Electric fields enhance Diels-Alderase catalysis in abyssomicin C biosynthesis.

Natural Diels-Alderases catalyse [4+2] cycloadditions by preorganizing substrates into reactive conformations. However, the roles of other catalytic factors, such as electrostatic effects, remain elusive. Here, we combine conceptual density functional theory (CDFT) descriptors and electric field analysis to unravel the electrostatic basis of activity in the Diels-Alderase AbyU. Previously, four different enzyme-substrate poses were identified, of which two showed catalytically favorable free energy barriers based on quantum mechanical/molecular mechanical (QM/MM) reaction simulations. Here, we show that atom-condensed Fukui functions can predict the reactivity from reactant conformations alone, focusing on the diene carbons involved in bond formation. The importance of the enzyme-diene interaction is supported by electric field analysis, which shows how reactivity of enzyme-substrate poses correlates with alignment of the enzyme field along the diene moiety. Our findings establish a basis for predicting and engineering Diels-Alderase activity based on electrostatic and electronic reactivity features.