All proteins contain carbon, hydrogen, oxygen and nitrogen and most contain sulfur as well. Proteins are built from building-block molecules called amino acids; there are twenty amino acids commonly found in proteins. Each type of amino acid is composed of a carboxyl group and an amino group ( ) bound to a common carbon. In addition, each amino acid has a side chain, referred to as its "R group;" it is the R group of an amino acid that differentiates it from all other amino acids. R groups can be polar, non polar, or electrically charged. Amino acids are joined together through condensation reactions, which form bonds, called peptide bonds, between the carboxyl and amino groups of successive amino acids. Chains of many acids are
