Amino Acid Metabolism By Dr. Mohammad

Gaseous nitrogen is chemically unreactive due to strong triple bond.
To reduce nitrogen gas to ammonia takes a strong enzyme --> reaction is called nitrogen fixation.
Only a few organisms are capable of fixing nitrogen and assembling amino acids from that.
Higher organisms cannot form NH4+ from atmospheric N2.
Bacteria and blue-green algae (photosynthetic procaryotes) can because they possess nitrogenase.
Enzyme has two subunits:
1) strong reductase - has Fe-S cluster that supplies e- to second subunit
2) two re-dox centers, one of which is a nitrogenase
Composed of iron and molybdenum that reduces N2 to NH4+
Reaction is ATP-dependent, but unstable in the presence of oxygen.
Enzyme is present in Rhizobium, symbiotic bacterium in roots of legumes (i.e. soybeans)
Nodules are pink inside due to presence of leghemoglobin (legume hemoglobin) that binds to oxygen to keep environment around enzyme low in oxygen (nitrogen fixation requires the absence of oxygen)
Plants and microorganisms can obtain NH3 by reducing nitrate (NO3-) and nitrite (NO2-) --> used to make amino acids, nucleotides, phospholipids.