The amino group of amino acids is typically derived from some inorganic nitrogen source in the environment, such as ammonia. Ammonia is most often incorporated through formation of the amino acids glutamate or glutamine by the enzymes glutamate dehydrogenase and glutamine synthetase, respectively (Figure5.26). When ammonia is present at high levels, glutamate dehydrogenase or other amino acid dehydrogenases are used. However, when ammonia is present at low levels, glutamine synthetase, with its energy-consuming reaction mechanism (Figure5.26b) and high affinity for substrate, is employed. We discuss control of the activity of glutamine synthetase in Section 5.18.