Principles of Biochemistry Volume 2 - A. Lehninger 1985

Bioenergetics and Metabolism
Biosynthesis of Amino Acids and Nucleotides
Certain amino acids must be obtained from the diet

The biosynthetic PATHWAYS OF AMINO acids and NUCLEOTIDES are discussed together in this chapter for several reasons. The molecules of both Amino Acids and nucleotides contain nitrogen atoms derived from the same biological sources. Furthermore, amino acids serve as precursors in the Biosynthesis of Nucleotides. There is yet another connection between amino acids and nucleotides: both classes of compounds play The Role of fundamental building blocks in the biochemistry of heredity. Nucleotides, which encode the elements of Nucleic Acids, serve to store and transmit Genetic information, whereas amino acids, the Building Blocks of Proteins, ensure its expression.

The biosynthetic pathways leading to the 20 amino acids found in proteins and the eight common nucleotides of nucleic acids are numerous and, for the most part, quite complex. Rather than providing a detailed description of all these pathways, we will focus our attention on the core metabolic principles that underlie them. Because amino acids and nucleotides are required by the Organism in relatively small quantities, metabolic flux is almost never as high here as it is in the pathways leading to CARBOHYDRATES or fats in animal Tissues. At the same time, ensuring the synthesis of proteins and nucleic acids requires that various amino acids and nucleotides be produced in the proper proportions and at the appropriate times, meaning that their respective biosynthetic pathways must be regulated very tightly and in a coordinated manner.

We have already seen from other Examples that the biosynthetic and degradative pathways of particular compounds are not identical. Similarly, they are not identical for amino acids and nucleotides. Moreover, in this case as well, biosynthetic and Catabolic pathways are regulated independently of one another.

Different species of living organisms vary widely in their ability to synthesize the 20 different amino acids. They also differ in their capacity to utilize Various Forms of nitrogen as amino group precursors. Humans and the albino rat, for example, can synthesize only 10 of the 20 amino acids required for METABOLISM/35.html">Protein Biosynthesis (Table 22-1). These 10 Amino acids are termed nonessential; the organism synthesizes them itself from ammonia and various carbon sources. The other 10 amino acids must be obtained from the diet and are termed essential. Higher plants are better equipped in this regard: they can synthesize all the amino acids required for Protein Synthesis. Furthermore, they can utilize not only ammonia but also nitrates as precursors for amino groups. Microorganisms vary dramatically in their ability to synthesize amino acids.

Class="center">Table 22.1. Nonessential and Essential Amino Acids for humans and the albino rat

¹⁾ Arginine is an essential amino acid only for young, growing animals.

Escherichia coli, for example, synthesizes all the amino acids required for protein synthesis from simple precursors, whereas lactic acid Bacteria are incapable of this and must acquire certain amino acids preformed from their environment.