Tema 4. Electron Transport. Cap. 4 pages

Transcription

1 Tema 4. Electron Transport Cap. 4 pages

2 The generation of energy for growth-related physiological processes in respiring prokaryotes is by coupling the flow of electrons in membranes to the creation of an electrochemical proton gradient. OUT IN A red # protons extruded -e A ox -n Eh = y p # e transfered yh + Coupling site yh + -e B ox B red

3 Who is responsible for the movement of the electrons through the membrane? The electron carriers are either proteins or lipids! 1) Flavoproteins which carry a hydrogen and an electron 2) Quinones are lipids which carry a hydrogen and an electron 3) Iron-sulfur proteins which carry only electrons 4) Cytochromes which carry only electrons The electrons are not carried in the protein, but in a non-protein portion called Prosthetic group

4 Prosthetic groups 1) Flavoproteins is a flavin, either flavin adenine dinucleotide (FAD) or flavin mononucleotide (FMN) 2) Iron-sulfur proteins is a cluster of iron-sulfur (FeS) 3) Cytochromes is heme.

5 Flavoproteins (Fp) Flavin comes from the Latin flavius, which means yellow. FMN The flavins (FAD, FMN) are synthesized by cells from the vitamin riboflavin (B 2 ). FMN

6 Quinones UQ coenzyme Q Their hydrophobic lipid nature allow them to transfer electrons to carries that are not mobile. n = 6 to 10 The isoprenoid side chains contribute the hydrophobicity. Menaquinones are derivatives of vitamin K and differ from UQ in being a naphthoquinone They also have a lower E h and are used predominately during anaerobic respiration menaquinone

7 Iron-sulfur proteins They contain non-heme iron and Usually acid-labile sulfur. These proteins cover a wide range of potentials -400mV to +350 mv. Example: ferredoxin

8 Cytochromes Cytochrome b Different types of heme: a, b, c, d, and o. The iron is the electron carrier and is oxidized to ferric or reduced to ferrous ion during e- transport. D and o have only been found in prokaryotic cytochrome oxidases They can usually be identified by spectrophotometry.

9 In Mitochondria Organization of the electron carriers Coupling sites Proton extrusion E o -320 mv E o mv NADH fp FeS UQ b c 1 c aa 3 O 2 Complex I Complex III Complex IV FeS succinate fp Complex II NADH dehydrogenase Ubiquinol-cytochrome c oxidoreductase Succinate dehydrogenase Cytochrome aa 3 oxidase

10 In bacteria Aerobic respiration AH 2 dehydrogenase quinone bc 1 c aa 3 O 2 b This creates flexibility, oxidases with different affinity to oxygen, they could differ in p. o Adaptability to environmental conditions: E. coli can use a reductase or an oxidase depending on oxygen availability. Anaerobic respiration AH 2 dehydrogenase quinone reductase Y

11 How to identify a coupling site? E o -320 mv E o mv NADH fp FeS UQ b c 1 c aa 3 O 2 Complex I FeS Complex III Complex IV succinate fp Complex II The # of ATP s made for every 2e- transfer to oxygen is called the P/O ratio. It is equal to the # of ATP formed per oxygen consumed. You could use P/2e- when the e- acceptor is not O 2 NADH O 2 P/O ratio = 3 succinate O 2 P/O ratio = 2 Each coupling site is characterized by a drop in midpoint potential of about 200 mv.

12 The Q loop Out In Site 1 2H + fp FeS 2 X [e-] NADH + H + NAD + Note that the electron carriers alternate between those that carry both H 2 and e- and those that only carry e-. QH 2 Q 2H + Site 2 2H + 2 X [e-] Cytochrome Site 3 ½ O 2 + 2H + oxidase H 2 O 2H + 2H +

13 Q cycle ( explains observations of e- transfer in mitochondria, chloroplasts, and many bacteria) Out 2 H + UQH 2 O UQ - UQ 2 UQH 2 1eb L b H 1e- P site N site UQH 2 UQ - UQ In 2 H + P site can by inhibited by myxothiazol and stigmatellin N site is inhibited by antimycin.

14 E. coli O 2 NO 3 fumarate NADH dehydrogenase NDH1 NADH dehydrogenase NDH2 MQ 3% 30% 74% UQ 60% 10% DMQ 37% 70% 16% b 562 o b 558 d High affinity to O 2 O 2 NDH1 and bo 8H + NDH2 and bd 2H +

15 Paracoccus denitrificans Non-fermenting G-, facultative anaerobe cyt, bb 3 NADH dehydrogenase NDH1 UQ bc 1 c aa 3 O 2 methanol dehydrogenase Nitrate reductase NO 3 Nitrite reductase NO 2 Nitric oxide reductase Nitrous oxide reductase NO N 2 O ½ N 2

16 Wolinella succinogenes G-, anaerobe isolated from the rumen. Out IN H 2 Ni FeS cty b fumarate 2 H + + 2H + MK cty b FeS FAD HCO 2 succinate MO MO FeS cty b H + + CO 2