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Microbial Bioenergetics

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Microbial Bioenergetics A bit of chemistry Contact Details Free energy change Gibb's Free Energy change is a concept which allows to predict if Dr Alexander Galkin a reaction is thermodynamically favorable A + B ↔ C ∆G<0 – spontaneous reaction →, EXErgonic Microbial energy metabolism Email: [email protected] ∆G>0 – no reaction, ENDErgonic 210BBC210 Office: MBC Room 01.442 A + B A + B Tel: (028) 90972166 CC Free energy change Adenosine triphosphate (ATP) Redox reactions (reduction-oxidation reactions) ∆Go = change of free energy of reaction at standard state conditions at ATP is regarded as a universal source of energy 1M concentration of reactants. But in reality concentrations may vary! occurring in all cell types. In animals it is produced during the degradation of foodstuff. A + B ↔ C ∆Go>0 – no reaction if we mix A, B and C at concentrations of 1M ([A]=[B]=[C]=1M) → ATP ADP + P i + energy ]C[ × ∆ G 0 +∆=∆ lnG However, if [A] [B] >>[C], real G<0 and × ]B[]A[ reaction will go from left to right → A + B → A + B Midpoint redox potential ( Eo) is a tendency In particular case of standard 0 1 0 red ox ox red G +∆=∆ lnG +∆= 0G of A to donate electrons conditions ∆G= ∆Go because × --------------------------- red 1 1 → - Ared Aox + 2e -→ Electrons transferred from A to B if Box + 2e Bred red ox Eo < Eo Enzymes accelerate the attainment of equilibrium, but not shift it or reverse reaction. Aox /A red Box /B red ∆ Direction of the reaction is defined by G. Some of the biological reactions have Burning (oxidation) of Cells can store energy during ATP can be used to drive any ∆Go>0, but due to the concentration component (in logarithm) ∆G<0. glucose to CO 2 and H 2O glucose oxidation can in form cellular processes produces heat only of ATP 1 Photosynthesis and Respiration Reduction of oxygen Oxidative phosphorylation (energy conversion) History +hν ↔↔↔ ↔↔↔ CO 2 + H 2O {CH 2O} + O2 {CH 2O}+ O2 CO 2 + H 2O + energy Photosynthetic organisms Animals or microorganisms W. A. Engelhardt, 1936-39 - measured inorganic and organic phosphate content definition of oxidative phosphorylation Capture of solar energy to use it for reduction Oxidation of food to obtain energy of carbon compounds Warburg vs Thunberg and Keilin - respiratory enzyme vs dehydrogenase Albert Lehninger – 1948 – mitochondria are the site of energy metabolism Animals David Green – 50s, isolation and reconstitution of electron transport chain - + ↔ Piter Mitchell – energy transduction in membranes Nobel Prize 1978 O2 +4e +4H 2H 2O Plants Mitochondria – respiring organelle Membranes of mitochondria and bacteria What are mitochondria? • An intracellular organelle. • There are 100 to 1000s of mitochondria/cell. • All mitochondria come from the mother. • Mitochondria have their own DNA. • Major functions of mitochondria: – Makes energy in the form of ATP. Also contain its own DNA and its own Endosymbiotic theory transcription/translational system 2 Glycolysis Mitochondrial DNA Locations Double stranded, circular Bacteria • No introns, 80 - 93% coding gene • No repeats • Glycolysis • Lack histone and DNA repair mechanism ⇒ – Cytoplasm Cytoplasm Cytoplasm damage, mutations (free radicals) • 37 gene: 22 tRNA, 2 rRNA & 13 protein • Krebs’ TCA – Mitochondrial matrix • Oxidative phosphorylation NUCLEOID A dynamic complex that consists of Plasma membrane several copies of mitochondrial DNA and – Inner mitochondrial membrane key maintenance proteins within the organelle. POLYCISTRONIC A form of gene organization that results ⇒⇒⇒ Compartmentalisation in transcription of an mRNA that codes for multiple gene products, each of which is independently translated from the Mitochondrial matrix mRNA. Electron transferring coenzyme Krebs cycle (TCA cycle) After glycolysis and TCA cycle 2 Carbon Not much ATP formed Lots of reduced coenzymes 4 Carbon 6 Carbon Per glucose molecule: 10 NADH 2 FADH 2 (!!!) At the same time: NAD + + 2e - +H + ↔ NADH Reoxidation of NADH releases energy Requires oxygen as oxidant This energy can be used for ATP synthesis Mitochondrial matrix 3 Electron transport chain and ATP Redox centres synthase Flavin Respiratory chain couples processes of Iron- sulphur centres (FeS-centres) oxidation and ATP synthesis Ubiquinone H+ Cytochromes in Inner membrane H+ out Flavin FeS clusters Ubiquinone 2 electrons + 2H + Complex I Complex I Complex II = succinate dehydrogenase from Krebs cycle Complex II = succinate dehydrogenase from Krebs cycle Membrane mobile redox carrier linking Complexes I and II with Complex III Complex III = bc 1 complex Proton-translocating Q-cycle in complex III Different n for different species ( n=6-10) Menaquinone and rhodoquinone in some bacteria and plastoquinone in chloroplasts - + ↔ FAD + 2e +2H FADH 2 Usually serves as intermediate of electron Always transfer one electron at the time transfer between 2e - donor and 1e- acceptor Electron is delocalised No free flavins in a cell !!! 