ABSTRACT
Amino acids (AAs) are organic substances containing both amino and acid groups and widely occur in nature. The history of their discoveries dates back to over 200 years ago when chemists identified AAs as components of protein and non-protein extracts. All proteinogenic AAs except glycine can have both L- and D-isomers. L-AAs are generally the most abundant physiological isomers and account for >99.9% of total AAs in the organisms, but some D-AAs (e.g., D-aspartate and D-serine) may have important functions, particularly in the neurological and immune systems of animals. Peptide-bound AAs and free AAs account for approximately 97% and 3% of total AAs in animals, respectively. In some plants, select AAs (e.g., glutamine and asparagine) are highly abundant in the free form, with free AAs representing 34.4% and 28.5% of the total AAs in potatoes and sweet potatoes, respectively. Dynamic changes in the concentrations of free AAs (particularly the arginine family of AAs) in the physiological fluids of animals during growth and development and under pathological conditions reveal an exciting aspect of multidisciplinary research on AAs. The diversity of AAs lies in their different side chains, which determine their different chemical properties (e.g., solubility, stability, reactions, and taste). The reactivity of the amino and carboxyl groups of AAs with select organic or inorganic reagents provides a basis for the analysis of AAs and peptides.
