An Overview of Antennal Esterases in Lepidoptera

Abstract

Lepidoptera are used as a model for the study of insect olfactory proteins. Among them, odorant degrading enzymes (ODEs), that degrade odorant molecules to maintain the sensitivity of antennae, have received less attention. In particular, antennal esterases (AEs; responsible for ester degradation) are crucial for intraspecific communication in Lepidoptera. Currently, transcriptomic and genomic studies have provided AEs in several species. However, efforts in gene annotation, classification, and functional assignment are still lacking. Therefore, we propose to combine evidence at evolutionary, structural, and functional level to update ODEs as well as key information into an easier classification, particularly of AEs. Finally, the kinetic parameters for putative inhibition of ODEs are discussed in terms of its role in future integrated pest management (IPM) strategies.

Keywords: Lepidoptera; antennal esterases; inhibition; olfactory system; semiochemicals; transcriptomic.

Figure 1

Schematic organization of proteins present in the olfactory system of Lepidoptera. It is possible to identify the four families, chemosensory proteins (CSPs), odorant-binding proteins (OBPs), odorant degrading enzymes (ODEs), and chemosensory receptors (Vogt, 2005; Pelosi et al., 2006, 2017; Rytz et al., 2013).

Figure 2

(A) Schematic representation of the olfactory mechanism in sensilla of Lepidoptera with emphasis on esterases. Compounds from the environment pass through cuticular pores toward the sensillar lymph. Here, OBPs bind and transport these molecules to odorant receptors (ORs) located in the dendritic membrane of olfactory neurons where they are activated. After cell excitation, the molecules are degraded by the action of ODEs (esterases). These enzymes can even act when the molecules enter to the sensillar lymph (Leal, 2013). (B) Reaction mechanism of the esterases in Lepidoptera. The ester hydrolysis occurs in a two-step reaction plus water addition. There is first a nucleophilic attack produced by the serine hydroxyl on the carbonyl carbon of the pheromone. The reaction is then stabilized by the histidine, and this amino acid is stabilized by the glutamic acid at the same time. A molecule of alcohol is then released, and the enzyme is acetylated. Second, the water molecule has affinity with the histidine residue and then acts as a nucleophile on the acetylated enzyme. Finally, a carboxylic acid is released, and the enzyme is free to start a new reaction. Importantly, there are two conserved glycines participating in the stabilization of the transition states in the oxyanion hole (Montella et al., 2012).

Figure 3

Phylogenetic tree of esterases. SlCXE13 (S. littoralis), SlitCXE13 (S. litura), SexiCXE13 (S. exigua), SinfCXE13 (S. inferens), ApolPDE1 (A. polyphemus), EsemCXE6 (Eriocrania semipurpurella), PJAPPDE1 (Popillia japonica), and DmelEST6 (Drosophila melanogaster). Phylogenetic analyses for esterases were performed by using MAFFT sever for multiple sequence alignments and FastTree software for phylogenetic relationships based on maximum-likelihood method (Price et al., 2010).

Figure 4

Modeled structures of esterases in Lepidoptera and partial alignment. (A) ApolPDE1 from A. polyphemus, (B) EsemCXE6 from E. semipurpurella, (C) SinfCXE13 from S. inferens, and (D) SlitCXE13 from S. litura. (E–H) Active sites of ApolPDE1, EsemCXE6, SinfCXE13, and SlitCXE13, respectively. (I) Partial alignment of amino acids sequences. Amino acids not shown are represented by two sequential dots. Oxyanion hole (G104-G105-A184) is indicated by red arrows. G181-X-S183-X-G185 motif is indicated by black arrows. The catalytic triad [S188-E(D)313-H432] is indicated by the blue arrows. The program Modeler 9.15 (Sali and Blundell, 1993; Webb and Sali, 2016) was used to build the three-dimensional structures and 4FNM (ApolPDE1), 5CH3 (SinfCXE13 and SlitCXE13), and 5THM (EsemCXE6) as templates were used to obtain these modeled structures. Moreover, molecular dynamic (MD) simulations were performed using the NAMD 2.9 (Phillips et al., 2005) so as to achieve a refinement of the modeled structure via the root mean square deviation (RMSD).