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IntroductionMicrobes transfer energy by moving electrons.

- Electrons move from substrate molecules onto energy carriers, then onto membrane protein carriers, and then onto oxygen or an alternative electron acceptor.

• Glucose NADH + FADH2 -> ETS in plasma membranes O2

• In soil, organisms tranfer electrons to Metals such as Fe3+.

• Some bacteria can donate electrons • to electrodes and power a fuel cell

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What is an electron transport system

(EST)?

Where is EST located?

What is a protonmotive force?

How are ATP generated?

What is oxidative phophorylation?

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The Electron Transport Chain

Series of electron carriers transfer electrons from NADH and FADH2 to a terminal electron acceptor

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A respiratory electron transport system includes at least 3 functional components:

1) An initial substrate oxidoreductase (or dehydrogenase)

2) A mobile electron carrier

3) A terminal oxidase

The ETS can be summarized as such:

Oxidoreductase Protein Complexes

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Coenzymes and cofactors are associated with oxidoreductase protein complexes and assist in moving electrons from NADH and FADH2 to O2

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Bacteria Cytoplasmic membraneEukaryotes Mitochondrial membrane

• Flavoproteins (FMNFMNH2)• Iron-sulfur proteins (Fe3+ Fe2+ )• Quinone (Q QH2 )• Cytochromes (Fe3+ Fe2+ )

Electron Transport Systems (ETS) is present in membrane

Electrons flow in cascading fashion from one carrier to an another carrier in membranes to a terminal electron acceptor

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ETS Function within a Membrane

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• Large difference in reduction potential between donor (NADH) and O2

(acceptor), a large amount of energy is released.

• Free energy change is proportional to reduction-potential difference between a donor and an acceptor (G =nFEo

’ ).

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A Bacterial ETS for Aerobic substrate Oxidation

Electron transfer is accompanied by the build up of protons across inner mitochondrial membrane

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Mitochondrial ETC

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In redox reactions, the G values are proportional to the reduction potential (E) between the oxidized form (e– acceptor) and its reduce form (e– donor)

- The reduction potential is a measure of the tendency of a molecule to accept electrons.

A reaction is favored by positive values of E, which yield negative values of G.

The standard reduction potential assumes all reactants and products equal 1 M at pH = 7.

Reduction potential and Free energy

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Proton Motive Force

The electron transport system generates a “proton motive force” that drives protons across the membrane.

- The PMF stores energy to make ATP.

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The transfer of H+ through a proton pump generates an electrochemical gradient of protons, called a proton motive force.

The Proton Motive Force

- It drives the conversion of ADP to ATP through ATP synthase.

- This process is known as the chemiosmotic theory.

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When protons are pumped across the membrane, energy is stored in two different forms:

• The electrical potential () arises from the

separation of charge between the cytoplasm

and solution outside the cell membrane.

• The pH difference (pH is the log ratio of external to internal chemical concentration of H+.

The relationship between the two components of the proton potential p is given by:

p = – 60pH

The Proton Motive Force

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Besides ATP synthesis, p drives many cell processes including: rotation of flagella, uptake of nutrients, and efflux of toxic drugs.

p Drives Many Cell Functions

Figure 14.9

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The electron transport proteins are called oxidoreductases.

They oxidize or extract electrons from a substrate (NADH, FADH2, H2, or Fe2+) and transfer them to next electron carrier in the membrane.

- Thus, they carry out discrete redox-reactions while electrons flow from one donor to next acceptor

Electron flow from a carrier with negative redox-potential to a carrier with positive redox-potential to a terminal electron acceptor

This flow of electrons results the generation of proton motive force across the membrane

The ETS: Summary

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A respiratory electron transport system includes at least 3 functional components:

1) An initial substrate oxidoreductase (or dehydrogenase)

2) A mobile electron carrier

3) A terminal oxidase

The ETS can be summarized as such:

Oxidoreductase Protein Complexes

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The oxidation of NADH and reduction of Q is coupled to pumping 4H+ across the membrane.

1) The substrate dehydrogenase receives a pair of electrons from an organic substrate, such as glucose, NADH, H2.

2) It donates the electrons ultimately to Flavoprotein (FMN/FMNH2) and Iron sulfur (Fe3+/Fe2+).

glucoseamino acidsfatty acidsnuleic acidsH2

Fe2+

NA

DH

-dehydrogenase com

plex

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Electrons from NADH-dehydrogenase complex

3) A mobile electron carrier, such as quinone pickups 2e- from previous electron donor and 2H+ cytoplasm (Q/QH2).

- There are many quinones, each with a different side chain; so for simplicity they are collectively referred to as Q and QH2.

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4) A terminal oxidase complex, which typically includes cytochromes, receive two electrons from quinol (QH2).

The 2H+ are translocated outside the membrane.

In addition, the transfer of the two electrons through the terminal oxidase complex is coupled to the pumping of 2H+.

- Totally 4 electrons are translocated across the membrane

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5) The terminal oxidase complex transfers the electrons to a terminal electron acceptor, such as O

Each oxygen atom receives two electrons and combines with two protons from the cytoplasm to form one molecule of H2O.

1/2 O2 + 2H+ → H2O

Thus, the E. coli ETS can pump up to 8H+ for each NADH molecule, and up to 6H+ for each FADH2 molecule.

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A Bacterial ETS for Aerobic NADH Oxidation

Figure 14.14

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The ATP synthase is a highly conserved protein complex, made of two parts:

The ATP Synthase

- Fo: Embedded in the membrane

- Pumps protons

- F1: Protrudes in the cytoplasm

- Generates ATP

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H+ Flux Drives ATP Synthesis: Oxidative Phophorylation

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Anaerobic Respiration

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Oxidized forms of nitrogen- Nitrate is successively reduced as follows:

NO3– → NO2

– → NO → 1/2 N2O → 1/2 N2

nitrite nitric oxide

nitrous oxide

- In general, any given species can carry out onlyone or two transformations in the series.

Oxidized forms of sulfur- Sulfate is successively reduced by many bacteria as follows:

SO42– → SO3

2– → 1/2 S2O32– → S0 → H2S

sulfite thiosulfate sulfur hydrogen sulfide

nitrate nitrogen gas

sulfate

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Anaerobic environments, such as the bottom of a lake, offer a series of different electron acceptors.- As each successive TEA is used up, its reduced form appears; the next best electron acceptor is then used, generally by a different microbe species.

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Lithotrophy:Oxidation of inorganic

compounds

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Lithotrophy is the acquisition of energy by oxidation of inorganic electron donors.

A kind of lithotrophy of great importance in the environment is nitrogen oxidation.

Lithotrophy

NH4+ → NH2OH → HNO2 → HNO3

ammonium hydroxylamine nitrous acid(nitrite)

nitric acid(nitrate)

1/2 O2 O2 1/2 O2

Surprisingly, ammonium can also yield energy under anaerobic conditions through oxidation by nitrite produced from nitrate respiration.

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Sulfur and metal oxidation

Lithotrophy

H2S → S0 → 1/2 S2O32– → H2SO4

hydrogensulfide

elementalsulfur

thiosulfate sulfuric acid

1/2 O2 1/2 O2 O2 + H2O

Microbial sulfur oxidation can cause severe environmental acidification, eroding structures.

- Problem is compounded by iron presence.

- Ferroplasma oxidizes ferrous sulfide:

FeS2 + 14Fe3+ + 8H2O → 15Fe2+ + 2SO42– + 16H+

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Sulfuric Acid Production: Science and Science Fiction

Figure 14.21