O

Figure 13.12. Synthesis of fatty acids.

300 atmospheres and temperatures of 500°C. Some nitrogen is fixed naturally by lightning, but most is fixed by various types of prokaryotes, which possess the complex enzyme called nitrogenase. Some of these nitrogen-fixing organisms are free-living while others form symbiotic relationships with plants: the root nodules of legumes are a good example of plants generating a special environment for their nitrogen-fixing symbionts.

Nitrogenase consists of two protein complexes: a reductase and an iron-molybdenum protein. Both proteins have iron-sulfur clusters. The reductase accepts electrons from donors such as ferredoxin (page 275), which can get its electrons through photosynthetic electron transport or from other reactions. The reductase passes electrons to the iron-molybdenum protein (also called dinitrogenase). Nitrogen gas is bound to the iron-molybdenum cofactor where it is reduced to ammonia. Ammonia in water forms ammonium ions NH4+, which can be incorporated into the amino acid glutamine and from there into other amino acids and other molecules.

The overall reaction is

N2 + 8e- + 8 H+ + 16 ATP ^ 2NH3 + H2 + 16 ADP + 16 Pi where e- represents an electron.

Nitrogenase is inhibited by oxygen. Plants with nitrogen-fixing symbiotic bacteria have evolved methods of restricting the oxygen concentration in the vicinity of the bacteria. For example, legumes (the pea family) surround the bacteria with cells that produce a molecule called leghaemoglobin, which is very similar to myoglobin (page 239). The leghaemoglobin binds oxygen, preventing it from reaching the nitrogenase.

Given ammonium ions, plants and bacteria can synthesise all 20 amino acids. Animals are more limited and must obtain some amino acids from their diet. Amino transferases (page 249) allow animals to move amino groups from an amino acid to an oxo-acid to generate a new amino acid, but there are some carbon skeletons that cannot be synthesized. Adult humans require histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine in their diet—these are the essential amino acids. Adult humans are in nitrogen balance—we excrete the same amount of nitrogen as we take in. Growing infants, however, have a net uptake of nitrogen: they take in more nitrogen than they excrete. Growth clearly demands more amino acids, and in this case the limited ability we have to synthesize arginine is insufficient and so it must be present in the diet as well.

Other molecules, such as the nucleic acid bases, are synthesized from amino acid starting materials.

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