ABSTRACT
The building blocks of peptides and proteins are amino acids. Most proteins consist of 20 standard amino acids that are coded by DNA; these amino acids are often called coded amino acids. They are joined into peptides and proteins by an amide bond called the peptide bond. The formation of this peptide bond is the condensation reaction in which water is released. What it is left of the amino acid as a part of the peptide or protein is called the residue. Besides standard amino acids, a large number of nonstandard amino acids are found in peptides and proteins, usually in limited amounts. The characteristics and properties of amino acids need to be understood in order to be able to comprehend the structure and behavior of polypeptides. Amino acids are not only structural elements of polypeptides, but they also have important precursor, transport, and metabolic roles. For example, histidine and tryptophan are precursors of the hormone histamine and the neurotransmitter serotonin, respectively. Carnitine can be acylated and, in this form, serves as a transporter of fatty acids across the mitochondrial membrane. Amino acids can be metabolized to release energy; some of them, called glucogenic amino acids, can also be converted into glucose to provide energy to the brain. Some of the amino acid derivatives are important in technology, medicine, and the food industry. Examples include the sodium salt of glutamic acid, sodium glutamate, as a food additive; aspartame, methylated dipeptide aspartyl-phenylalanine, as an articial sweetener; and l-DOPA, l-dihydroxyphenylalanine, as a drug for the treatment of Parkinson’s disease.
