title: Identification of (Z)-3:(E)-2-Hexenal Isomerases Essential to the Production of the Leaf Aldehyde in Plants.
abstract: The green odor of plants is characterized by green leaf volatiles (GLVs) composed of C6 compounds. GLVs are biosynthesized from polyunsaturated fatty acids in thylakoid membranes by a series of enzymes. A representative member of GLVs (E)-2-hexenal, known as the leaf aldehyde, has been assumed to be produced by isomerization from (Z)-3-hexenal in the biosynthesis pathway; however, the enzyme has not yet been identified. In this study, we purified the (Z)-3:(E)-2-hexenal isomerase (HI) from paprika fruits and showed that various plant species have homologous HIs. Purified HI is a homotrimeric protein of 110 kDa composed of 35-kDa subunits and shows high activity at acidic and neutral pH values. Phylogenetic analysis showed that HIs belong to the cupin superfamily, and at least three catalytic amino acids (His, Lys, Tyr) are conserved in HIs of various plant species. Enzymatic isomerization of (Z)-3-hexenal in the presence of deuterium oxide resulted in the introduction of deuterium at the C4 position of (E)-2-hexenal, and a suicide substrate 3-hexyn-1-al inhibited HI irreversibly, suggesting that the catalytic mode of HI is a keto-enol tautomerism reaction mode mediated by a catalytic His residue. The gene expression of HIs in Solanaceae plants was enhanced in specific developmental stages and by wounding treatment. Transgenic tomato plants overexpressing paprika HI accumulated (E)-2-hexenal in contrast to wild-type tomato plants mainly accumulating (Z)-3-hexenal, suggesting that HI plays a key role in the production of (E)-2-hexenal in planta.
authors.affiliation: Graduate School of Agricultural Science, Kobe University, 1-1 Rokkodai, Nada, Kobe 657-8501, Japan.
authors.forename: Mikiko
authors.initials: M
authors.lastname: Kunishima
authors.affiliation: Graduate School of Agricultural Science, Kobe University, 1-1 Rokkodai, Nada, Kobe 657-8501, Japan. Electronic address: yamauchi@kobe-u.ac.jp.
authors.forename: Yasuo
authors.initials: Y
authors.lastname: Yamauchi
authors.affiliation: Graduate School of Agricultural Science, Kobe University, 1-1 Rokkodai, Nada, Kobe 657-8501, Japan.
authors.forename: Masaharu
authors.initials: M
authors.lastname: Mizutani
authors.affiliation: Graduate School of Agricultural Science, Kobe University, 1-1 Rokkodai, Nada, Kobe 657-8501, Japan.
authors.forename: Masaki
authors.initials: M
authors.lastname: Kuse
authors.affiliation: Graduate School of Agricultural Science, Kobe University, 1-1 Rokkodai, Nada, Kobe 657-8501, Japan.
authors.forename: Hirosato
authors.initials: H
authors.lastname: Takikawa
authors.affiliation: Graduate School of Agricultural Science, Kobe University, 1-1 Rokkodai, Nada, Kobe 657-8501, Japan.
authors.forename: Yukihiro
authors.initials: Y
authors.lastname: Sugimoto
mesh_terms: D000447:Aldehydes / Q000378:metabolism*; D000595:Amino Acid Sequence; D002212:Capsicum / Q000737:chemistry; D008401:Gas Chromatography-Mass Spectrometry; D007535:Isomerases / Q000737:chemistry / Q000378:metabolism*; D018551:Solanum lycopersicum / Q000378:metabolism; D018515:Plant Leaves / Q000378:metabolism*; D030821:Plants, Genetically Modified; D066244:Proton Magnetic Resonance Spectroscopy
chemical_list: D000447:Aldehydes; C051750:2-hexenal; D007535:Isomerases
keywords: enzyme mechanism; enzyme purification; lipid peroxidation; plant biochemistry; polyunsaturated fatty acid (PUFA)