NITROGEN FIXATION

       N2 is quite stable (The N≡N has a bond energy of 945 kJ mol−1, versus 351 kJ mol−1 for C−O bond).

       Requires a substantial amount of energy to fix N2 into NH3.

       Can be done by a few strains of bacteria diazotrophs (such as certain marine cyanobacteria and bacteria that is symbiotic in the roots of legumes).

       Diazotrophs contain nitrogenase that catalyzes:

N2 + 8H+ + 8e + 16 ATP + 16 H2O 2 NH3 + H2 + 16 ADP + 16 Pi

 

NITROGENASE

A multi-subunit complex of two proteins (see Fig.20-41)

1. A Fe-protein homodimer g2 containing one [4Fe-4S] cluster and 2 ATP binding sites.

2. A MoFe-protein α2β2 hetero-tetramer containing Mo-Fe cofactor and a P-cluster (consisting of two iron-sulfur clusters).

 

Mechanism of Nitrogenase

       The required electrons are derived from photosynthesis or oxidative electron transport.

       See Fig. 20-43 for electron transfer from Fe-protein to MoFe-protein to N2, and ATP utilization.

       2 ATP are utilized for the transfer of one electron.

       2 ATP bind to reduced Fe-protein and are hydrolyzed as the electron is transferred to MoFe-protein.

       The ATP hydrolysis causes a conformational change of Fe-protein, altering its E from -0.29 to -0.40 V. This enables the subsequent reduction of N2 to 2NH3 (E = -0.34 V for N2 + 6H+ + 6e 2 NH3)

       This occurs in three steps of 2e-reduction:

NN + 2H+ + 2e HN=NH (Diimine)

HN=NH + 2H+ + 2e H2NNH2 (Hydrazine)

H2NNH2 + 2H+ + 2e 2 NH3

       The step-wise reduction of N2 occurs on the MoFe-protein.

       However, for every N2 reduced to 2 NH3, at least one additional N2 goes through a 2e-futile cycle via diimine:

N2 + 2H+ + 2e HN=NH

HN=NH + H2 N2 + 2 H2

       Therefore, the overall reaction for the net formation of 2 NH3 from one N2 is:

N2 + 8H+ + 8e + 16 ATP + 16 H2O 2 NH3 + H2 + 16 ADP + 16 Pi

 

WHAT HAPPENS TO FIXED NH3?

1. Key Role of Glutamine Synthetase

       Glutamine synthetase catalyzes the formation of glutamine from ammonia, ATP, and glutamate.

Glutamate + ATP + NH4+ Glutamine + ADP + Pi

       Glutamine is a major amino group donor and a storage form of NH3.

       Glutamine synthetase is a key to the control of nitrogen metabolism, and is sensitive to activity regulation.

       Mammalian enzyme is activated by αketoglutarate, the product of glutamate oxidative deamination (see below). The activation will prevent the accumulation of NH3.

       Bacterial enzyme consists of 12 identical subunits arranged in two layers of hexamer.

       Sensitive to allosteric inhibitions by (1) histidine, tryptophan, carbamoyl phosphate, AMP, and CTP (all end products derived from glutamine) and (2) alanine, serine, and glycine (markers for cellular nitrogen level).

       E. coil glutamine synthetase is sensitive to covalent modification by adenylylation of a tyrosine residue involving a regulatory protein PH, adenylyltransferase, uridylyltransferase, and uridylyl-removing enzyme. (See Fig. 20-29).

2. Other enzymatic reactions

Including mitochondrial glutamate dehydrogenase (catalyzes a reversible reaction) and carbamoyl phosphate synthetase I, etc.

Glu + NAD+ (or NADP+) + H2O NH4+ + a-ketoglutarate + NAD(P)H + H+

2ATP + HCO3 + NH3 Carbamoyl phosdphate + 2ADP + Pi

 

NITROGEN CYCLE

 

Describes the interconversion of nitrogen in the biosphere.

 

 

Ammonification

By plants, fungi, and many bacteria.

Nitrate Reductase: NO3 + 2 H+ + 2 e NO2 + H2O

Nitrite Reductase: NO2 + 7 H+ + 6 e NH3 + 2 H2O

 

Anammox

In some newly discovered strains of anaerobic bacteria, which contain anammoxosome.

NH4+ + NO2 N2 + 2 H2O