Recent advances of Diels-Alderases involved in natural product biosynthesis

Frequent occurrence of [4+2] adducts in the secondary metabolites suggested involvement of Diels-Alderases (DAases) in their biosynthesis. However, a limited number of DAases were reported before early 2000s. Advancements in whole-genome sequencing and searching tool of the biosynthetic gene clusters of the secondary metabolites facilitate the identification of plausible DAases. Thus, during past 5 years, nine DAases have been characterized by genetic and biochemical analyses. These include a detailed functional analysis of SpnF that solely catalyzes [4+2] cycloaddition, a structural analysis of spirotetramate-forming enzyme PyrI4 complexed with the corresponding cycloadduct, and DAases catalyzing decalin formation and macrocyclic pyridine formation. Together with decalin-forming enzymes and macrocyclic pyridine-forming enzymes, these results provided sufficient data to discuss catalytic mechanism of DAases and nature's strategy for molecular diversification of linear chain intermediates derived from polyketide and ribosomal peptide biosynthetic machinery.