Glutamate synthase from Escherichia coli K-12 exhibits NH3-dependent activity. NH3-dependent activity is increased approximately 5-fold in apoglutamate synthase lacking flavin and non-heme iron. Whereas glutamine plus 2-oxoglutarate have the capacity to reoxidize the chemically reduced flavoenzyme, no such reoxidation is obtained with 2-oxoglutarate plus NH3. These results establish that the glutamine- and NH3-dependent syntheses of glutamate occur by different pathways of electron transfer from NADPH. The NH3-dependent activity of native and apoglutamate synthase exhibits similar catalytic properties. Some properties of apoglutamate synthase are similar to those of glutamate dehydrogenase. These properties include pH optima for synthesis and oxidative deamination of glutamate, inactivation by alkylating reagents and p-mercuribenzoate, an enhanced rate of inactivation by alkylating reagents and p-mercuribenzoate at low pH, 2-oxoglutarate protection against inactivation by p-mercuribenzoate, and reactivation of p-mercuribenzoate-treated enzyme by 2-mercaptoethanol. 2-Oxoglutarate protects against alkylation of glutamate synthase by iodo [1-14C]acetamide and reduces incorporation of methyl [1-14C]carboxamide into the small subunit of the enzyme.