Optimization of factors influencing enzyme activity and product selectivity and the role of proton transfer in the catalytic mechanism of patchoulol synthase.

The patchoulol synthase (PTS) from Pogostemon cablin is a versatile sesquiterpene synthase and produces more than 20 valuable sesquiterpenes by conversion of the natural substrate farnesyl pyrophosphate (FPP). PTS has the potential to be used as a biocatalyst for the production of valuable sesquiterpenes such as (-)-patchoulol. The objective of the present study is to develop an efficient biotransformation and to characterize the biocatalytic mechanism of the PTS in detail. For this purpose, soluble PTS was prepared using an optimized cultivation protocol and continuous downstream process with a purity of 98%. The PTS biotransformation was then optimized regarding buffer composition, pH-value, and temperature for biotransformation as well as functional and kinetic properties to improve productivity. For the bioconversion of FPP, the highest enzyme activity was reached with the 2-(N-morphlino)ethanesulfonic acid (MES) buffer containing 10% (v/v) glycerol and 10 mM MgCl2 at pH 6.4 and 34°C. The PTS showed an unusual substrate inhibition for sesquiterpene synthases indicating an intermediate sesquiterpene formed in the active center. Deuteration experiments were used to gain further insights into the biocatalytic mechanism described in literature. Thus it could be shown that a second substrate binding site must be responsible for substrate inhibition and that further protonation and deprotonation steps are involved in the reaction mechanism.

Bioproduction of α-humulene in metabolically engineered Escherichia coli and application in zerumbone synthesis.

Zerumbone is a sesquiterpene ketone with potent anti-cancerogenic activities, produced in several ginger species of the Zingiberaceae familiy. We have investigated the biotechnological production of α-humulene, a precursor of zerumbone. By implementing a heterologous mevalonate pathway in combination with the α-humulene synthase expression, we effectively synthesized α-humulene from glucose in Escherichia coli. In this study, we developed a practical and efficient in situ separation method for α-humulene by comparison of extractive and adsorptive strategies. By the in situ adsorption of the product to the hydrophobic resin Amberlite® XAD4 we were able to increase α-humulene yield by 2310% to 60.2 mg/L. Furthermore we present an easy applicable, short subsequent chemical process for the conversion of α-humulene to zerumbone by using transition metal catalysis. To reduce process steps, the chemical reaction was carried out in the same solvent as the eluting solvent that was used to elute α-humulene from the adsorbent resin. By allylic oxidation of α-humulene with manganeseII chloride as a catalyst and tert.-butylhydroperoxide as an oxidizing agent we were able to synthetize zerumbone with a selectivity of 51.6%. Product and byproducts of the oxidation reaction were identified by GC-MS.

Heterologous Expression, Purification, and Biochemical Characterization of α-Humulene Synthase from Zingiber zerumbet Smith.

The α-humulene synthase from Zingiber zerumbet Smith was expressed as a polyhistidine-tagged protein in an E. coli BL21(DE3) strain. Induction time and inductor (isopropyl-ß-D-thiogalactopyranoside) concentration were optimized. The enzyme was successfully purified directly from cell lysate by NTA affinity column chromatography and careful selection of coordinated metal ion and imidazole elution conditions. Bioactivity assays were conducted with the natural substrate farnesyl diphosphate (FDP) in a two-phase system with in situ extraction of products. The conversion of FDP to α-humulene (~94.5%) and ß-caryophyllene (~5.5%) could be monitored by gas chromatography-flame ionization detection (GC-FID). Optimal pH and temperature as well as kinetic parameters K M and k cat were determined using a discontinuous kinetic assay.

Characterisation of a Recombinant Patchoulol Synthase Variant for Biocatalytic Production of Terpenes.

The patchoulol synthase (PTS) is a multi-product sesquiterpene synthases which is the central enzyme for biosynthesis of patchouli essential oil in the patchouli plant. Sesquiterpene synthases catalyse the formation of various complex carbon backbones difficult to approach by organic synthesis. Here, we report the characterisation of a recombinant patchoulol synthase complementary DNA (cDNA) variant (PTS var. 1), exhibiting significant amino acid exchanges compared to the native PTS. The product spectrum using the natural substrate E,E-farnesyl diphosphate (FDP) as well as terpenoid products resulting from conversions employing alternative substrates was analysed by GC-MS. In respect to a potential use as a biocatalyst, important enzymatic parameters such as the optimal reaction conditions, kinetic behaviour and the product selectivity were studied as well. Adjusting the reaction conditions, an increased patchoulol ratio in the recombinant essential oil was achieved. Nevertheless, the ratio remained lower than in plant-derived patchouli oil. As alternative substrates, several prenyl diposphates were accepted and converted in numerous compounds by the PTS var. 1, revealing its great biocatalytic potential.