Functions of Protein and Individual Amino Acids
Traditionally amino acids have been described as ketogenic and glucogenic, that is, they tend to give rise to acetoacetate or carbohydrate intermediates.
In light of the present knowledge of interrelated metabolic pathways, these terms are obsolete. Nonetheless, it is perhaps useful to remember that phenylalanine, tyrosine, leucine and isoleucine are degraded in part to acetoacetate whereas other amino acids are degraded chiefly to pyruvate, oxaloacetate, alpha-ketoglutarate, succinate and fumarate.
The dietary requirements of certain of the amino acids are influenced by the intake of other nutrients. For example, phenylalanine is converted to tyrosine in the animal cell.
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 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.
Glutamic acid cysteine, and glycine are components of a tripeptide glutathione, which functions in cellular oxidation-reduction reactions.
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.
Functions of Protein and Individual Amino Acids
