the proton motive force

• The transfer of H+ through a proton pump generates an electrochemical gradient of protons, called a proton motive force.

The Proton Motive Force

Figure 14.5

- It drives the conversion of ADP to ATP through ATP synthase.- This process is known as the chemiosmotic theory.

• 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

The Proton Motive Force

Figure 14.6

• Besides ATP synthesis, Dp drives many cell processes including: rotation of flagella, uptake of nutrients, and efflux of toxic drugs.

Dp Drives Many Cell Functions

Figure 14.9

• 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

The Respiratory ETS

Figure 14.11

Figure 14.13

A Bacterial ETS for Aerobic NADH Oxidation

Figure 14.14

• Animation: A bacterial electron transfer system

Click box to launch animation

ETS

• The F1Fo ATP synthase is a highly conserved protein complex, made of two parts:

The F1Fo ATP Synthase

- Fo: Embedded in the membrane

- Pumps protons- F1: Protrudes in the cytoplasm

- Generates ATP

Figure 14.17

H+ Flux Drives ATP SynthesisFigure 14.18AB

• Animation: ATP Synthase Mechanism

Click box to launch animation

The F1Fo ATP Synthase

• 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

• 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 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.

• Hydrogenotrophy is the use of molecular hydrogen (H2) as an electron donor.

Hydrogenotrophy

- 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