Glutamate / glutamine / ammonia metabolic hub (GLUL, GLS/GLS2, GLUD1/GLUD2)

Glutamate, glutamine and ammonia sit at the crossroads of amino-acid, nitrogen and carbon metabolism, and three enzyme activities interconvert them. Glutamine synthetase (GLUL) is the sole route of net ammonia fixation into organic form: it condenses L-glutamate + ammonia + ATP to L-glutamine, the body's principal non-toxic nitrogen carrier and a nitrogen donor for nucleotide, amino-sugar and amino-acid synthesis; it detoxifies ammonia in perivenous hepatocytes, astrocytes and muscle. Running the opposite direction, the phosphate-activated mitochondrial glutaminases GLS (kidney-type, KGA/GAC) and GLS2 (liver-type, LGA/GAB) hydrolyse L-glutamine back to L-glutamate + ammonia, initiating glutaminolysis and supplying glutamate for the astrocytic glutamate-glutamine cycle, renal ammoniagenesis and cancer glutamine addiction. Glutamate is then reversibly deaminated by mitochondrial glutamate dehydrogenase — the ubiquitous GLUD1 and the hominoid-specific, brain/testis GLUD2 — to 2-oxoglutarate (alpha-ketoglutarate) + ammonia with NAD(P)+, linking amino-acid nitrogen to the TCA cycle for energy and anaplerosis and, in pancreatic beta-cells, coupling amino-acid sensing to insulin secretion. Together GLUL and GLS establish the glutamate<->glutamine interconversion cycle, while GLUD channels glutamate carbon into the TCA cycle and releases its nitrogen as ammonia (destined for the urea cycle). Inherited defects highlight the hub's importance: GLUL deficiency (congenital glutamine deficiency / neonatal encephalopathy), GLS deficiency (epileptic encephalopathy) and gain-of-function (developmental delay), and GLUD1 activating mutations (the hyperinsulinism-hyperammonemia syndrome).