Illinois State University
BSC 260
2/3/12
Metabolism 2
Aerobic Respiration
Oxidation using O2 as the terminal electron acceptor
Higher ATP yield than fermentations
ATP produced at the expense of the proton motive force, which is generated by electron transport
Respiration and Electron Carriers
Electron Transport Systems
Membrane associated
Mediate transfer of electrons
Conserve some of the energy released during transfer and use it to synthesize ATP
Many oxidation–reduction enzymes are involved in electron transport
NADH dehydrogenases: proteins bound to inside surface of cytoplasmic membrane; active site binds NADH and accepts 2 electrons and 2 protons that are passed to flavoproteins
Flavoproteins: contains flavin prosthetic group (e.g., FMN, FAD) that accepts 2 electrons and 2 protons but only donates the electrons to the next protein in the chain
Cytochromes
Proteins that contain heme prosthetic groups
Accept and donate a single electron via the iron atom in heme
Iron–Sulfur Proteins
Contain clusters of iron and sulfur
Example: ferredoxin
Reduction potentials vary depending on number and position of Fe and S atoms
Quinones
Hydrophobic non-protein-containing molecules that participate in electron transport
Accept electrons and protons but pass along electrons only
The Proton Motive Force
Membrane proteins utilize energy from electron transport to pump protons across cytoplasmic membrane establishing proton gradient
Protons originate from NADH and the dissociation of water
Results in generation of pH gradient and an electrochemical potential across the membrane (the proton motive force)
ATP synthase (ATPase): complex that converts proton motive force into ATP; two components
The Citric Acid Cycle
Citric acid cycle (CAC): pathway through which pyruvate is completely oxidized to CO2
Initial steps (glucose to pyruvate) same as glycolysis
Per glucose molecule, 6 CO2 molecules released and NADH and FADH generated
Plays a key role in catabolism and biosynthesis
Energetics advantage to aerobic respiration
The citric acid cycle generates many compounds available for biosynthetic purposes
(-Ketoglutarate and oxalacetate (OAA): precursors of several amino acids; OAA also converted to phosphoenolpyruvate, a precursor
of glucose
Succinyl-CoA: required for synthesis of cytochromes, chlorophyll, and other tetrapyrrole compounds
Acetyl-CoA: necessary for fatty acid biosynthesis
Microorganisms demonstrate a wide range of mechanisms for generating energy
Fermentation
Aerobic respiration
Anaerobic respiration
Chemolithotrophy
Phototrophy
Anaerobic Respiration
The use of electron acceptors other than oxygen
Examples include nitrate (NO3(), ferric iron (Fe3+), sulfate (SO42(), carbonate (CO32(), certain organic compounds
Less energy released compared to aerobic respiration
Dependent on electron transport, generation of a proton motive force, and ATPase activity
Chemolithotrophy
Uses inorganic chemicals as electron donors
Examples include hydrogen sulfide (H2S), hydrogen gas (H2), ferrous iron (Fe2+), ammonia (NH3)
Typically aerobic
Begins with oxidation of inorganic electron donor
Uses electron transport chain and proton motive force
Autotrophic; uses CO2 as carbon source
Phototrophy: uses light as energy source
Photophosphorylation: light-mediated ATP synthesis
Photoautotrophs: use ATP for assimilation of CO2 for biosynthesis
Photoheterotrophs: use ATP for assimilation of organic carbon for biosynthesis
V. Essentials of Anabolism
Biosynthesis of Sugars and Polysaccharides
Prokaryotic polysaccharides are synthesized from activated glucose
Adenosine diphosphoglucose (ADPG)
Precursor for glycogen biosynthesis
Uridine diphosphoglucose (UDPG)
Precursor of some glucose derivatives needed for biosynthesis of important polysaccharides
Examples: N-acetylglucosamine,
N-acetylmuramic acid
Gluconeogenesis
Synthesis of glucose from phosphoenolpyruvate
Pentoses are formed by the removal of one carbon atom from a hexose
Biosynthesis of Amino Acids
Biosynthesis of amino acids and nucleotides often involves long, multistep pathways
Amino acid biosynthesis
Carbon skeletons come from intermediates of glycolysis or citric acid cycle
Ammonia is incorporated by glutamine dehydrogenase or glutamine synthetase
Amino group transferred by transaminase and synthase
Biosynthesis of Fatty Acids
and Lipids
Fatty acids are biosynthesized two carbons at a time (Figure 4.27)
Acyl carrier protein (ACP) involved
ACP holds the growing fatty acid as it is being synthesized
In Bacteria and Eukarya, the final assembly of lipids involves addition of fatty acids to glycerol
In Archaea, lipids contain phytanyl side chains instead of fatty acids
Phytanyl biosynthesis is distinct from that of fatty acids
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