The reduction of atmospheric nitrogen (N2) to ammonia (NH3) is called biological nitrogen fixation and it occurs in relatively few living systems. These include some free-living soil bacteria, such as Klebsiella and Azotobacter, cyanobacteria, and Rhizobium, which is a symbiont on the roots of leguminous plants. The infecting bacterium assumes a modified form, called a bacteroid, inside the cells of infected plants.

Reducing nitrogen (as occurs in nitrogen fixation), requires energy and electrons with low-potential (electron carriers with very low E0'). The enzymes involved are very sensitive to oxygen and must be studied only under anaerobic conditions. In root nodules of plants, the anaerobic environment is provided by the protein leghemoglobin, which binds any O2 that makes its way into the nodules.

The mechanisms appear quite similar among the species examined to date. In Klebsiella pneumoniae, the overall reaction is as follows:

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

The number of ATPs required is actually uncertain. The low-potential electron carriers are either ferredoxin or flavodoxin (a low potential flavoprotein). Note that hydrogen is a product of the reaction.

The enzyme system for nitrogen fixation consists of two separate proteins. Nitrogenase (also called component I or molybdenum-iron protein) catalyzes the reduction of N2. The other, nitrogenase reductase (also called component II or iron protein), transfers electrons from ferredoxin or flavodoxin to nitrogenase (Figure 20.4)

Both proteins contain iron-sulfur clusters. Nitrogenase also contains a tightly bound iron-molybdenum cofactor. Some variations on this theme are known. Some bacteria, such as Azotobacter, contain more than one nitrogenase complex. Of the three systems in Azotobacter, one uses vanadium instead of molybdenum and another uses only iron.

Crystal structures of nitrogenase and nitrogenase reductase (Figure 20.5) are now available and may help scientists understand how the enzyme system for nitrogen fixation operates.

1. Biological Nitrogen Fixation

2. Nitrogen Fixation

3. Component I

4. Component II

5. Nitrate Uptake and Reduction