Environmentally benign process for the preparation of antimicrobial α-methylene-β-hydroxy-γ-butyrolactone (tulipalin B) from tulip biomass

Identification of tuliposides K-M in tulip bulbs via an enzyme reaction-based screening method using a tuliposide-converting enzyme

Tuliposides (Pos) are major defense-related secondary metabolites in tulip, having 4-hydroxy-2-methylenebutanoyl and/or (3S)-3,4-dihydroxy-2-methylenebutanoyl groups at the C-1 and/or C-6 positions of d-glucose. The acyl group at the C-6 position is converted to antimicrobial lactones (tulipalins) by an endogenous Pos-converting enzyme. Based on this enzyme activity, we examined tulip bulb extracts and detected HPLC peaks that disappeared following the reaction by the Pos-converting enzyme. Spectroscopic analyses of the three purified compounds revealed that one of them was a glucose ester-type Pos, while the other two were identified as a glucoside ester-type Pos. These compounds were designated as PosK, L, and M. They were specific to bulbs, with the highest content in the outermost layer, but they were markedly less abundant than PosG, the minor bulb Pos we identified earlier. The study results suggest that tulip bulbs contain at least four minor Pos in addition to the major 6-PosA. Although PosK-M were present in almost all of the tested tulip cultivars, they were detected in only a few wild species, indicative of their potential utility as chemotaxonomic markers in tulip. Identification of PosK-M as 6-PosA derivatives unveils the biosynthetic diversity of Pos, the well-known group of secondary metabolites in tulip.

Occurrence of Z-2-oxo-4-methyl-3-pentene-1,5-dioic acid and its regioisomer 4-methylene-2-oxo-glutaric acid in tulip tissues

Although Z-2-oxo-4-methyl-3-pentene-1,5-dioic acid (Z-OMPD) has been identified as a major dicarboxylic acid in tulip tissues, its biosynthetic pathway has not been elucidated. Herein, Z-OMPD was isolated from tulip leaves and chemically synthesized. Comparisons of these samples revealed that Z-OMPD exists as a tautomeric mixture at physiological pH. As a regioisomer of Z-OMPD, we enzymatically and chemically prepared 4-methylene-2-oxo-glutaric acid (4-MEOG) for the first time. Using these compounds as standards, the occurrence of Z-OMPD and 4-MEOG in various tissues of the tulip cultivar "Murasakizuisho" was evaluated directly and by 2,4-dinitrophenylhydrazone derivatization. Z-OMPD was found to be abundant in the aerial tissues, whereas 4-MEOG was almost absent from all tissues. Stability analyses of Z-OMPD and 4-MEOG revealed that no double bond isomerization occurred at physiological pH, suggesting that enzyme systems are responsible for Z-OMPD biosynthesis in tulip tissues.

Identification of tuliposide G, a novel glucoside ester-type tuliposide, and its distribution in tulip

Tuliposides (Pos) are major defensive secondary metabolites in tulip (genus Tulipa), having 4-hydroxy-2-methylenebutanoyl and/or (3S)-3,4-dihydroxy-2-methylenebutanoyl groups at the C-1 and/or C-6 positions of d-glucose. The acyl group at the C-6 position is converted to antimicrobial lactones, tulipalins, by tuliposide-converting enzymes (TCEs). In the course of a survey of tulip tissue extracts to identify novel Pos, we found a minute high-performance liquid chromatography peak that disappeared following the action of a TCE, and whose retention time differed from those of known Pos. Spectroscopic analyses of the purified compound, as well as its enzymatic degradation products, revealed its structure as 5″-O-(6-O-(4'-hydroxy-2'-methylenebutanoyl))-β-d-glucopyranosyl-(2″R)-2″-hydroxymethyl-4″-butyrolactone, which is a novel glucoside ester-type Pos. We gave this compound the trivial name 'tuliposide G' (PosG). PosG accumulated in bulbs, at markedly lower levels than 6-PosA (the major Pos in bulbs), but was not found in any other tissues. Quantification of PosG in bulbs of 52 types of tulip, including 30 cultivars (Tulipa gesneriana) and 22 wild Tulipa spp., resulted in the detection of PosG in 28 cultivars, while PosG was present only in three wild species belonging to the subgenus Tulipa, the same subgenus to which tulip cultivars belong, suggesting the potential usefulness of PosG as a chemotaxonomic marker in tulip.

