Inorganic nitrogen from ammonia can be converted into organic nitrogen by incorporating into the amino groups of amino acids through a series of reactions called Nitrogen Fixation.
| NH3 + a-ketoglutarate -----> glutamate |
| NH3 + glutamate -----> glutamine |
| The a-amino group on glutamate serves as the sourec of a-amino group in the synthesis of many other amino acids. |
| "Glutamate dehydrogenase" catalyzes the reductive amination of a-ketogultarate to form glutamate. This reaction only occur in certain soil bacteria. |
| In green plant and other bacteria, glutamate can be synthesized by another enzyme called "glutamate synthase" which obtain its a-amino group from glutamine. |
| Conversion of glutamate to glutamine by amidation which is catalyzed by an enzyme called " glutamine synthase". |
| Glutamine synthase is a tightly regulated enzyme because this enzyme controls the flow of ammonia nitrogen into organic amino group. |
| Glutamine synthase is a very large and complex enzyme. |
| Activity of glutamine synthase is negatively controlled by at least 8 nitrogenous compounds. Most of these compound received their amide group directly from glutamine. |
| The regulation of glutamine synthase is a perfect example of end-product inhibition or feed-back inhibition. |
| Additional mode of regulation of enzyme: covalent modification of enzyme protein molecule. |
| Adenylation and de-adenylation (of specific tyrosine residues) of glutamine synthase can cause changes in the activity of glutamine synthase.Excess nitrogen > More adenylation > Less active glutamine synthase > Less nitrogen fixation.Nitrogen limitation > Less adenylation > More active glutamine synthase > More nitrogen fixation (ammonia + glutamate --> glutamine). |
| The adenylation and de-adenylation reactions themselves are regulated by the nitrogen supply in the cell which is "sensed" by the cell in the form of glutamine-to-a-ketoglutarate ratio. |
Nitrogen
cycle and nitrogen fixation.
Nitrogen fixation is the reduction of atmospheric nitrogen (N2) to organic ammonia (NH4+).
Nitrogen fixation occurs only in small number of microorganisms.
Nitrogen fixation involves an enzyme complex called nitrogenase.
N2 + 8H+ + 8e- + 16 ATP ---> 2 NH3 + H2 + 16 ADP + 16 Pi
