Glutamine is a special aminoacid, because it serves as the donor of amino groups in many biosynthetic reactions; indeed few enzymes are able to use ammonia as a substrate, e.g. carbamyl phosphate synthase, or glycine synthase. Accordingly, glutamine is the most abundant free aminoacid in the blood serum. The essential metabolic conversions of Gln are as follows:
Renal ammoniagenesis. An important function of glutamine, which consumes a very significant amount of this aminoacid is renal ammoniagenesis. The kidney participates to the acid-base balance by retaining bicarbonate and by producing new bicarbonate; this result is accomplished via ammoniagenesis and requires that ammonium ion is excreted in the urine (together with chloride). In this way, acid is eliminated in the urine and serum bicarbonate is increased. The reactions of renal ammoniagenesis require two successive deamination steps, catalyzed by glutaminase and glutamate dehydrogenase; the α-ketoglutarate thus produced is oxidized in the Krebs cycle.
Glutamine as a donor of amino groups. In principle, every aminoacid could donate its amino group via transamination; in practice, however, transaminases transfer amino groups from aminoacids to α-ketoglutarate. The donor of amino groups in many metabolic pathways is glutamine; e.g. this aminoacid plays crucial roles in the biosynthesis of purine bases and in that of asparagine.