Those metabolic reactions involved in the synthesis, utilization and/or degradation of the primary or standard 20 amino acids found in proteins. Amino acids are described as non-essential or essential depending on whether humans can or can not synthesize them, respectively. Chemically, they are classified based on the polarity of their side chain or R group, as non-polar and hydrophobic and hydrophilic and charged or polar uncharged. Amino acid metabolic pathways are also listed with their KEGG entries.