title: Does nature click? Theoretical prediction of an enzyme-catalyzed transannular 1,3-dipolar cycloaddition in the biosynthesis of lycojaponicumins A and B.
abstract: 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.
authors.affiliation: School of Chemistry and Molecular Biosciences, University of Queensland , Brisbane, Queensland 4072, Australia.
authors.forename: Elizabeth H
authors.initials: EH
authors.lastname: Krenske
authors.forename: Ashay
authors.initials: A
authors.lastname: Patel
authors.forename: K N
authors.initials: KN
authors.lastname: Houk
mesh_terms: D000470:Alkaloids / Q000096:biosynthesis* / Q000737:chemistry; D053961:Azabicyclo Compounds / Q000737:chemistry; D055162:Biocatalysis; D003500:Cyclization; D004798:Enzymes / Q000378:metabolism*; D015394:Molecular Structure; D011789:Quantum Theory; D013816:Thermodynamics
chemical_list: D000470:Alkaloids; D053961:Azabicyclo Compounds; D004798:Enzymes; C587803:lycojaponicumin A; C587805:lycojaponicumin B