Lecture 21

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Lecture 21
2010-11-14 16:31:04

Bacterial Metabolic Diversity -1
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  1. Chemolitotroph
    • Energy and electrons from the oxidation of inorganic compounds
    • Most chemolitotrophs are autotrophs (carbon from fixation of CO2)
    • Can be aerobic and use O2, or anaerobic using NO3-, SO42-, etc.
  2. Acidithiobacillus ferrooxidans
    • Aerobic oxidation of ferrous iron (Fe2+) to ferric iron (Fe3+)
    • Lives in acidic conditions (pH 2-3)
    • Ferric iron froms insoluble ferric hydroxide [Fe(OH)3] in water; red-brown color
    • Ferrous iron as an electron donor- Fe2+ to Fe3+ is an oxidation
  3. ATP Production in A. ferrooxidans
    • 1. Rusticyanin (periplasmic, copper-containing protein) oxidizes Fe2+ to Fe3+ in a single electron transition
    • 2. Rysticyanin then transfers the electron to cytochrome c, which then transfers the electron to cytochrome a
    • 3. Cytochrome a then passes the electron to O2, and at that step, protons are pumped out of the cytoplasm
  4. Anaerobic Respiration
    • Respiration using electron acceptor other than O2
    • Membrane-associated electron transport chain similar to that of aerobes
  5. Anaerobic Respiration in Escherichia coli
    • 1. Nitrate is just one of several inorganic nitrogen compmounds used by bacteria in anaerobic respiration
    • 2. Nitrate reductase is the enzyme that catalyzes the first step
    • - dual control of nitrate reductase (absence of oxygen and presence of NO3+)
    • 3. Only two proton-transporting steps occur during nitrate reduction, compared to three for oxygen respiration
  6. Nitrogen Fixation
    • The use of N2 as a source of cell nitrogen (for amino acids, nucleotides, etc.)
    • Fixation of N2 (gas from air) into ammonia (NH3)
    • This form of metabolism is strictly bacterial (no eukaryotes)
  7. Mechanism of Bacterial Nitrogen Fixation
    N2 + 8H+ + 8e- + 16-24 ATP → 2NH3 + H2 + 16-24 ADP + 16-24 Pi
  8. Protection of Enzyme from Oxygen
    • 1. Rapid removal of oxygen by respiration
    • 2. Formation of a slime layer that functions as a diffusion barrier for oxygen (Azotobacter)
    • 3. Compartmentalization of nitrogenase in specialized cells, heterocysts, in some cyanobacteria.
  9. nif Regulon
    • Regulon: composed of many operons or genes
    • nifD and nifK- code for the two subunits of dinitrogenase
    • nifH- cod for dinitrogenase reductase
    • NifL- oxygen sensing transcriptional regulator that represses nif gene expression in presence of oxygen
    • NtrC- transcriptional activator of NifA (active when levels of ammonia are low)
    • NifA- activates transcription of nif genes