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Summary Metabolic diversity - different electron donors
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- Rijksuniversiteit Groningen (RuG)
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HC8 Metabolic diversity II –Different electron donors (BOOK)
Chapter 14.7 – 14.11
CH14 Metabolic Diversity of Microorganisms
14.7 Principles of Respiration
In all forms of respiration, low-potential electron donors (more electronegative) are oxidised and the
resulting electrons are driven through ETCs by their affinity for a high-potential electron acceptor
(more electropositive).
Respiration can be understood as the coupling of two redox half reactions. The more electronegative
half reaction will proceed as oxidation and the more electropositive half reaction as a reduction.
These reactions can support growth provided that sufficient energy is released for the production of
ATP.
Anaerobic respirations are those that have electron acceptors other than oxygen. These do differ
from fermentations because fermentations do not require an external electron acceptor and they
generate ATP as a result of substrate-level phosphorylation rather than by harnessing an proton
motive force.
However, aerobic respiration is more favourable because more energy is available due to the fact
that the O2/H2O couple is the most electropositive.
Biosynthesis reactions require both ATP and reducing power. Reducing power in the cell is typically in
the form of NADH. Reducing power is used to reduce inorganic compounds so that they can be used
as sources in new cell material.
Assimilative and dissimilative
Assimilative and dissimilative metabolisms differ markedly. In assimilative metabolism, energy is
consumed and only enough of the inorganic compound is reduced to satisfy the needs for
biosynthesis. In dissimilative metabolism, energy is conserved, a large amount of the electron
acceptor is reduced and the reduce product remains a small molecule which is then excreted from
the cell.
14.8 Hydrogen (H2) oxidation
A few chemotlithotrophs lack autotrophic capacities and grow as mixotrophs, meaning that they use
their inorganic electron donor for energy conservation but assimilate organic carbon as their carbon
source.
Hydrogenase catalyzes the reaction from oxygen and hydrogen to water.
Some bacteria have two types of hydrogenases, one membrane integrated and one cytoplasmic
(instead of binding H2 for use as an electron donor, it catalyzes the reduction of NAD+ to NADH).
Aerobic hydrogen bacteria must have a backup metabolism to H2 oxidation, an in nature they likely
shift between chemoorganotrophic and chemolithotrophic lifestyles as nutrients in their habitats
allow.
, 14.9 Oxidation of Sulfur Compounds
The most common sulfur compounds used as electron donors are hydrogen sulfide (H2S) elemental
sulfur (S0) and thiosulfate (S2O32-). However in most cases the final oxidation product is sulfate (SO 42-).
There are diverse pathways for conserving energy from the oxidation of sulfur compounds. One is
the sox system. There are four key proteins in the sox system, all present in the periplasm:
- SoxXA
- SoxYZ
- SoxB
- SoxCD
It begins when SoxXA forms a heterodisulfide bond between the sulfur compound to be oxidised and
the carrier protein, SoxYZ. The sulfur compound remains bound to the carrier thourghout the
pathway and being ultimately released by SoxB. SoxCD mediates the removal of 6 electrons from the
sulfur compound bound to the carrier. Electrons from the system are funnelled into the ETC while
protons are released to acidify the external environment.
Some sulfur-oxidizing microbes that store sulfur granules also use components of the Sox system but
lack the key enzyme SoxCD which means that the sulfur atom bound to SoxYZ is added to a growing
sulfur granule in the periplasm. The sulfur in the granule can be reductively activated and
transported to the cytoplasm where it is eventually oxidised to sulfite (SO 32-) by the reverse activity of
DsrAB.
Sulfite reducties can then oxidise sulfite and transfer the electrons to the ETC.
14.10 Iron (Fe2+) oxidation
The aerobic oxidation of ferrous iron (Fe2+) to ferric iron (Fe3+) supports growth of
chemolithotrophic iron bacteria. However, at acidic pH only a small amount of energy is available
from this reaction.
Because the reduction potential of the Fe2+/Fe3+ couple is so high, steps in electron transport to
oxygen can obviously be few. Iron oxidation begins in the outer membrane where the organism
contacts ferrous iron which is than oxidised to ferric iron which is a one electron transition. This
reaction is thought to be pulled forward by the consumption of ferric iron in Fe(OH)3 formation.
The nature of the proton motive force in highly acidic environments is interesting because a large
gradient of protons already exists across the cytoplasmic membrane. However, the organism cannot
make ATP from this preformed proton motive force in the absence of an electron donor. This is
because H+ ions that enter the cytoplasm via ATPase must be consumed in order to maintain the
internal pH within acceptable limits.
Autotrophy is supported by the Calvin cycle and because the high potential of the electron donor,
much energy must be consumed in revere electron flow reactions to obtain the reducing power
(NADH) to drive the CO2 fixation.
Ferrous iron can also be oxidezed under anoxic conditions in which Fe2+ is used either as an electron
donor in energy metabolism and/or as a reductant for CO2 fixation.
14.11 Nitrification
Nitrification is the process in which nitrifying bacteria oxidize ammonia and nitrite. Nitrification
consists of two different sets of reactions:
1. Catalyse oxidation of ammonia to nitrite
2. Catalyse oxidation of nitrite to nitrate
Most nitrifying microbes are only capable of catalysing one of the two reactions mentioned above.
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