Free energy simulations unravel luciferin-chirality dependence in firefly bioluminescence—A pKa-controlled asymmetric catalysis

Abstract

Firefly bioluminescence contains a series of complicated chemical reactions starting from the enzymatic oxidation of D-luciferin (D-LH2) by luciferase (Luc). Why firefly bioluminescence uses D-LH2 not L-LH2 has never been clearly interpreted. We theoretically investigated the two steps where LH2-chirality takes effect: adenylation of LH2 to adenylluciferin (ALU) with the aid of ATP and Mg2+, and deprotonation of ALU. Free energy simulations showed that both D- and L-LH2 react with ATP via a reversible SN2 reaction. The co-factor Mg2+ decreases the activation energy by an entropic trap. The theoretical estimated pKa of D-ALU (4.5) and L-ALU (22.2) in Luc, indicating deprotonation at the chiral carbon effectively occurs for D-ALU but not L-ALU under the physiological pH. This pKa-controlled asymmetric catalysis was further analyzed via the substrate-residue interactions. This work unravels the LH2-chirality dependence in firefly BL for the first time, and provides another residue-regulation strategy for designing BL systems and other asymmetric catalysts.