1 Introduction Microbes 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. • GlucoseNADH + FADH2 -> ETS in plasma membranesO 2 • In soil, organisms tranfer electrons to Metals such as Fe 3+ . • Some bacteria can donate electrons • to electrodes and power a fuel cell
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1 Introduction Microbes transfer energy by moving electrons. - Electrons move from substrate molecules onto energy carriers, then onto membrane protein.
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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.
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