Does Nature Click? Theoretical Prediction of an Enzyme-Catalyzed Transannular 1,3-Dipolar Cycloaddition in the Biosynthesis of Lycojaponicumins A and B

Biosynthetic 1,3-dipolar cycloadditions are rare. No enzymes have yet been identified whose function is to catalyze this class of reactions. Recently, however, a 1,3-dipolar cycloaddition was proposed as a key step in the biosynthesis of two Lycopodium alkaloids, lycojaponicumins A and B. The lycojaponicumins' fused bicyclic tetrahydroisoxazole ring system was proposed to originate from a transannular 1,3-dipolar cycloaddition between a nitrone and an enone in a nine-membered macrocycle. We have used quantum mechanical calculations to predict whether this cycloaddition could constitute a feasible step in a biosynthetic pathway. Our calculations define a general computational approach for analyzing whether a putative biosynthetic reaction is likely to be enzyme-catalyzed. The quantum mechanically predicted rate of the uncatalyzed reaction in water is compared with the rate enhancement theoretically achievable when the reaction is catalyzed by a theozyme (theoretical enzyme). Density functional theory calculations (M06-2X) predict that the uncatalyzed transannular 1,3-dipolar cycloaddition of the putative lycojaponicumin precursor in water is moderately facile (ΔG(++) = 21.5 kcal/mol, k = 10(-3) s(-1)) and that an enzyme could accelerate the cycloaddition by placing hydrogen bond donors around the enone while maintaining an otherwise nonpolar active site. The theoretical enzyme-catalyzed process has ΔG(++) ≈ 17 kcal/mol, corresponding to a 2000-fold rate enhancement, and the predicted kcat (2 s(-1)) is similar to those of known enzymes involved in secondary metabolic pathways. Thus, theory predicts that the proposed transannular 1,3-dipolar cycloaddition is a plausible step in a biosynthetic pathway leading to the lycojaponicumins and suggests that dipolar cycloadditions can be accelerated by enzyme catalysis.