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 In soil, organisms tranfer electrons to Metals such as Fe3+. Some bacteria can donate electrons to electrodes and power a fuel cell

What is an electron transport system (EST). Where is EST located What is an electron transport system (EST)? Where is EST located? What is a protonmotive force? How are ATP generated? What is oxidative phophorylation?

The Electron Transport Chain Series of electron carriers transfer electrons from NADH and FADH2 to a terminal electron acceptor 3

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: 4

Coenzymes and cofactors are associated with oxidoreductase protein complexes and assist in moving electrons from NADH and FADH2 to O2 5

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 Flavoproteins (FMNFMNH2) Iron-sulfur proteins (Fe3+  Fe2+ ) Quinone (Q QH2 ) Cytochromes (Fe3+  Fe2+ )

ETS Function within a Membrane 7

D 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’ ).

A Bacterial ETS for Aerobic substrate Oxidation Electron transfer is accompanied by the build up of protons across inner mitochondrial membrane 9

Mitochondrial ETC

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. 11

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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.

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. 14

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 15

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 16

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 17

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:

NADH-dehydrogenase complex 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+). NADH-dehydrogenase complex glucose amino acids fatty acids nuleic acids H2 Fe2+ The oxidation of NADH and reduction of Q is coupled to pumping 4H+ across the membrane. 19

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

 The 2H+ are translocated outside the membrane. 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 21

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. 22

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A Bacterial ETS for Aerobic NADH Oxidation Figure 14.14 24

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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 26

H+ Flux Drives ATP Synthesis: Oxidative Phophorylation 27

Anaerobic Respiration

Oxidized forms of nitrogen - Nitrate is successively reduced as follows: NO3– → NO2– → NO → 1/2 N2O → 1/2 N2 nitric oxide nitrous oxide nitrogen gas nitrate nitrite - 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 29

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. 30

Oxidation of inorganic compounds Lithotrophy: Oxidation of inorganic compounds

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. 32

Lithotrophy Sulfur and metal oxidation H2S → S0 → 1/2 S2O32– → H2SO4 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+ 33

Sulfuric Acid Production: Science and Science Fiction Figure 14.21 34