Microbial Metabolism

Microbial Metabolism Microbiology 274 March 22, 2007 Free energy of reactions Temperature ∆G = ∆H - T∆S Free Energy: How much energy during a reaction is available to work Entropy: Enthalpy: How much heat is lost or gained during the reaction How much randomness is lost or gained during the reaction Page 152 in your text Free energy of reactions • Exergonic reaction: A+B • Endergonic reaction: A+B ∆G°’ is negative C+D ∆G°’ is positive C+D Figure 8.5 Metabolism • Organisms need to synthesize many complex organic molecules • Synthesis from monomers to polymers these reactions are endergonic and require a source of energy ATP ATP is used as a source of energy to change endergonic reactions to exergonic reactions Energy Coupling A+B ATP C+D ADP + Pi A+B C+D enzyme Figure 8.6 Enzymes as catalysts for reactions • Enzymes are proteins • Highly specific for the reaction that they catalyze • Reduce the energy of activ ation (Ea ) of a reaction C6H12O6 + O2 CO2 + H2O Ea ∆G°’ Figure 8.14 is similar Back to ATP… ATP is constantly depleted by cells ADP + Pi = ATP ∆G = 7.3 kcal/mole In order to make more ATP, cells need to input energy… Oxidation-Reduction Reactions • Loss of Electrons = Oxidation (LEO) • Gain of Electrons = Reduction (GER) • Electron Donors and Electron A cceptors Acceptor + ne- Donor Page 153 in your text Standard Reduction Potential • Measure of the tendency of a donor to lose electrons (E’0) 2H+ + 2e- H2 NAD+ + 2H+ + 2e1 /2O2 + 2H+ + 2e- E’0 = -420 mV NADH + H2 E’0 = -320 mV H2O E’0 = +820 mV E coli Electron Transport System H+ cytochrome d cytochrome o H+ H+ periplasmic space Q FeS FAD cytoplasm + 2H+ + 1/2O2 2H + /2O2 H 2O NADH + H+ NAD+ H2O Figure 9.15 E coli Electron Transport System • Cytochrome d has a high affinity for oxygen, but does not pump hydrogen across the membrane – Active pathway when environmental oxygen is low • Cytochrome o has a low affinity for oxygen and acts as a hydrogen pump – Active pathway when environmental oxygen is high Proton Motive Force • Accumulation of protons on one side of a biological membrane creates an electrochemical imbalance = potential energy H H+ H+ H+ H+ H+ H+ H+ H+ + + H+ H + H + + H H H + + H+ H+ + H+ H H+ H+ H H H+ + + + H H H + H H+ H+ H+ + + + + H + + + H H + + + + H H H H H H H H + H+ H+ H+ H + H+ H+ H+ H + H+ H+ H+ Proton motive force drives ATP Synthase H H+ H+ H+ H+ H+ H+ H+ H+ + + H+ H + H + + H H H H+ + H+ H+ + H+ H H+ H+ H H+ + + + H H H + H H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ + H+ H + ATP Synthase H + H+ H+ H+ H+ H + H+ ATP ADP H+ H+ = Oxidative Phosphorylation ATP Yield from the Aerobic Oxidation of Glucose by Eukaryotic Cells • Glycolytic Pathway – Substrate-level phosphorylation (ATP) – Oxidative phosphorylation (NADH x 2) ATP 4-6 ATP • Pyruvate to Acetyl CoA – Oxidative phosphorylation (NADH x 2) • Tricarboxylic Acid Cycle – Substrate-level phosphorylation (ATP) – Oxidative phosphorylation (NADH x 6) – Oxidative phosphorylation (FADH2 x 2) Total Aerobic Yield ATP ATP 18 ATP ATP 36-38 ATP Protein Catabolism • Bacteria can secrete proteases to digest proteins to amino acids • Amino acids are converted to pyruvate, acetyl CoA or a TCA cycle intermediate… Synthesis of NADH and FADH2 and subsequent oxidative phosphorylation Lipid Catabolism Fats lipases Glycerol Dihydroxyacetone phosphate Fatty acids β-oxidation cycle Acetyl CoA, NADH, FADH2 Glycolysis (continued) Glyceraldehyde 3- P Pi Dihydroxyacetone- P NAD+ dehydrogenase NADH + H+ 1,3-Bisphosphoglycerate ADP ATP phosphoglycerate kinase 3-Phosphoclycerate phosphoglycerate mutase 2-Phosphoglycerate Anaerobic Respiration • Use of electron acceptors other than oxygen • Examples: – Nitrate (NO3-) – Sulfate (S042-) – Carbon Dioxide (C02) Figure 8-7 Denitrification e.g Pseudomonas sp Nitrate (NO3-) Nitrite (NO2-) Nitrate (NO3-) Nitrite (NO2-) (N20) dinitrogen gas (N2) Ammonia (NH3) nitrous oxide Terminal oxidase = Nitrate reductase synthesis of this enzyme is turned on when oxygen is absent, inhibited when oxygen present Take-Home Message • Organisms require ATP to grow • ATP is synthesized as a result of the catabolism of a variety of nutritional sources, through a variety of metabolic pathways • ATP synthesis is linked to the transfer of electrons (energy) from molecules of high redox potential to molecules of low redox potential • The presence of an efficient electron acceptor allows increased ATP synthesis Methanogens Obligate anaerobic archaeobacteria reduce CO2 methane CO2 + H + H = CH4 + H2O Methanogens often live in the same environment as H2 producing bacteria (fermentation) and will use it as it is produced