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Bacterial Physiology (Micr430)
Lecture 2
Membrane Bioenergetics
(Text Chapter: 3)
Bioenergetics
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Bioenergetics is the subject of a field of
biochemistry that concerns energy flow
through living systems.
Membrane bioenergetics focuses on
energy flow involving biological
membranes
The Chemiosmotic Theory
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Energy-transducing membranes pump
protons across the membrane, thereby
generating an electrochemical gradient
of protons across the membrane (the
proton potential)
This proton potential can be used to do
useful work when the protons return
across the membrane to the lower
potential.
Reaction Types
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Exergonic reactions are energy-yielding
and are thermodynamically favored.
Endergonic reactions require energy
(consuming energy)
Proton Circuit
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Cell membrane is similar to a battery in that it
maintains a potential difference between the
inside and outside;
The difference is that the flows is one of
protons rather than electrons;
Protons are translocated to the cell surface,
driven there by either chemical or light
energy through a proton pump (reactions 1);
The protons return through a special proton
transporters (reactions 2) that do work.
The Proton Current
Fig. 3.1
Electrochemical Energy of
Protons
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When bacteria translocate protons across the
membrane to the outside surface, energy is
conserved in the proton gradient that is established;
Energy in the proton gradient is both electrical and
chemical;
The electrical energy exists because a positive charge
has been moved across the membrane, creating a
charge separation, i.e., the membrane potential;
When the proton moves back into the cell toward the
negatively charged surface of the membrane, the
membrane potential is dissipated (energy is reduced
and work can be done).
Electrochemical Energy of
Protons
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The same description applies to chemical
energy: Energy is required to move the
proton against its concentration gradient;
This energy is stored in the concentration
gradient, which is called chemical energy;
When the proton returns to the lower
concentration side of the membrane, the
energy in the concentration gradient is
dissipated and work can be done.
The sum of the changes in electrical and
chemical energies is called electrochemical
energy.
Cell Energetics
From Gardner, Boston U.
Video clip
http://www.youtube.com/watch?v=Idy2XAlZIVA
Proton Motive Force
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The electrochemical work that is performed
when an ion crosses a membrane is a
function of both the membrane potential, DΨ,
and the difference in concentration between
the solutions separated by the membrane;
For one mole of protons:
DmH+ = FDΨ + RT ln[H+]in/[H+]out
J
Where FDΨ represents the electrical energy,
RT ln[H+]in/[H+]out represents the chemical
energy
Proton Motive Force
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To express the equation in milivolts (mV),
divide both sides by the Faraday constant
(F ͌ 96,500C)
For one mole of protons:
electrical
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chemical
Dp = DmH+/F = DΨ – 60DpH
mV (at 30°C)
Where Dp represents the proton motive force.
Bacteria maintain an average Dp of -140 to 200 mV (note it is an negative value).
Proton and Sodium Currents
and Work can be Done
Proton and Sodium Currents
and Work can be Done
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Processes that can be driven by proton
and sodium potentials:
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The Na+/H+ antiporter (3)
The H+/solute symporter (4)
The Na+/solute symporter (5)
Flagella movement (6)
Synthesis of ATP by ATP synthase (7)
PMP in Neutrophiles,
Acidophiles and Alkaliphiles
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For neutrophilic bacteria, the DΨ contributes
approximately 70 or 80% to Dp.
For acidophiles, DΨ is positive thus lowers
Dp, and Dp is due entirely to the DpH.
An opposite situation holds for aerobic
alkaliphilic bacteria. In these bacteria, DpH is
one to two units negative, so Dp is due
entirely to the DΨ.
Ionophores
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An ionophore is a lipid-soluble molecule usually
synthesized by microorganisms to transport ions
across the lipid bilayer of the cell membrane. There
are two broad classifications of ionophores.
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Small molecules (mobile ion carriers) that bind to a
particular ion, shielding its charge from the surrounding
environment, and thus facilitating its crossing of the
hydrophobic interior of the lipid membrane.
Channel formers that introduce a hydrophilic pore into the
membrane, allowing ions to pass through while avoiding
contact with the membrane's hydrophobic interior.
Ionophores are important research tools for
investigating membrane bioenergetics.
Examples of Ionophores
The ATP synthase
The ATP synthase