β-Deuterium Isotope Effects on Firefly Luciferase Bioluminescence

Enzymatic promiscuity and the evolution of bioluminescence

Bioluminescence occurs when an enzyme, known as a luciferase, oxidizes a small-molecule substrate, known as a luciferin. Nature has evolved multiple distinct luciferases and luciferins independently, all of which accomplish the impressive feat of light emission. One of the best-known examples of bioluminescence is exhibited by fireflies, a class of beetles that use d-luciferin as their substrate. The evolution of bioluminescence in beetles is thought to have emerged from ancestral fatty acyl-CoA synthetase (ACS) enzymes present in all insects. This theory is supported by multiple lines of evidence: Beetle luciferases share high sequence identity with these enzymes, often retain ACS activity, and some ACS enzymes from nonluminous insects can catalyze bioluminescence from synthetic d-luciferin analogues. Recent sequencing of firefly genomes and transcriptomes further illuminates how the duplication of ACS enzymes and subsequent diversification drove the evolution of bioluminescence. These genetic analyses have also uncovered candidate enzymes that may participate in luciferin metabolism. With the publication of the genomes and transcriptomes of fireflies and related insects, we are now better positioned to dissect and learn from the evolution of bioluminescence in beetles.

Luciferin Regeneration in Firefly Bioluminescence via Proton-Transfer-Facilitated Hydrolysis, Condensation and Chiral Inversion

Firefly bioluminescence is produced via luciferin enzymatic reactions in luciferase. Luciferin has to be unceasingly replenished to maintain bioluminescence. How is the luciferin reproduced after it has been exhausted? In the early 1970s, Okada proposed the hypothesis that the oxyluciferin produced by the previous bioluminescent reaction could be converted into new luciferin for the next bioluminescent reaction. To some extent, this hypothesis was evidenced by several detected intermediates. However, the detailed process and mechanism of luciferin regeneration remained largely unknown. For the first time, we investigated the entire process of luciferin regeneration in firefly bioluminescence by density functional theory calculations. This theoretical study suggests that luciferin regeneration consists of three sequential steps: the oxyluciferin produced from the last bioluminescent reaction generates 2-cyano-6-hydroxybenzothiazole (CHBT) in the luciferin regenerating enzyme (LRE) via a hydrolysis reaction; CHBT combines with L-cysteine in vivo to form L-luciferin via a condensation reaction; and L-luciferin inverts into D-luciferin in luciferase and thioesterase. The presently proposed mechanism not only supports the sporadic evidence from previous experiments but also clearly describes the complete process of luciferin regeneration. This work is of great significance for understanding the long-term flashing of fireflies without an in vitro energy supply.

Theoretical Development of Near-Infrared Bioluminescent Systems

The luciferin/luciferase system of the firefly has been used in bioluminescent imaging to monitor biological processes. In order to enhance the efficiency and expand the application range, some efforts have been made to tune the light emission, especially the effort to obtain NIR light. However, those case-by-case studies have not together revealed the nature and mechanism of the color tuning. In this paper, we theoretically investigated the fluorescence of all kinds of typical oxyluciferin analogues. The present systematical modifications of both oxyluciferin and luciferase indicate that the essential factor affecting the emission color is the charge distribution (or the electric dipole moment) on the oxyluciferin, which impacts on the charge transfer to form the light emitter and, subsequently, influence the strength and wavelength of the emission light. More negative charge distributed on the "thiazolone moiety" of the oxyluciferin or its analogues leads to a redshift. Based on this conclusion, we theoretically designed optimal pairs of luciferin analogue and luciferase for emitting NIR light, which could inspire new synthetic procedures and practical applications.

Modeling Chemical Reactions by QM/MM Calculations: The Case of the Tautomerization in Fireflies Bioluminescent Systems

In less than half a century, the hybrid QM/MM method has become one of the most used technique to model molecules embedded in a complex environment. A well-known application of the QM/MM method is for biological systems. Nowadays, one can understand how enzymatic reactions work or compute spectroscopic properties, like the wavelength of emission. Here, we have tackled the issue of modeling chemical reactions inside proteins. We have studied a bioluminescent system, fireflies, and deciphered if a keto-enol tautomerization is possible inside the protein. The two tautomers are candidates to be the emissive molecule of the bioluminescence but no outcome has been reached. One hypothesis is to consider a possible keto-enol tautomerization to treat this issue, as it has been already observed in water. A joint approach combining extensive MD simulations as well as computation of key intermediates like TS using QM/MM calculations is presented in this publication. We also emphasize the procedure and difficulties met during this approach in order to give a guide for this kind of chemical reactions using QM/MM methods.

β-Deuterium Isotope Effects on Firefly Luciferase Bioluminescence

A 5,5-d2 -luciferin was prepared to measure isotope effects on reactions of two intermediates in firefly bioluminescence: emission by oxyluciferin and elimination of a putative luciferyl adenylate hydroperoxide to dehydroluciferin. A negligible isotope effect on bioluminescence provides further support for the belief that the emitting species is the keto-phenolate of oxyluciferin and rules out its excited-state tautomerization, one potential contribution to a bioluminescence quantum yield less than unity. A small isotope effect on dehydroluciferin formation supports a single-electron-transfer mechanism for reaction of the luciferyl adenylate enolate with oxygen to form the hydroperoxide or dehydroluciferin. Partitioning between the dioxetanone intermediate (en route to oxyluciferin) and dehydroluciferin is determined, not by the fate of the hydroperoxide, but by that of the radical formed from luciferyl adenylate, and the kinetic isotope effect (KIE) reflects H-atom abstraction by superoxide.