Biochemistry - The Chemical Reactions of Living Cells Volume 3 - D. Metzler 1980

Metabolism of Nitrogen-Containing Compounds
Synthesis and Catabolism of Compounds Belonging to the Glutamic Acid Family
Proline and Arginine

The five-carbon Skeleton of glutamic acid directly gives rise to Proline, Ornithine, and Arginine. The corresponding reactions are shown in Fig. 14-2. Arginine, in turn, participates in The Urea Cycle (Fig. 14-4) and serves as a precursor in polyamine Biosynthesis.

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FIG. 14-4. Biosynthesis of citrulline, arginine, and urea. Solid arrows indicate reactions directly related to AMINO ACID DEAMINATION and urea synthesis.

The ATP-dependent reduction of the y-carboxyl group of glutamate by NADPH (reaction в in Fig. 14-2) belongs to a standard type of biosynthetic reaction that is the reverse of the oxidation reaction shown in Fig. 8-I. Like the latter reaction, the ATP-dependent reduction apparently proceeds via The formation of an intermediate acyl phosphate or acyl adenylate. The reduction product, glutamic acid semialdehyde, spontaneously cyclizes and is converted to proline through further reduction (Fig. 14-2).

If the amino group is blocked by Acetylation (Fig. 14-2, step 2) prior to the reduction of glutamate to the semialdehyde, cyclization is prevented. The y-aldehyde group can be converted into an amino group via Transamination, and the removal of the blocking acetyl group leads to the formation of ornithine¹). The latter is converted into arginine As a result of the reactions shown in Fig. 14-4. These reactions not only provide the pathways for arginine biosynthesis operating in all organisms, but also ensure the synthesis of urea, the principal nitrogenous end product in mammals and several other organisms. An interesting feature has been observed in Neurospora; when grown on a minimal medium, its Cells accumulate large amounts of ornithine and arginine, over 98% of which are sequestered in cytoplasmic vesicles [33b].