Eleven amino acids are derived from intermediates of glycolysis, the pentose phosphate pathway or the citric acid cycle. The other nine, the essential amino acids, must be obtained from the diet.

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NH4+ is introduced into the biosynthetic pathway of many amino acids when glutamate dehydrogenase catalyzes a form of reductive amination of the carbonyl group of α-ketoglutarate, an intermediate in the citric acid cycle, leading to the biosynthesis of L-glutamate, an amino acid. Chiral enzyme and coenzyme catalysis in the reduction step introduces the chiral center. Glutamine synthetase may then convert the glutamate into another amino acid, glutamine. Glutamate then often serves an amine group donor with other α-keto acids, such as pyruvate or oxaloacetate, in transamination reactions leading to the synthesis of other amino acids, such as alanine and aspartate.