tricarboxylic acid cycle. (2011). In Encyclopædia Britannica. Retrieved from http://www.britannica.com/EBchecked/topic/604852/tricarboxylic-acid-cycle