Two key amino acids for incorporation of the ammonia into biochemicals in general are Glu and Gln (Figure 14-3). Briefly, Glu is made from ammonia and α–ketoglutarate in a reduction reaction catalyzed by glutamate dehydrogenase, with the reducing power coming from NADPH. Gln is then made from Glu by amidation of the side-chain carboxylic acid using NH3. Amino groups of most other amino acids derive from glutamate by transamination. Gln donates its side-chain nitrogen in many biosynthetic reactions-for example, in the synthesis of Trp and His, and in the biosynthesis of nucleotides