Transcript Respiration, Lithotrophy & Photosynthesis

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.
• Glucose NADH + FADH2 -> ETS in plasma
membranes O2
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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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Oxidoreductase Protein Complexes
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:
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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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Electron Transport Systems (ETS) is
present in membrane
Bacteria  Cytoplasmic membrane
Eukaryotes  Mitochondrial membrane
Electrons flow in cascading fashion
from one carrier to an another carrier in
membranes to a terminal electron
acceptor
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Flavoproteins (FMNFMNH2)
Iron-sulfur proteins (Fe3+  Fe2+ )
Quinone (Q QH2 )
Cytochromes (Fe3+  Fe2+ )
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ETS Function within a Membrane
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D
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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 (DG =nFDEo’ ).
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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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Reduction potential and Free energy
In redox reactions, the DG 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 DG.
The standard reduction potential assumes all
reactants and products equal 1 M at pH = 7.
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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 Proton Motive Force
The transfer of H+ through a proton pump generates
an electrochemical gradient of protons, called a
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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The Proton Motive Force
When protons are pumped across the membrane, energy is
stored in two different forms:
•The electrical potential (Dy) arises from the
separation of charge between the cytoplasm
and solution outside the cell membrane.
• The pH difference (DpH) is the log ratio of external to
internal chemical concentration of H+.
The relationship between the two components of the proton
potential Dp is given by:
Dp = Dy – 60DpH
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Dp Drives Many Cell Functions
Besides ATP synthesis, Dp drives many cell
processes including: rotation of flagella, uptake of
nutrients, and efflux of toxic drugs.
Figure 14.9
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The ETS: Summary
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
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Oxidoreductase Protein Complexes
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:
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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+).
The oxidation of NADH and reduction of Q is coupled to
pumping 4H+ across the membrane.
NADH-dehydrogenase
complex
glucose
amino acids
fatty acids
nuleic acids
H2
Fe2+
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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.
Electrons from NADH-dehydrogenase
complex
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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 O2.
- 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
The ATP synthase is a highly conserved protein
complex, made of two parts:
- 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
nitrate
nitrite
nitric
oxide
nitrous
oxide
nitrogen
gas
- In general, any given species can carry out only
one or two transformations in the series.
Oxidized forms of sulfur
- Sulfate is successively reduced by many
bacteria as follows:
SO42– → SO32– → 1/2 S2O32– → S0 → H2S
sulfate
sulfite
thiosulfate
sulfur
hydrogen
sulfide
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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
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.
1/2 O2
O2
1/2 O2
NH4+ → NH2OH → HNO2 → HNO3
ammonium
hydroxylamine nitrous acid
(nitrite)
nitric acid
(nitrate)
Surprisingly, ammonium can also yield energy under
anaerobic conditions through oxidation by nitrite produced
from nitrate respiration.
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Lithotrophy
Sulfur and metal oxidation
1/2 O2
1/2 O2
O2 + H2O
H2S → S0 → 1/2 S2O32– → H2SO4
hydrogen
sulfide
elemental
sulfur
thiosulfate
sulfuric acid
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
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