Nonspecific Heme-Binding Cyclase, AbmU, Catalyzes [4 + 2] Cycloaddition during Neoabyssomicin Biosynthesis

Diels-Alder (DA) [4 + 2]-cycloaddition reactions rank among the most powerful transformations in synthetic organic chemistry; biosynthetic examples, however, are few and far between. We report here a heme-binding cyclase, AbmU, that catalyzes an essential [4 + 2] cycloaddition during neoabyssomicin scaffold assembly. In vivo genetic and in vitro biochemical analyses strongly suggest that AbmU catalyzes an intramolecular and stereoselective [4 + 2] cycloaddition to form a spirotetronate skeleton from an acyclic substrate featuring both a terminal 1,3-diene and an exo-methylene group. Biochemical assays and X-ray diffraction analyses reveal that AbmU binds nonspecifically to a heme b cofactor and that this association does not play a catalytic role in AbmU catalysis. A detailed study of the AbmU crystal structure reveals a unique mode of substrate binding and reaction catalysis; His160 forms a H-bond with the C-1 carbonyl O-atom of the acyclic substrate, and the imidazole of the same amino acid directs the tetronate moiety of acyclic substrate toward the terminal Δ^10,11, Δ^12,13-diene moiety, thereby facilitating intramolecular DA chemistry. Our findings expand upon what is known about mechanistic diversities available to biosynthetic [4 + 2] cyclases and help to lay the foundation for the use of AbmU in possible industrial applications.