Metabolic pathways 185

FIG. 6-30 Micrograph of the cyanobacterium Anabaena a/lindrica showing vegetative cells and a heterocyst (en­larged cell) in which nitrogen fixation occurs. (400 X).

gen fixation, the incorporation of nitrogen atoms from N₂ gas into protein, requires the breaking of an N=N triple bond. This is a very strong bond that is extremely difficult to break.

In the biological fixation of nitrogen, the triple bond of molecular nitrogen is enzymatically broken by nitrogenase. This is a complex enzyme system. An iron-containing compound such as ferredoxin first obtains electrons from the breakdown of organic molecules or from photosynthetic light reactions and carries them to a protein, nitrogen reductase, which channels them to another protein, dinitrogenase. With | the transfer of six electrons and the use of twelve ATP and four water molecules, nitrogenase converts ni­trogen gas into two molecules of ammonia.

Nitrogen-fixing bacteria, called Rhizobium and Bradyrhizobium, live mutualistically in the nodules on the roots of legume plants (FIG. 6-30). Within the nodule, leghemoglobin, a protein produced by the plant, provides controlled amounts of oxygen so that aerobic energy-yielding metabolism can be carried out without inactivating nitrogenase, which is sensi­tive to oxygen exposure. When growing alone, Rhizo­bium and Bradyrhizobium require oxygen for their me­tabolism and are unable to fix nitrogen. When they live within root nodules, Rhizobium and Bradyrhizo- [ Uum survive in this oxygen-free environment by uti­lizing the metabolites of the plant. Other nitrogen-fixing bacteria, such as Azotobacter I and Beijerinckia, are free living. Nitrogen-fixing I cyanobacteria have specialized cells, called hetero-

I cysts, that contain the nitrogenase (FIG. 6-31). The j heterocyst provides protection for nitrogenase against molecular oxygen, which is produced photo-synthetically by cyanobacteria and which denatures nitrogenase.

FIG. 6-31 Bacterial cells of Bradyrhizobium japonicum within a nodule of a soybean produce nitrogenase, which results in the conversion of molecular nitrogen to ammonia.

Methanogenesis

Some archaebacteria are able to use hydrogen and carbon dioxide to generate the ATP and molecules that compose their cellular structures. The metabo­lism of these archaebacteria produces methane and they are therefore called methanogens (FIG. 6-32). Other methanogens use fatty acids instead of carbon dioxide for the production of methane. Hydrogen gas, carbon dioxide, and fatty acids were available at the time life evolved on Earth. The methanogenic ar-

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