Structural and Computational Bases for Dramatic Skeletal Rearrangement in Anditomin Biosynthesis

Abstract

AndA, an Fe(II)/α-ketoglutarate (αKG)-dependent enzyme, is the key enzyme that constructs the unique and congested bridged-ring system of anditomin (1), by catalyzing consecutive dehydrogenation and isomerization reactions. Although we previously characterized AndA to some extent, the means by which the enzyme facilitates this drastic structural reconstruction have remained elusive. In this study, we have solved three X-ray crystal structures of AndA, in its apo form and in the complexes with Fe(II), αKG, and two substrates. The crystal structures and mutational experiments identified several key amino acid residues important for the catalysis and provided insight into how AndA controls the reaction. Furthermore, computational calculations validated the proposed reaction mechanism for the bridged-ring formation and also revealed the requirement of a series of conformational changes during the transformation.

Enzymatic Cascade Reactions in Biosynthesis

“…[90] Thesecond are aset of maturations of the anditomin scaffold by AntA and AntF to create alternate ring connectivity within the starting pentacyclic framework. [91] Thetetracyclic ent-kaurenoic acid (Scheme 35) undergoes one hydroxylation at C 7 to the 7-a-OH by the plant enzyme Cyp88A. In the process of as econd hydroxylation to the gemdiol and thereby the aldehyde,ar earrangement intervenes,i nitiated by C 6 À Hc leavage to yield the C 6 radical.…”

“…Thei ron-based oxygenase AndA then sets off ar adical-based framework rearrangement that alters connectivity,and sets up afused glactone and an ew carbon bridge on the way to the bicyclooctane unit. [91] Thec ascade is set in motion by homolytic cleavage of one of the methyl C 12 ÀHb onds by the Fe IV =Oa ctive site oxidant. That primary radical is converted into an exomethylene group as the neighboring C 8 ÀOe ther bond is cleaved to generate a5 ' carbon radical and adjacent ketone.Reaction of the C 12 olefin terminus with that 5' carbon radical creates the new CH 2 bridge with unpaired electron density at C 8 .A nother olefin-radical coupling ensues to set the final hexacyclic connectivity before HC transfer by reducing agent generates product andiconin, with an embedded bicyclo[2.2.2]octane moiety.…”

“…No oxygenation of the scaffold has occurred. The X‐ray crystal structure of AndA in complex with its substrates and computational calculations have recently provided further insight in how AndA initiates this cascade reaction . The enzyme AndF, which occurs further along in the pathway after the action of a Baeyer–Villigerase, is likewise a mononuclear nonheme oxygenase family member that also enables radical‐based nonoxygenative rearrangement outcomes to get to anditomin at the end of 12 enzymatic steps in this pathway (Scheme ) …”

Interrogating chemical mechanisms in natural products biosynthesis using quantum chemical calculations

“…A case for which a clear prediction between mechanisms could be made is that shown in Figure . Two plausible rearrangement mechanisms were proposed as means of constructing the core polycyclic framework of andiconin (a precursor to anditomin) . The radical reactant is thought to be generated from a CH‐activation reaction.…”

Biomimetic Synthesis of Meroterpenoids by Dearomatization‐Driven Polycyclization

“…[1] Over 100 congeners have been isolated and characterized, including tropolactone D( 1), [2] berkeleyone A( 2)( antiinflammatory activity), [3] preterretonin A( 3), [4] asperterpene A( 4)( ap otent BACE-1 inhibitor), [5] terretonins (antimicrobial), [6] austins, [7] andrastins, [8] preandiloid A( 5), [9] and anditomin (6). [10] Despite the promising therapeutic potential of this family,m any compounds remain untested or the biological assays are limited in scope,largely due to the small quantities of natural samples available via isolation methods. Although recent elegant syntheses of the DMOA-derived meroterpenoid family members berkeleyone A(2)and select andrastin/terretonin congeners have been reported by the Maimone and Newhouse laboratories, [11] ag eneral biomimetic strategy to access the majority of DMOA/FPP natural product family remains elusive.…”