The dietary requirement for phenylalanine therefore is a function of the total aromatic amino acid content of the diet.
Similarly, methionine may function metabolically as a precursor of other sulfur-containing amino acids so that both of the dietary methionine and cystine determine the requirement for methionine.
The methionine metabolic pathway supports numerous functions within the central nervous system beyond providing an essential amino acid precursor for protein synthesis.
These functions include neurotransmitter synthesis, methyl group donation, polyamine precursor, osmotic protection, antioxidant synthesis, and DNA salvage synthesis.
The relationship between tryptophan and nicotinic acid is another important example. Tryptophan may be metabolized to form nicotinic acid, and in so doing, contributes to the total amount of the vitamin available for cellular metabolism.
Many of the amino acids are precursors of other significant compounds required in metabolic processes. For example, tyroxine and therefore, phenylalanine give rose to the hormones tyroxine and epinephrine.
L-arginine is a precursor for biosynthesis of other amino acids present in protein (glutamate and proline) or not present in proteins (ornithine, citruline). Arginine is a precursor of urea, polyamines, creatine and nitric acid. Polyamines are involved in cell mitosis.
Glutamic acid cysteine, and glycine are components of a tripeptide glutathione, which functions in cellular oxidation-reduction reactions. Glutamic acid can undergo conversion to alpha-ketoglutarate and ammonium. As such, glutamate is an oxidative substrate in several cell types.
Sulfur containing amino acids give rise to taurine a bile acid component. Tryptophan may be metabolized to form serotonin (5-hydroxytryptamine), a tissue hormone that is found predominantly in serum, blood platelets, gastrointestinal mucosa and nerve tissue.
Methionine provides methyl groups for synthesis of choline, creatine and methylation of nicotinamide to its major excretion product N’-methylnicotinamide.
Glycine contributes to the porphyrin ring of hemoglobin and, along with serine, provides part of the structure of the purine and pyrimidines of the nuclei acids.
Two hydroxylated amino acids – hydroxyproline and hydroxylysine – are important constituents of collagen; approximately 12 percent of the total amino acids content of collagen is hydroxyproline.
What are the functions of protein and amino acids?