Isolation and identification of tuliposides D and F from tulip cultivars

6-Tuliposides A (6-PosA) and B (6-PosB) are major defensive secondary metabolites in tulip cultivars (Tulipa gesneriana), having an acyl group at the C-6 position of d-glucose. Although some wild tulip species produce 1,6-diacyl-glucose type of Pos (PosD and PosF), as well as 6-PosA/B, they have not yet been isolated from tulip cultivars. Here, aiming at verifying the presence of PosD and PosF in tulip cultivars, tissue extracts of 25 cultivars were analyzed by high-performance liquid chromatography (HPLC). Although no HPLC peaks for PosD nor PosF were detected in most cultivars, we found two cultivars giving a minute HPLC peak for PosD and the other two cultivars giving that for PosF. PosD and PosF were then purified from petals of cultivar 'Orca' and from pistils of cultivar 'Murasakizuisho', respectively, and their identities were verified by spectroscopic analyses. This is the first report that substantiates the presence of 1,6-diacyl-glucose type of Pos in tulip cultivars.

Substrate specificity of tuliposide-converting enzyme, a unique non-ester-hydrolyzing carboxylesterase in tulip: Effects of the alcohol moiety of substrate on the enzyme activity

6-Tuliposides A (PosA) and B (PosB) are glucose esters accumulated in tulip (Tulipa gesneriana) as major defensive secondary metabolites. Pos-converting enzymes (TgTCEs), which we discovered previously from tulip, catalyze the conversion reactions of PosA and PosB to antimicrobial tulipalins A (PaA) and B (PaB), respectively. The TgTCEs, belonging to the carboxylesterase family, specifically catalyze intramolecular transesterification, but not hydrolysis. In this report, we synthesized analogues of Pos with various alcohol moieties, and measured the TgTCE activity together with a determination of the kinetic parameters for these analogues with a view to probe the substrate recognition mechanism of the unique non-ester-hydrolyzing TgTCEs. It was found that d-glucose-like structure and number of the hydroxyl group in alcohol moiety are important for substrate recognition by TgTCEs. Among the analogues examined, 1,2-dideoxy analogues of PosA and PosB were found to be recognized by the TgTCEs more specifically than the authentic substrates by lowering K_m values. The present results will provide a basis for designing simple, stable synthetic substrate analogues for crystallographic analysis of TgTCEs.

One-Step Enzymatic Synthesis of 1-Tuliposide A Using Tuliposide-Converting Enzyme

6-Tuliposides A (6-PosA) and B (6-PosB) are major secondary metabolites in tulip (Tulipa gesneriana), having an acyl group at the C-6 position of D-glucose. They serve as precursors of the antimicrobial α-methylene-γ-butyrolactones tulipalins A (PaA) and B (PaB). The conversions of 6-PosA/6-PosB to PaA/PaB are catalyzed by tuliposide-converting enzymes A and B (TCEA and TCEB), respectively. A minor Pos, 1-PosA, which has the acyl group at the C-1 position of D-glucose, has been identified in some wild tulip species, but availability of this compound is limited. Here, by using the TCEs, we established a facile enzymatic process for 1-PosA synthesis from the naturally occurring 1,6-diacyl-glucose type of Pos (PosD and PosF). We first discovered that TCEA and TCEB react preferentially with PosD and PosF, respectively, to form 1-PosA and the corresponding Pa derived from the 6-acyl group, demonstrating that the TCEs specifically acted on the 6-acyl group, but not the 1-acyl group, of the substrates. Using TCEB, 300 mg of PosF was completely converted to 1-PosA and PaB in 10 min at room temperature. Then, 160 mg of 1-PosA (75% molar yield) was purified by column chromatography. This one-step enzymatic process dramatically improves accessibility to 1-PosA.

Molecular diversity of tuliposide B-converting enzyme in tulip (Tulipa gesneriana): identification of the third isozyme with a distinct expression profile

6-Tuliposide B (PosB), a major secondary metabolite that accumulates in tulip (Tulipa gesneriana), is converted to the antibacterial lactone, tulipalin B (PaB), by PosB-converting enzyme (TCEB). TgTCEB1 and TgTCEB-R, which encode TCEB, are specifically expressed in tulip pollen and roots, respectively, but are hardly expressed in other tissues (e.g. leaves) despite the presence of substantial PosB-converting activity, suggesting the existence of another TCEB isozyme. Here, we describe the identification of TgTCEB-L (“L” for leaf), a paralog of TgTCEB1 and TgTCEB-R, from leaves via native enzyme purification. The enzymatic characters of TgTCEB-L, including catalytic activity and subcellular localization, were substantially the same as those of TgTCEB1 and TgTCEB-R. However, TgTCEB-L did not exhibit tissue-specific expression. Identification of TgTCEB-L explains the PosB-converting activity detected in tissues where TgTCEB1 and TgTCEB-R transcripts could not be detected, indicating that tulip subtilizes the three TgTCEB isozymes depending on the tissue.

