Chapter 2 - Amino Acids

Abstract

This chapter focuses on amino acids, which are the monomeric units, or building blocks of proteins joined by a specific type of covalent linkage. The properties of proteins depend on the characteristic sequence of component amino acids, each of which has distinctive side chains. Amino acid polymerization requires elimination of a water molecule, as the carboxyl group of one amino acid reacts with the amino group of another amino acid to form a covalent amide bond. The repetition of this process with many amino acids yields a polymer, known as a polypeptide. The amide bonds linking amino acids to each other are known as peptide bonds. The sequence of amino acids in a protein is dictated by the sequence of nucleotides in a segment of the DNA in the chromosomes, and the uniqueness of each living organism is due to its endowment of specific proteins. Almost all of the naturally occurring amino acids in proteins are L-α-amino acids. The principal 20 amino acids in proteins have an amino group, a carboxyl group, a hydrogen atom, and an R-group attached to the α-carbon. A useful classification of the amino acids is based on the solubility (that is, ionization and polarity) characteristics of the side chains (R-groups). The R-groups fall into four classes, which include nonpolar (hydrophobic); polar, negatively charged (acidic); polar, positively charged (basic); and polar, neutral (unionized). Glycine is the smallest amino acid and has an H atom as its R-group. It is the only α-amino acid that is notoptically active. Glycine is used for the biosynthesis of many nonprotein compounds, such as porphyrins and purines.