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
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.

Glycolysis glucose is broken
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.

3 ways bacteria use glucose
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.

Krebs cycle detailed
Krebs Cycledetailed

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.

About coenzyme a
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. coenzymes.htm

Electron transport
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
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 transport1
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.

Chemiosmosis electron transport used to make atp
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
Proton motive force

Electrochemical gradient establishes a voltage across the membrane. Flow of protons (instead of electrons) does work (the synthesis of ATP).

Overview of aerobic metabolism
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
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