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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