Denitrification is reduction of nitrate (NO3-) to nitrogen gas or to organic nitrogen compounds. Two types of denitrification may be distinguished - dissimilative and assimilative.
1. Dissimilative Denitrification
The dissimilative pathway returns nitrogen atoms to the atmosphere by reducing nitrate to nitrogen gas. Several intermediates are involved:
NO3- -----> NO2- -----> NO -----> N2O -----> N2 gas
The diagnostic enzyme for denitrification is nitrate reductase. The best-studied nitrate reductase is from E. coli, although this organism only converts nitrate to nitrite and does not do the subsequent reaction steps.
E. coli will only reduce nitrate under anaerobic conditions. Recall that oxygen is the favored electron acceptor (because it has a higher reduction potential). When oxygen is not available, E. coli utilizes the next best electron acceptor available, i.e. nitrate.
Expression of the nar genes encoding nitrate reductase is tightly regulated. These genes are activated only when nitrate is present and no oxygen is present. This makes sense since there is no need to make proteins necessary to reduce nitrate if it is not present. In addition, this system ensures that the "favorite" electron acceptor (oxygen) will be utilized preferentially.
2. Major Denitrifiers:
Denitrification is done only by microorganisms, usually facultative anaerobes, and predominantly in two genera:
4. Uses in Sewage Treatment/Bioremediation - Dissimilative denitrification is desirable in sewage treatment and bioremediation. Dissimilative denitrifiers aid in converting organic nitrogen to nitrogen gas that escapes to the atmosphere. This method allows for very clean disposal of nitrogenous pollutants.
5. Individual Steps in Dissimilatory Nitrate Reduction
First step: Reduction of Nitrate to Nitrite
2HNO3 ------> 2HNO2 +4e
2NADH-H -----> 2NAD
Enzyme: Dissimilatory Nitrate Reductase
Second step: Reduction of Nitrite to Nitric oxide
2HNO2 -----> 2NO +2e
NADH-H ------> NAD
Enzyme: Dissimilatory Nitrite Reductase
Third Step: Reduction of Nitric Oxide to Nitrous Oxide
2NO ------>N2O +2e
NADH-H ----> NAD
Enzyme: Dissimilatory Nitric Oxide Reductase
Fourth Step: Reduction of Nitrous Oxide to Dinitrogen
N2O ---------> N2 +2e
NADH-H ------> NAD
Enzyme: Dissimilatory Nitrous Oxide Reductase
Measurement:
By difference calculations: (N balances).
By use of acetylene - reduction: (N2O) is a competitive inhibitor of acetylene.
Overall Reactions:
Nitrate-->Nitrite-->Nitric Oxide-->Nitrous Oxide-->Dinitrogen
As reductions are energy yielding, 24 ATPs are generated per mole of nitrate reduced.
Ecological factors:
Back to Nitrogen Cycle.
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