SUMMARY

• 31.1 The Nitrogenase Complex Fixes Nitrogen

  Microorganisms use ATP and reduced ferredoxin, a powerful reductant, to reduce N₂ to NH₃. A molybdenum–iron cluster in nitrogenase catalyzes the fixation of N₂, a very inert molecule. Higher organisms consume the fixed nitrogen to synthesize amino acids, nucleotides, and other nitrogen-containing biomolecules. Ammonia exists as ammonium ion (NH₄⁺) in aqueous solutions. The major points of entry of NH₄⁺ into metabolism are glutamine or glutamate.

• 31.2 Amino Acids Are Made from Intermediates of Major Pathways

  Human beings can synthesize 11 of the basic set of 20 amino acids. These 11 amino acids are called nonessential, in contrast with the essential amino acids, which must be supplied in the diet. The pathways for the synthesis of nonessential amino acids are quite simple. Glutamate dehydrogenase catalyzes the reductive amination of α-ketoglutarate to glutamate. A transamination reaction takes place in the synthesis of most amino acids. Alanine and aspartate are synthesized by the transamination of pyruvate and oxaloacetate, respectively.

  Tetrahydrofolate, a carrier of activated one-carbon units, plays an important role in the metabolism of amino acids and nucleotides. This coenzyme carries one-carbon units at three oxidation states, which are interconvertible. The major donor of activated methyl groups is S-adenosylmethionine, which is synthesized by the transfer of an adenosyl group from ATP to the sulfur atom of methionine. S-Adenosylhomocysteine is formed when the activated methyl group is transferred to an acceptor. It is hydrolyzed to adenosine and homocysteine, and the latter is then methylated to methionine to complete the activated methyl cycle.

• 31.3 Feedback Inhibition Regulates Amino Acid Biosynthesis

  Most of the pathways of amino acid biosynthesis are regulated by feedback inhibition, in which the committed step is allosterically inhibited by the final product. The regulation of branched pathways requires extensive interaction among the branches and includes both negative and positive regulation.