Molecular diversity of tuliposide B-converting enzyme in tulip (Tulipa gesneriana): identification of the root-specific isozyme

6-Tuliposide B (PosB) is a glucose ester accumulated in tulip (Tulipa gesneriana) as a major secondary metabolite. PosB serves as the precursor of the antimicrobial lactone tulipalin B (PaB), which is formed by PosB-converting enzyme (TCEB). The gene TgTCEB1, encoding a TCEB, is transcribed in tulip pollen but scarcely transcribed in other tissues (e.g. roots) even though those tissues show high TCEB activity. This led to the prediction of the presence of a TCEB isozyme with distinct tissue specificity. Herein, we describe the identification of the TgTCEB-R gene from roots via native enzyme purification; this gene is a paralog of TgTCEB1. Recombinant enzyme characterization verified that TgTCEB-R encodes a TCEB. Moreover, TgTCEB-R was localized in tulip plastids, as found for pollen TgTCEB1. TgTCEB-R is transcribed almost exclusively in roots, indicating a tissue preference for the transcription of TCEB isozyme genes.

Function and application of a non-ester-hydrolyzing carboxylesterase discovered in tulip

Plants have evolved secondary metabolite biosynthetic pathways of immense rich diversity. The genes encoding enzymes for secondary metabolite biosynthesis have evolved through gene duplication followed by neofunctionalization, thereby generating functional diversity. Emerging evidence demonstrates that some of those enzymes catalyze reactions entirely different from those usually catalyzed by other members of the same family; e.g. transacylation catalyzed by an enzyme similar to a hydrolytic enzyme. Tuliposide-converting enzyme (TCE), which we recently discovered from tulip, catalyzes the conversion of major defensive secondary metabolites, tuliposides, to antimicrobial tulipalins. The TCEs belong to the carboxylesterase family in the α/β-hydrolase fold superfamily, and specifically catalyze intramolecular transesterification, but not hydrolysis. This non-ester-hydrolyzing carboxylesterase is an example of an enzyme showing catalytic properties that are unpredictable from its primary structure. This review describes the biochemical and physiological aspects of tulipalin biogenesis, and the diverse functions of plant carboxylesterases in the α/β-hydrolase fold superfamily.

Molecular identification of tuliposide B-converting enzyme: a lactone-forming carboxylesterase from the pollen of tulip

6-Tuliposides A (PosA) and B (PosB), which are the major secondary metabolites in tulip (Tulipa gesneriana), are enzymatically converted to the antimicrobial lactonized aglycons, tulipalins A (PaA) and B (PaB), respectively. We recently identified a PosA-converting enzyme (TCEA) as the first reported member of the lactone-forming carboxylesterases. Herein, we describe the identification of another lactone-forming carboxylesterase, PosB-converting enzyme (TCEB), which preferentially reacts with PosB to give PaB. This enzyme was isolated from tulip pollen, which showed high PosB-converting activity. Purified TCEB exhibited greater activity towards PosB than PosA, which was contrary to that of the TCEA. Novel cDNA (TgTCEB1) encoding the TCEB was isolated from tulip pollen. TgTCEB1 belonged to the carboxylesterase family and was approximately 50% identical to the TgTCEA polypeptides. Functional characterization of the recombinant enzyme verified that TgTCEB1 catalyzed the conversion of PosB to PaB with an activity comparable with the native TCEB. RT-qPCR analysis of each part of plant revealed that TgTCEB1 transcripts were limited almost exclusively to the pollen. Furthermore, the immunostaining of the anther cross-section using anti-TgTCEB1 polyclonal antibody verified that TgTCEB1 was specifically expressed in the pollen grains, but not in the anther cells. N-terminal transit peptide of TgTCEB1 was shown to function as plastid-targeted signal. Taken together, these results indicate that mature TgTCEB1 is specifically localized in plastids of pollen grains. Interestingly, PosB, the substrate of TgTCEB1, accumulated on the pollen surface, but not in the intracellular spaces of pollen grains.

Organopolymerization of naturally occurring Tulipalin B: a hydroxyl-functionalized methylene butyrolactone

Naturally occurring, OH-containing, tri-functional Tulipalin B has been successfully polymerized by N-heterocyclic carbene and phosphazene superbase catalysts into polymers with M_n up to 13.2 kg mol⁻¹.