glutamate. The a-ketoglutarate can then be used again to incorporate more ammonia. ■ amino group, p. 26

Aromatic Amino Acids

Synthesis of aromatic amino acids such as tyrosine, phenylala-nine, and tryptophan requires a multistep branching pathway (figure 6.31). This serves as an excellent illustration of many important features of the regulation of amino acid synthesis.

The pathway begins with the formation of a 7-carbon compound, resulting from the joining of two precursor metabolites, erythrose 4-phosphate (4-carbon) and phosphoenolpyru-vate (3-carbon). These precursors originate in the pentose phosphate pathway and glycolysis, respectively. The 7-carbon compound is converted through a series of steps until a branch point is reached. At this juncture, two options are possible. If synthesis proceeds in one direction, tryptophan is produced. In the other direction, another branch point is reached; from there, either tyrosine or phenylalanine can be made.

When a given amino acid is provided to a cell, it would be a waste of carbon, energy, and reducing power for that cell to continue synthesizing it. But when only one product of a branched pathway is present, how does the cell control synthesis? In the pathway for aromatic acid biosynthesis, this partly occurs by regulating the enzymes at the branch points. Tryptophan acts as a feedback inhibitor of the enzyme that directs the branch to its synthesis; this sends the pathway to the steps leading to the synthesis of the other amino acids, tyrosine and phenylalanine. Likewise, these two amino acids each inhibit the first enzyme of the branch leading to their synthesis.

In addition, the three amino acids each control the first step of the full pathway, the formation of the 7-carbon compound. Three different enzymes can catalyze this step; each has the same active site, but they have different allosteric sites. Each aromatic amino acid acts as a feedback inhibitor for one of the enzymes. If all three amino acids are present in the environment, then very little of the 7-carbon compound will be synthesized. If only one or two of those amino acids are present, then proportionally more of the compound will be synthesized.

Nucleotide Synthesis

Nucleotide subunits of DNA and RNA are composed of three units: a 5-carbon sugar, a phosphate group, and a nitrogenous base, either a purine or a pyrimidine (see figure 2.22). They are synthesized as ribonucleotides, but these can then be converted to deoxyribonucleotides by replacing the hydroxyl group on the 2' carbon of the sugar with a hydrogen atom. ■ purine, p. 31, ■ pyrimidine, p. 32

The purine nucleotides are synthesized in a distinctly different manner from the pyrimidine nucleotides. Purine nucleotides are synthesized on the sugar phosphate component in a very complex process. In fact, nearly every carbon and nitrogen of the purine ring comes from a different source (figure 6.32).

From glycolytic pathway

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