Substrate specificity of benzamide synthetase involved in 4-hydroxy-3-nitrosobenzamide biosynthesis

Glutamine-dependent amidotransferase (Gn-AT) catalyzes the transfer of the amido nitrogen of glutamine to an acceptor substrate to produce glutamate and an aminated product. Although most Gn-ATs are involved in biosyntheses of primary metabolites, some are important in the synthesis of secondary metabolites. Recently, a Gn-AT (NspN) was discovered in the biosynthetic pathway of 4-hydroxy-3-nitorosobenzamide in Streptomyces murayamaensis. NspN converts 3-amino-4-hydroxybenzoic acid (3,4-AHBA) to 3-amino-4-hydroxybenzamide. Here, we report the amino-acceptor substrate specificity of NspN. NspN could use several benzoic acid derivatives as amino-acceptor substrates to produce corresponding benzamides and catalyze the amidation of several carboxylate-type phenylpropanoids, such as p-coumaric acid, cinnamic acid and caffeic acid. NspN showed the highest activity toward its natural substrate 3,4-AHBA among the substrates examined. NspN and related bacterial Gn-ATs may be useful in developing combinatorial biosynthetic strategies for benzamide derivatives, which could, in turn, be used as therapeutic agents for a variety of diseases.