A Single Enzyme Transforms a Carboxylic Acid into a Nitrile through an Amide Intermediate

The biosynthesis of nitriles is known to occur through specialized pathways involving multiple enzymes; however, in bacterial and archeal biosynthesis of 7-deazapurines, a single enzyme, ToyM, catalyzes the conversion of the carboxylic acid containing 7-carboxy-7-deazaguanine (CDG) into its corresponding nitrile, 7-cyano-7-deazaguanine (preQ₀). The mechanism of this unusual direct transformation was shown to proceed via the adenylation of CDG, which activates it to form the newly discovered amide intermediate 7-amido-7-deazaguanine (ADG). This is subsequently dehydrated to form the nitrile in a process that consumes a second equivalent of ATP. The authentic amide intermediate is shown to be chemically and kinetically competent. The ability of ToyM to activate two different substrates, an acid and an amide, accounts for this unprecedented one-enzyme catalysis of nitrile synthesis, and the differential rates of these two half reactions suggest that this catalytic ability is derived from an amide synthetase that gained a new function. Double duty: In bacterial and archeal biosynthesis of 7-deazpurines, a single enzyme, ToyM, catalyzes the two-step conversion of the carboxylic acid 7- carboxy-7-deazaguanine (CDG) into the corresponding nitrile, 7-cyano-7-deazaguanine (preQ₀). The mechanism of this unusual direct transformation proceeds via the adenylation of CDG. This activates it to form the amide intermediate 7-amido-7-deazaguanine (ADG), which is in turn activated to form the nitrile.