Natural [4 + 2]-Cyclases

Abstract

[4 + 2]-Cycloadditions are increasingly being recognized in the biosynthetic pathways of many structurally complex natural products. A relatively small collection of enzymes from these pathways have been demonstrated to increase rates of cyclization and impose stereochemical constraints on the reactions. While mechanistic investigation of these enzymes is just beginning, recent studies have provided new insights with implications for understanding their biosynthetic roles, mechanisms of catalysis, and evolutionary origin.

Figure 1

Structure of the active site of macrophomate synthase with enolpyruvate coordinated to the catalytic Mg²⁺ ion (PDB: 1IZC). The active site is located in a solvent-exposed cavity on the surface of the enzyme (grey) with the Mg²⁺ ion coordinated by the carboxylates of Asp211 (D211) and Glu185 (E185), two water ligands and enolpyruvate as shown. The crystal structure does not have the 2-pyrone substrate 1 bound; however, modeling simulations suggest that it may bind relative to enolpyruvate in a configuration conducive to a Diels-Alder reaction similar to that sketched here.

Figure 2

The hexaketide 34 is an analog of the predicted reactant for the [4 + 2]-cyclization during biosynthesis of dihydromonacolin L bound to LovB (30, see Scheme 4). This hex-aketide can undergo an intramolecular [4 + 2]-cycloaddition to produce four diastereomers in different relative proportions depending on the reaction conditions as shown.^,

Figure 3

Examples of thiopeptides that have been used to study lynchpin biosynthesis.

Figure 4

Biosynthetic gene cluster for spinosyn A from Saccaropolyspora spinosa.

Figure 5

Biosynthetic pathway for spinosyn A.

Figure 6

Selected examples of spirotetronate and spirotetramate natural products. The spirotetronate/tetramate heterocycles are highlighted in red, whereas the decalin ring systems are highlighted in blue.

Figure 7

Structures of the monomeric units of SpnF (PDB: 4PNE), AbyU (PDB: 5DYQ) and PyrI4 (PDB: 5BU3) showing the lidding regions (in teal) that occlude the active sites (in red) in the closed complexes. The structure of PyrI4 has the product species 97 bound in the active site and is rendered in gold. The β-barrel structure of AbyU and PyrI4 is highlighted in purple.

Figure 8

A partial list of natural products currently under investigation for the presence of a potential Diels-Alderase in the biosynthetic pathway.

Scheme 1

Proposed mechanisms for the reaction catalyzed by macrophomate synthase.

Scheme 2

Proposal for the macrophomate synthase-catalyzed reaction between oxaloacetate and methyl coumalate.

Scheme 3

Summary of abbreviated mechanisms proposed for the reaction catalyzed by riboflavin synthase.

Scheme 4

Biosynthetic pathway for lovastatin.

Scheme 5

Biosynthetic pathway for solanapyrones A (43), B (44), D (45) and E (46). Desmethylprosolanapyrone I (39) is constructed from 8 acetate units and a methyl from SAM under the action of the polyketide synthase Sol1.

Scheme 6

Current mechanistic hypotheses for the reactions catalyzed by TclM as a paradigm for the biosynthesis of lynchpin heterocycles in macrocyclic thiopetide antibiotics.^,

Scheme 7

Mechanism proposed for the nonenzymatic cyclization of 62.

Scheme 8

Biogenesis of the tetronate (X = O, Z = O) and tetramate (X = NH, Z = S) functionalities prior to cyclization to form the spirotetronate and spirotetramate ring systems, respectively.

Scheme 9

Reaction catalyzed by VstJ in the biosynthesis of versipelostatin (74).

Scheme 10

Reactions catalyzed by AbyU.

Scheme 11

Sequential reactions catalyzed by PyrE3 and PyrI4 in the biosynthesis of pyrroindomycin A (75).