Lecture 28 Summary

Nitrogenase and nitrogen fixation

Compared to the other most common elements found in living systems, carbon and nitrogen are considerably less easily assimilated by living organisms. This is directly related to the fact that the major inorganic forms of these elements, CO₂ and N₂, are both very stable molecules, hence relatively unreactive. This is especially true of diatomic nitrogen, which has a bond energy of 225 kcal/mol (945 kJ/mol).

We have seen that the assimilation of carbon dioxide is accomplished by photosynthetic organisms via the activity of RuBisCO and the operation of the Calvin cycle. This is the sole route (with only very minor exceptions) by which net conversion of carbon dioxide to bioorganic compounds occurs. The ability of organisms to fix nitrogen in the form of metabolically useful compounds is even more restricted. In fact, only a few types of bacteria, termed diazotrophs, can carry out this kinetically demanding process, which is catalyzed by an enzyme complex called nitrogenase [EC 1.18.6.1]. Nitrogenase is a fascinating enzyme with a number of unusual features, including a molybdenum-containing cofactor.

Many diazotrophic bacteria live in symbiotic association with certain species of plants. Legumes, such as beans, clover, and alfalfa have specialized structures in which root nodule cells are found in association with diazotrophs of the genus Rhizobium. The image above shows the root nodules of soybean.

Structure and function of nitrogenase complex

Nitrogenase complex consists of two separable oligomeric proteins: a dimeric Fe protein or "reductase" component, and a heteroterameric MoFe protein or "nitrogenase" component. The Fe protein contains a [4Fe-4S] cluster that accepts electrons from reduced ferredoxin. These electrons are passed on to the MoFe protein, which contains two prosthetic groups - a "P cluster" consisting of two linked [4Fe-4S] clusters that accepts electrons from the reductase, and a FeMo cofactor which is the actual site of nitrogen reduction

Assimilation of amino groups for biosynthesis

Glutamate synthase [EC 1.4.1.13].

• Catalyzes a reductive amination
• Occurs in microorganisms, plants, and lower animals
• Electrons contributed from NADPH or ferredoxin
• Glutamate synthase from Azosprillum brasilense is well characterized

Glutamine synthetase [EC 6.3.1.2]

Study questions

• Compare and the contrast the reaction carried out by nitogenase with the industrial Haber process.

Page updated 12-27-06

References

1. Berg, Tymoczko, and Stryer. Biochemistry (BTS): 6th edition (2007, Freeman) Ch.24, pp.679-685.