An overview of bacterial catabolism
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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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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.


Fructose: same atoms, but rearranged.

Glucose, showing numbering system.

http://nov55.com/scie/fructose.gif

http://www.biotopics.co.uk/as/glucosehalf.png


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


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.


Usually used in addition to EMP or Entner-Doudoroff.

http://www.cellml.org/examples/images/metabolic_models/the_pentose_phosphate_pathway.gif


Krebs Cycledetailed

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


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


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


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.


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!


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


Electron Transport molecules


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.


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


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!


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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