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.
Figure 14.5
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
Figure 14.6
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
The Respiratory ETS
• ETS proteins such as cytochromes associate
electron transfer with small energy transitions,
which are mediated by cofactors.
• Energy transitions typically involve these kinds of
molecular structures:
•
- Metal ions, such as iron or copper, held in
place with amino acid residues
•
- Conjugated double bonds and
heteroaromatic rings, such as the nicotinamide
ring of NAD+/NADH
Figure 14.11
Figure 14.13
A Bacterial ETS for Aerobic NADH Oxidation
Figure 14.14
ETS
• Animation: A bacterial electron transfer
system
Click box to launch animation
The F1Fo ATP Synthase
• The F1Fo 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
Figure 14.17
H+ Flux Drives ATP Synthesis
Figure 14.18AB
The F1Fo ATP Synthase
• Animation: ATP Synthase Mechanism
Click box to launch animation
• 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
• 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.
Figure 14.20
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.
Hydrogenotrophy
• Hydrogenotrophy is the use of molecular hydrogen
(H2) as an electron donor.
- H2 has sufficient reducing
potential to donate e– to
nearly all biological
electron acceptors.
- Including chlorinated
organic molecules, via
dehalorespiration
- Which has potential
for bioremediation
Figure 14.24