Download The Proton Motive Force

BSC 260
2/1/12
Metabolism
Metabolism-The sum total of all chemical reactions that occur in a cell
catabolism
Energy-releasing metabolic reactions
anabolism
Energy-requiring metabolic reactions
Nutrients
Supply of monomers (or precursors of) required by cells for growth
Macronutrients
Nutrients required in large amounts
Carbon
Required by all cells
Typical bacterial cell ~50% carbon (by dry weight)
Major element in all classes of macromolecules
Nitrogen
Typical bacterial cell ~12% nitrogen
Key element in proteins, nucleic acids, and many more
cell constituents
Other Macronutrients
Phosphorus, Sulfur, Potassium, Magnesium, Calcium, and
Sodium
Micronutrients
Nutrients required in trace amount
Iron
Key component of cytochromes and FeS proteins involved in
electron transport
Cells produce siderophores (iron-binding agents) to obtain
iron from insoluble mineral form
Culture Media
Nutrient solutions used to grow microbes in the laboratory
Two broad classes
Defined media: precise chemical composition is known
Complex media: composed of digests of chemically undefined
substances (e.g., yeast and meat extracts)
Selective Media
Contains compounds that selectively inhibit growth of some
microbes but not others
Differential Media
Contains an indicator, usually a dye, that detects particular chemical
reactions occurring during growth
Enzymes
Typically proteins (some RNAs)
Highly specific
Typically rely on weak bonds
Examples: hydrogen bonds, van der Waals
forces, hydrophobic interactions
Active site: region of enzyme that binds substrate
Increase the rate of chemical reactions by 108 to 1020 times the
spontaneous rate
Oxidation–Reduction and Energy-Rich Compounds
Energy from oxidation–reduction (redox) reactions is used in
synthesis of energy-rich compounds (e.g., ATP)
Redox reactions occur in pairs (two half reactions; Figure 4.8)
Electron donor: the substance oxidized in a redox reaction
Electron acceptor: the substance reduced in a redox reaction
Essentials of Catabolism
Two reaction series are linked to energy conservation in
chemoorganotrophs: fermentation and respiration (Figure 4.13)
Differ in mechanism of ATP synthesis
Fermentation: substrate-level phosphorylation; ATP directly
synthesized from an energy-rich intermediate
Respiration: oxidative phosphorylation; ATP produced from proton
motive force formed by transport of electrons
Glycolysis
Glucose consumed
Two ATPs produced
Fermentation products generated
Some harnessed by humans for consumption
Respiration and Electron Carriers
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
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
(e.g., NADH dehydrogenases, flavoproteins, iron–sulfur proteins,
cytochromes)
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
Quinones: Hydrophobic non-protein-containing molecules that
participate in electron transport
Accept electrons and protons but pass along electrons only
The Proton Motive Force
Electron transport system oriented in cytoplasmic membrane so that
electrons are separated from protons
The final carrier in the chain donates the electrons and protons to
the terminal electron acceptor
During electron transfer, several protons are released on outside of
the membrane
Results in generation of pH gradient and an electrochemical
potential across the membrane (the proton motive force)
Terminal oxidase; reduces O2 to H2O
ATP synthase (ATPase): complex that converts proton motive force into
ATP; two components
F1: multiprotein extramembrane complex, faces cytoplasm
Fo: proton-conducting intramembrane channel
Reversible; dissipates proton motive force
The Citric Acid Cycle
Citric acid cycle (CAC): pathway through which pyruvate is
completely oxidized to CO2
Per glucose molecule, 6 CO2 molecules released and NADH and
FADH generated
Several high energy electrons stored in carrier molecules
Energetics advantage to aerobic respiration
Catabolic Diversity
Microorganisms demonstrate a wide range of mechanisms for
generating energy (Figure 4.22)
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