Biological Nitrogen Fixation

nitrogen fixation is the conversion of nitrogen (nitrogen to ammonia ) the ability to fix nitrogen is limited to some prokaryotes. all naturally occurring fixers (diazotrophs) switch to fixation only when reduced nitrogen levels falls.

The nitrogen fixer are either free living organism or fix in symbiotic association with plants. some of them are also photosynthetic like plant (oxygenic) while others can fix carbon by anoxygenic photosynthesis.

these organisms range from obligate anaerobes to obligate aerobes .a few example of nitrogen fixing organisms are given  :-    

 obligate aerobes               -azotobacter vinelandii

 obligate anaerobes           -clostridium pasteuianum

 facultative anaerobes       -klebsiella pneumoniae

 microaerobes                    -Azospirillum

 photosynthetic/aerobic     -nostoc

 photosynthetis/anaerobic -rhodopseudomonas

free living and symbiotic nitrogen fixation in :-

(a)heterocyst of nostoc (enlarged cell);

(b)root nodule 

(c)stem nodules of leguminous plant .

biological nitrogen fixation is catalysed by nitrogenase .it is a metaalloenzyme which is a reversible complex of -protein( nitrogenase reductase) and fe- mo protein(denitrogenate ).the fe-protein is a homodimer that binds a single cluster via pair of cysteine residues from each subunit .it is also has two ATP binding and hydrolyzing sites.

The fe mo protein is a tetramer of two a and b-subunits. Each a -subunit  has a deeply embedded FE MO cofactor (femoco) which is formed from 4fe:3S,MO 3fe:3S and homocitrate while each B subunit has two bridged 4fe:3S cluster (p cluster ) that can be consider as an 8fe:8S cluster.

The enzyme nitrogenase has low turnover .it reduces only3 molecules of nitrogen gas/sec to compensate for its inefficiency ,nitrogen fixing cells express more nitrogenase amounting to almost 5% of the cellular proteins the enzymatic activities of nitrogenase include nitrogen reduction ,ATP dependent hydrogenase and reductant dependent ATPase . The ionic reaction for nitrogen and accompanying proton reduction is:

Thereaction consumes 2ATP/e- transferred from the fe -option to the fe-mo protein. ATP is required for kinetic rather than thermodynamic reasons. nitrogenase also reduce a number of triple bonded compounds  such as azide, acetylene and cyanide. the immediate source of reductant to fe protein in most cases is ferredoxin or flavodoxin and ATP is generated by oxidative or substrate level phosphorylation depending on the organism.

nitrogenase from all organism is characterized  as an oxygen labile some organism have not developed specialized strategies for protection ;they fix nitrogen only under anaerobic condition on the other hand many nitrogen fixing organism express protein which help in reducing the levels of free oxygen such as leghemoglobin ,uptake hydrogenase, branched electron transport chain, etc

nitrate assimilation

nitrate assimilation is a two step reduction of nitrate to ammonia. it is the major route by which green plant, algae and many microorganism acquire reduced nitrogen .the reduction is catalyzed sequentially by nitrate and nitrite reductases. we shall now describe the distribution ,structure ,cofactor requirements and reaction  catalyzed by these reductases.

nitrate reduction(NR) is either pyridine nucleotide or ferredoxin dependent enzyme. it catalyzed a  two electron reduction:

The pyridine nucleotide (NADH) dependent enzyme is present in the cytosol of eukaryotes. it is a dimeric enzyme which has multiple redox carries bound to it including molybdopterin. it is a rare example of a molybdenum requiring enzyme from eukaryotes. the molybdenum cofactor is necessary both for the assembly of NR subunit and enzyme activity. nitrate reduction in higher plant occurs in leaves, root, embryo aleurone cells and pollen cell. the flow of electrons from NADH to molybdopterin is as follows: