Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download BIOL E240!
Document related concepts
no text concepts found
Transcript
BIOL E240! Catabolism and Energy flow! Overview! 1. Energy flow! 2. Energy conversion! 3. Major catabolic pathways (anaerobic and aerobic)! 4. Example of catabolic switch in bacteria: E. coli growth ! ! Prevalent role of carbon in biomolecules! Biomolecules are carbon-based molecules:! C, H, O, & N make > 95% of all living organisms! ! Carbon represents 50% of the living matter after desiccation! Prevalent role of carbon in biomolecules! Why is carbon so important?! Carbon atoms are engaged in covalent bonds with a very specific geometry. Therefore, carbon is ! a versatile building block of biomolecules.! ! The carbon tetravalence provides the means for a wide range of functional group s arrangements ! within a biomolecule.! ! The versatility of carbon in forming single and double bonds has an impact on the structure and ! the chemical activity of biomolecules.! ! Energy interconversion in living organisms! A living organism is characterized by a continuous chemical activity that leads to constant exchange of matter and energy with its surrounding.! CATABOLISM! ANABOLISM! Classification based on energy and carbon sources ! Chemotrophs derive their energy ! from the oxidation of a fuel and ! require a source of organic nutrients. ! They cannot fix CO2 into organic ! compounds. In other words they are! heterotrophs.! Lehninger, 4th edition, Freeman! Both E. coli and humans are chemoorganoheterotrophs! Energy flow and chemical recycling! There is a constant flow of matter and energy between living organisms! ! Spontaneity of biochemical reactions! Reaction Coupling! The ATP system! Energy-providing processes and ATP synthesis! The ATP-forming system is an exergonic process with electron-accepting and -donating reactions:! ! The electron-donating process is defined by a flow of electrons from a donor substrate (fuel ! molecule) to the first electron carrier (generally nicotinamide adenine nucleotide, NAD). ATP is ! synthesized during this process by substrate level phosphorylation. ! ! The electron-accepting process is defined by the flow of electrons from the first electron carrier to ! the final acceptor. During this process ATP is synthesized by electron transfer phosphorylation.! The ensemble of catabolic pathways responsible for the conversion of fuel molecules into ! ATP is called cellular respiration.! ! Species that depend on oxygen as the final electron acceptor are said to use aerobic respiration! ! Species that depend on a final electron acceptor other than oxygen are said to use anaerobic ! respiration ! Electron transfer-REDOX reactions! Aerobic respiration: Overview! Fuel Molecules! (proteins, lipids, carbohydrates)! Electron-donating process:! Phases I & II! 1° electron carrier! Electron-accepting process:! Phase III ! Final electron acceptor! Glycolysis! Substrate level phosphorylation pathway shared by aerobic organisms and many anaerobic organisms ! Glycolysis! Citric Acid Cycle! Under aerobic conditions the end product of glycolysis (pyruvate) is further oxidized in the TCA! Electron Transport Chain! In eukaryotes, the mitochondrial inner membrane contains the ETC. The ETC components are ! found in the plasma membrane of bacteria! Energy released by the transfer of 2 electrons from ! 1 mole of NADH + H+ to O2 ΔG° = -220 kJ/mol ! The energy cost to move 10 moles of H+ across the membrane is about 200 kJ ! ! Proton motive force = Chemical potential energy + Electrical potential energy! ATP synthesis! The γ-chain rotates by 120° increments! ! Each full turn of the γ-chain:! - leads to the synthesis of 3 ATP ! - requires the transport of 10 H+ ! ATP synthesis during both aerobic and anaerobic respiration relies on an ion gradient (usually H+) ! across a membrane created by the transfer of electrons through a series of proteins and on an ! ATP synthase.! Fate of NADH under anaerobic conditions: fermentation! NADH is converted back to NAD+ by a process called fermentation.! The fermentative pathways can be either linear of branched. ! Branched fermentative pathway! The branched pathway produces more ATP and more oxidized products than the linear pathway.! The oxidation of a portion of the reduced electron carriers (NAD(P)H) is coupled to the reduction ! of H+ into H2 ! The relative abundance of the fermentation products and ATP is dictated by external growth conditions! Catabolic switch in E. coli! What are the consequences of an absence of oxygen on E. coli catabolism? ! - Repression of many genes coding for enzymes that catalyze the various steps of the TCA! In particular genes coding for: succinate dehydrogenase, succinyl-CoA complex, and ! α-ketoglutarate dehydrogenase. ! ! - Activation of the gene coding for fumarase ! reductase! ! - E. coli does not induce a full TCA cycle.! ! ! - A branched form of the TCA pathway is activated! - Lower production of reduced electron carriers ! (NADH).! ! - ATP must come exclusively from glycolysis! and other forms of substrate level phosphorylation ! ! Catabolic switch in E. coli! Low to very low oxygen concentration! Acetyl CoA has 2 alternative fates (example of branched fermentative pathway):! Conversion to acetate: additional ATP is produced but NADH is not consumed! Conversion to ethanol: energy production is sacrificed in favor of NADH consumption! ! By modulating the amount of ethanol and acetate secreted E. coli balances its requirement to ! regenerate NAD+ with its need for energy ! Catabolic challenges in prokaryotes! Prokaryotes face challenges to regulate their catabolism. ! - They do not store large amount of fuel molecules (carbon source). They are dependent on the ! availability and abundance of fuel molecules. ! Rapid switches between catabolic pathways depending on the carbon sources present in the ! environment ! - They are also dependent on the availability and abundance of electron acceptors from the ! environment! For example, some bacteria need to switch from an aerobic to an anaerobic respiration when ! oxygen level becomes low! ! ! - They are affected by the pH of the external environment (risk of decoupling the electron ! transport from ATP synthesis)! - They are affected by the density of the bacterial population (quorum sensing, cell ! specialization in biofilms) ! Example: the acetate switch! E. Coli culture growing in an aerated flasks with glucose as their sole source of carbon! What are the consequences of an increase of acetate in ! the environment on the bacterial population?! Wolfe, Microbiology & Mol. Biol. Review, 2005, 12-50! - Undissociated acetate from the environment is a weak ! lipophilic acid that permeates the membrane and start to ! accumulate into the bacteria. ! ! - Inside bacteria, acetate dissociates into protons and anions.! Protons acidify the cytoplasm and anions increase the ! internal osmotic pressure.! Conclusion: acetate becomes toxic.! Do bacteria respond to intracellular acetate toxicity by activating an acetate secretion pathway?! No! There is a rising energy cost associated with the active transport of acetate against its ! concentration gradient.! ! E. Coli up-regulate the synthesis of an enzyme converting acetate and ATP into acetyl-CoA and Pi! Acetate is consumed and used as a source of carbon to form Ac-CoA and thus ATP via oxidative ! phosphorylation (off-setting the energy cost associated with Ac-CoA synthesis)!
