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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.
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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
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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
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Anaerobic Respiration
- Respiration using electron acceptor other than O2
- Membrane-associated electron transport chain similar to that of aerobes
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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
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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)
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Mechanism of Bacterial Nitrogen Fixation
N2 + 8H+ + 8e- + 16-24 ATP → 2NH3 + H2 + 16-24 ADP + 16-24 Pi
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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.
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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
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