Deciphering substrate promiscuity and specificity of indolethylamine N-methyltransferase family enzymes from amphibian toads.

N-methylation is a crucial post-modification process in natural product biosynthesis and also contributes to the metabolism of various physiological substances, such as neurotransmitter, hormone, and trace elements. In this study, we identified seven indolethylamine N-methyltransferase (INMT) family enzymes from the amphibian toad Bufo gargarizan with distinct catalytic properties. Among these enzymes, BNMT 1, BNMT 5, BNMT 6 and BNMT 7 exhibited notable promiscuity, demonstrating the ability to methylate multiple derivatives of indolethylamine, phenylethylamine, phenylethanolamine, and nicotinamide. Conversely, BNMT 3 and BNMT 4 exhibited more specific substrate preferences, targeting particular phenylethylamine, phenylethanolamine, and nicotinamide-type substrates. Additionally, one enzyme, BNMT 11, exhibiting high specificity towards phenylethanolamines. By employing molecular docking and mutating key amino acids, we provided a rational explanation for the promiscuity and specificity mechanisms exhibited by these enzymes. This research offers valuable insights into the catalytic mechanisms of INMT family enzymes in B. gargarizans, as well as other organisms. Moreover, the identification of these broadly substrate-specific enzymes holds promise for leveraging synthetic biology in the production of a wide variety of naturally occurring N-methylated compounds.