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An overview of bacterial catabolism

An overview of bacterial catabolism. Model compound: glucose, C 6 H 12 O 6 Aerobic metabolism With oxygen gas (O 2 ) Fermentation and anaerobic respiration – later Four major pathways Glycolysis, Krebs cycle, electron transport, and chemiosmosis

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An overview of bacterial catabolism

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  1. An overview of bacterial catabolism • Model compound: glucose, C6H12O6 • Aerobic metabolism • With oxygen gas (O2) • Fermentation and anaerobic respiration – later • Four major pathways • Glycolysis, Krebs cycle, electron transport, and chemiosmosis • The Goal: gradually release the energy in the glucose molecule and use it to make ATP. • The carbons of glucose will be oxidized to CO2.

  2. Fructose: same atoms, but rearranged. Glucose, showing numbering system. http://nov55.com/scie/fructose.gif http://www.biotopics.co.uk/as/glucosehalf.png

  3. Glycolysis: glucose is broken • Glucose is activated • Glucose Glu-6-P • 2 ATPs are “invested” • Glu (6 C’s) broken into two 3-C pieces • 2 oxidations steps • NAD NADH • 4 ATPs are produced, net gain: 2 ATPs • 2 molecules of pyruvic acid are produced. http://members.tripod.com/beckysroom/glycolysis.jpg

  4. 3 ways bacteria use glucose EMP = Embden- Meyerhof-Parnas pathwayTraditional glycolysis, yields 2 ATP plus 2 pyruvic acids. Pentose PhosphateComplicated pathway, produces 5 carbon sugars and NADPH for use in biosynthesis. Entner-DoudoroffYields only 1 ATP per glucose, but only used by aerobes such as Pseudomonas which make many ATP through aerobic respiration.

  5. Usually used in addition to EMP or Entner-Doudoroff. http://www.cellml.org/examples/images/metabolic_models/the_pentose_phosphate_pathway.gif

  6. Krebs Cycledetailed http://www.personal.kent.edu/~cearley/PChem/Krebs1.gif

  7. What happens: carbons of glucose oxidized completely to CO2 Preliminary step: pyruvic acid oxidized to acetyl-CoA. Things to note: several redox steps make NADH, FADH2 One ATP made OAA remade, allowing cycle to go around again. http://www.sp.uconn.edu/~bi107vc/images/mol/krebs_cycle.gif

  8. About Coenzyme A The vitamin CoA is way bigger than the organic acids acted on by the enzymes. CoA serves as a handle; an acid attaches to it, chemistry is done on the acid. Acids (e.g. acetate, succinate) attach to this –SH group here. This piece here = acetyl group. www.gwu.edu/~mpb/ coenzymes.htm

  9. Electron transport • Metabolism to this point, per molecule of glucose: • 2 NADH made during glycolysis, 8 more through the end of Krebs Cycle (plus 2 FADH2) • What next? • If reduced NAD molecules are “poker chips”, they contain energy which needs to be “cashed in” to make ATP. • In order for glycolysis and Krebs Cycle to continue, NAD that gets reduced to NADH must get re-oxidized to NAD. • What is the greediest electron hog we know? Molecular oxygen. • In Electron transport, electrons are passed to oxygen so that these metabolic processes can continue with more glucose. • Electron carriers in membrane are reversibly reduced, then re-oxidized as they pass electrons (or Hs) to the next carrier.

  10. About Hydrogens • A hydrogen atom is one proton and one electron. • In biological redox reactions, electrons are often accompanied by protons (e.g. dehydrogenations) • In understanding metabolism, we are not only concerned with electrons but also protons. • Also called hydrogen ions or H+ • H+ (hydrogen ions) and electrons are opposites!! Don’t get them confused!

  11. Electron transport Electrons are passed carrier to carrier, releasing energy. • All occurs at the cell membrane • NADH is oxidized to NAD • H’s are passed to next electron carrier; NAD goes, picks up more H. Process can’t continue without an electron acceptor at the end. In aerobic metabolism, the acceptor is molecular oxygen. http://employees.csbsju.edu/hjakubowski/classes/ch331/oxphos/oxphos.gif

  12. Electron Transport molecules

  13. Chemiosmosis: electron transport used to make ATP Energy released during electron transport used to “pump” protons (against the gradient) to the outside of the membrane. The membrane acts as an insulator; protons can only pass through via the ATPase enzyme; the energy released is used to power synthesis of ATP from ADP and Pi. Creates a proton current (pmf) much like the current of electrons that runs a battery.

  14. Proton motive force Electrochemical gradient establishes a voltage across the membrane. Flow of protons (instead of electrons) does work (the synthesis of ATP). http://www.energyquest.ca.gov/story/images/chap04_simple_circuit_light.gif

  15. Overview of aerobic metabolism • Energy is in the C-H bonds of glucose. • Oxidation of glucose (stripping of H from C atoms) produces CO2 and reduced NAD (NADH) • Energy now in the form of NADH (“poker chips”) • Electrons (H atoms) given up by NADH at the membrane, energy released slowly during e- transport and used to establish a proton (H+) gradient across the membrane • Energy now in the form of a proton gradient which can do work. • Electrons combine with oxygen to produce water, take e- away. • Proton gradient used to make ATP • Energy now in the form of ATP. Task is completed!

  16. Adding it up: per glucose • Glycolysis: 2 ATP and 2 NADH • Krebs Cycle: 2 ATP, 2 x 4 NADH, 2 x FADH2 • Electron transport: each NADH results in chemiosmotic production of 3 ATPs (2 for FADH2) • 10 x 3 = 30; 2 x 2 = 4; plus 2 from glycolysis. • Total aerobic synthesis of ATP starting from glucose • About 36 ATP

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