Current developments and challenges in the search for a naturally selected Diels-Alderase

Abstract

Only a very few examples of enzymes known to catalyze pericyclic reactions have been reported, and presently no enzyme has been demonstrated unequivocally to catalyze a Diels-Alder reaction. Nevertheless, research into secondary metabolism has led to the discovery of numerous natural products exhibiting the structural hallmarks of [4+2] cycloadditions, prompting efforts to characterize the responsible enzymatic processes. These efforts have resulted in a growing collection of enzymes believed to catalyze pericyclic [4+2] cycloaddition reactions; however, in each case the complexity of the substrates and catalytic properties of these enzymes poses significant challenges in substantiating these hypotheses. Herein we consider the principles motivating these efforts and the enzymological systems currently under investigation.

Figure 1. Macrophomate synthase

Macrophomate synthase produces macrophomic acid from oxaloacetate and 2-pyrone. While the catalytic cycle may involve a Diels-Alder reaction, mounting evidence suggests a stepwise process of addition instead. Processes leading to C-C bond formation are highlighted in red.

Figure 2. Riboflavin synthase

Riboflavin synthase catalyzes the disproportionation of two molecules of 6,7-dimethyl-8-ribityllumazine (R denotes the ribityl moiety) to generate riboflavin and 5-amino-6-ribitylamino-2,4(1H,3H)-primidinedione. Discovery of a pentacyclic intermediate has suggested a Diels-Alder reaction during the catalytic cycle, though other mechanisms are also possible. Processes leading to C-C bond formation are highlighted in red.

Figure 3. Thiopeptide biosynthesis

The pyrimidine lynchpin motif of thiopeptides such as thiocillin I could arise via an enzyme-catalyzed aza-Diels-Alder reaction. However, stepwise mechanisms of addition have also been proposed. Processes leading to C-C bond formation are highlighted in red.

Figure 4. LovB, SpnF and solanapyrone synthase

The enzymes LovB, SpnF, and solanapyrone synthase are involved in polyketide biosynthesis and have been shown either through direct kinetic characterization or stereochemical analysis to participate in [4+2] cycloaddition reactions (red).