Figure 5.

Predicted two-step pathway for the biosynthesis for BMAA in cycads. A postulated route for BMAA biosynthesis supported by cycad EST analysis is shown. In this simple, two-step scheme, BMAA synthesis begins with (a) the transfer of NH₃ to β-substituted alanine, where X = phosphoserine, cysteine, o-acetylserine or cyanoalanine, to form (b) an intermediate. The reaction is catalyzed by a cysteine synthase-like enzyme. This step is followed by transfer of a methyl group from S-adenosylmethionine (Ad-S-CH₃) to the new amine group by a methyltransferase, which would lead to the formation of (c) BMAA. Candidate cycad genes encoding probable cysteine synthase-like enzymes and methyltransferase, as well as S-adenosylmethionine-regenerating enzymes that were identified in the cycad EST collection are listed in Table 2.