Principles of Biochemistry Volume 2 - A. Lehninger 1985

Bioenergetics and Metabolism
Biosynthesis of Amino Acids and Nucleotides
Biosynthesis of Essential Amino Acids

The biosynthetic PATHWAYS OF AMINO acids essential for humans and the albino rat were elucidated through biochemical and genetic studies on microorganisms capable of synthesizing these Amino Acids. We will not discuss them in detail here, limiting ourselves to just a few General Remarks.

As a rule, the synthesis pathways of Essential Amino Acids are more complex and longer than those of non-essential ones: the former typically involve from five to fifteen steps, whereas the latter rarely exceed five. The inability of higher animals to synthesize essential amino acids is attributed to the lack of one or two Enzymes required for this synthesis. The greatest complexity is characteristic of the pathways leading to the synthesis of essential amino acids such as phenylalanine, Tryptophan, and Histidine, whose molecules contain benzene rings or heterocycles. The synthesis of such rings (especially the two condensed rings of tryptophan) is a complex process involving a whole series of enzymatic steps.

Five amino acids that are essential for animals are synthesized by plants and microorganisms from non-essential amino acids: Threonine, Methionine, and Lysine are synthesized from aspartate, while Arginine and histidine are synthesized from glutamate. Isoleucine is produced in Bacteria from the essential amino acid threonine.