4 Cytochromes Respiratory chain cytochromes Reduced-oxidised spectra of Cytochrome c Reduced-oxidised spectra -------- a ------- b ------- c mitochondrial membranes Hugo Theorell Nobel prize 1955 Otto Warburg Nobel Prize 1931 Redox enzymes and biological Nature and mode of action of the oxidation Cyt c (Fe 3+ ) + e - ↔ Cyt c (Fe 2+ ) respiratory enzymes Protein part and haem part containing Fe ion Cytochromes a, b, c … + number As separate proteins (e.g. cytochrome c) or as subunits of enzymatic complexes (Complex II, III, IV) Easy to observe reduction/oxidation as change in optical spectra Electron transport chain Electron transport chain Structure of respiratory chain Values of redox potential of respiratory chain components II I III IV V fall downstream from NADH to oxygen. bc 1 complex NADH:ubiquinone Succinate oxidoreductase Cytochrome c dehydrogenase oxidase ATP synthase 5 Complex I (NADH:ubiquinone oxidoreductase) + + ↔ NADH + H + 4H in + UQ ↔ + + NAD + UQH 2 + 4H out Mammalian enzyme : 45 subunits Bacterial enzyme : 14 subunits Flavin = FMN 8 FeS clusters Tightly-bound semiquinones as intermediates of electron transfer Classical inhibitors : Rotenone, piericidine, MPP + 6 ATP synthase ATP synthase ATP-synthase Nobel prize 1997 John Walker Bacterial enzyme : 8 subunits Proton flow through Fo part is coupled with ATP synthesis in the F 1 part Yoshida Lab http://www.res.titech.ac.jp/~seibutu/ + ~10H per 3 molecules of ATP Kinosita Lab http://www.k2.phys.waseda.ac.jp/Movies.html ATP synthesis in mitochondria ATP transport ATP synthesis (in mitochondria!) Matrix Intermembrane ADP Respiration space ATP synthesis in mitochondrial matrix ATP Electron transport Needs to be transported out of H+ chain: Matrix Generates ∆p mitochondria Requires ATP-ADP transporter ATP-ADP transporter Integral membrane protein H+ ATP synthase: Consumes ∆p ATP and ADP transport coupled Inter-membrane space ADP Energy requiring ATP processes in the cell 7 Mitochondrial respiration How to study electron transport chain? Oxidative phosphorylation inhibitors History: Isolated mitochondria + substrates + oxygen • I – Rotenone – Ubiquinone-like structure Some compounds block oxygen consumption – • II – Oxaloacetate respiration inhibitors – Succinate-like structure Some compounds stop ATP synthesis but not respiration, • III - Antimycin A they break the link between respiration and ATP – Fungicide and insecticide - - synthesis – uncouplers • IV - Cyanide (CN ), azide (N 3 ), carbon monoxide (CO), nitric oxide (NO) – Similar electronic structures to O 2 Protonmotive force Mitochondrial respiration Oxidative phosphorylation Respiratory control ratio H+/2e - stoichiometry of respiratory chain complexes H+/ATP stoichiometry of ATP synthase ADP/O ratio – how much ADP can be converted to ATP per molecule of oxygen Reversibility of reactions = reverse electron transfer Reactive oxygen species generation 8 Aerobic respiration Aerobic respiration Aerobic respiration (in mammals!) Glucose (C ) Inner mitochondrial membrane ~1400m 2 →→→ 6 + 2 ADP C6H12 O6 + 6O 2 6CO 2 + 6H 2O 4 NAD Glycolysis We consume ~380 l of oxygen per day 10 NADH ~30 ADP ATP ADP 4 NADH + 2 H + 2 ATP e- ATP turnover ~60 kg/day Carbon dioxide: 2 CO 2 →→→ 2 Ac CoA (2 C 2) e- 3 × 10 23 H+ per second through ATP Released in Pyruvate AcCoA 2 FADH 2 UQ synthase and Krebs’ TCA 2 FAD e- 4 CO 2 Oxygen: 90% ATP is synthesised during oxidative 2 FADH 2 Required at the end of Krebs’ Cyt c phosphorylation TCA oxidative phosphorylation 2 GDP e- ~30 ATP 2 GTP O2 Glucose: 6 NAD + Oxidative phosphorylation Chemical energy from food 6 NADH + 6 H + Bacterial energy metabolism Membranes of mitochondria and bacteria What to do if food is scarce? Live in various environment Able metabolise different substrates Can adopt to the changing environment Some bacteria can grow on low carbon substrates like methanol as its sole source of energy. In P. denitrificans methanol is oxidised in periplasm and electrons are transferred directly to cytochrome oxidase 9 Simplification Bacterial energy metabolism Bypassing Complex I = NADH dehydrogenase Type 1 bc 1 (III) and cytochrome c oxidase (IV) bo 3 oxidase (quinol oxidase) (eukaryotes) together (in mitochondria ) (E.coli, Burkholderia phymatum) NADH dehydrogenase Type 2 In many bacteria efficiency of respiration (ATP:O ratio) is (E.coli, Rhodobacter capsulatus, etc) P lower than in mitochondria QH 2 More simple machinery of H +/e - transport Q Bypassing Q N Shortening or branching of the chain QH 2 Proton translocation No proton translocation (not even a membrane 2QH + O + 4H + ↔ 2Q + 2H O + 4H + protein) 2 2 in 2 out Direct oxidation of quinol
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