Bacterial Physiology (Micr430)

Lecture 2Membrane Bioenergetics

(Text Chapter: 3)

Bioenergetics

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

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

Exergonic reactions are energy-yielding and are thermodynamically favored.

Endergonic reactions require energy (consuming energy)

Proton Circuit

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

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

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

The electrochemical work that is performed when an ion crosses a membrane is a function of both the membrane potential, Ψ, and the difference in concentration between the solutions separated by the membrane;

For one mole of protons:

H+ = FΨ + RT ln[H+]in/[H+]out J

Where FΨ represents the electrical energy, RT ln[H+]in/[H+]out represents the chemical energy

Proton Motive Force

To express the equation in milivolts (mV), divide both sides by the Faraday constant (F B 96,500C)

For one mole of protons:

p = H+/F = Ψ – 60pH mV (at 30°C)

Where p represents the proton motive force.

Bacteria maintain an average p of -140 to -200 mV (note it is an negative value).

electrical chemical

Proton and Sodium Currents and Work can be

Done

Proton and Sodium Currents and Work can be

Done

Processes that can be driven by proton and sodium potentials: 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

For neutrophilic bacteria, the Ψ contributes approximately 70 or 80% to p.

For acidophiles, Ψ is positive thus lowers p, and p is due entirely to the pH.

An opposite situation holds for aerobic alkaliphilic bacteria. In these bacteria, pH is one to two units negative, so p is due entirely to the Ψ.

Ionophores

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.

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