FAD-dependent hexenal isomerases in Lepidoptera evolved convergently with plant-derived hexenal isomerases

Green leaf volatiles (GLVs) are six carbon volatile organic compounds that mediate plant responses to environmental stresses. The quantity and composition of emitted GLVs can vary with stress type, allowing plants to fine-tune their volatile blends. Additionally, insect herbivores are capable of modulating these emissions. A key mechanism underlying this plasticity is the conversion of Z-3-hexenal to E-2-hexenal by the enzyme (3Z):(2E)-hexenal isomerase (Hi), which reshapes GLV profiles and influences multi-trophic interactions. Here, we investigate the evolutionary origin, functional diversification, and catalytic mechanisms of Lepidopteran Hi homologs, which belong to the GMC oxidoreductase family. Phylogenetic analysis of 34 lepidopteran species identified a distinct GMCβ subclade enriched in Hi homologs, largely confined to the Apoditrysia lineage. Functional assays showed species-specific variation in Hi activity, with Manduca sexta Hi-1 displaying the highest activity under identical protein concentrations, both in vitro and in planta. Structural modeling and site-directed mutagenesis revealed that Hi activity requires a flavin adenine dinucleotide (FAD) cofactor enabling the identification of key residues critical for FAD binding. Comparative phylogenetics further suggests that Hi enzymes in plants and Lepidoptera evolved independently from unrelated enzyme families, representing a case of functional convergence during the Cretaceous angiosperm radiation.