Type III Polyketide synthases from Gerbera Hybrida

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http://urn.fi/URN:ISBN:978-951-51-2950-5
Title: Type III Polyketide synthases from Gerbera Hybrida
Author: Kontturi, Juha
Contributor: University of Helsinki, Faculty of Agriculture and Forestry, Department of Agricultural Sciences
Publisher: Helsingin yliopisto
Date: 2017-02-10
URI: http://urn.fi/URN:ISBN:978-951-51-2950-5
http://hdl.handle.net/10138/174094
Thesis level: Doctoral dissertation (article-based)
Abstract: The type III polyketide synthases (PKSs) are a group of enzymes involved in the production of structurally diverse secondary metabolites, polyketides in plants. Compounds, such as chalcones, chromones, stilbenes and coumarins possess a variety of important biological roles for the plants, such as protection against oxidative stress, pests and diseases, herbivory and UV irradiation. From the human use perspective, polyketides or their derivatives serve as antifungal compounds, immunosuppressants, anticancer drugs, antibiotics and insecticides. Type III PKS are small (40-45 kDa) proteins functioning as homodimers. They catalyze the sequential decarboxylative condensation reaction between malonyl-CoA and a variety of CoA-linked starter molecules in a biosynthesis, which closely resembles fatty acid biosynthesis. Gerbera (Gerbera hybrida) expresses three genes encoding chalcone synthases (CHS), which are enzymes involved in the biosynthesis of flavonoids in plants. In addition, it has three genes encoding 2-pyrone synthases (G2PS1-3), which differ from the CHSs with altered substrate specificity and the amount of decarboxylative malonyl-CoA condensation reactions. It was previously shown that G2PS1 is a triketide synthase, responsible for the biosynthesis of 4-hydroxy-6-methyl-2-pyrone (triacetolactone, TAL), a putative precursor for two antimicrobial compounds, gerberin and parasorboside in gerbera. In this study, the G2PS2 and G2PS3 were functionally characterized as pentaketide synthases and their role in the biosynthesis of another antimicrobial compound, 4-hydroxy-5methylcoumarin (HMC) was demonstrated. Using protein modelling and mutagenesis studies, the structural differences in the active site cavity of gerbera 2-pyrone synthases were demonstrated. The gerbera type III PKS family was extended by characterizing two additional 2-pyrone synthases and determining their expression patterns. The newly discovered GASCL1 and GASCL2 are anther specific chalcone synthase like enzymes (ASCLs), which in many other plant species have been shown to take part to the biosynthesis of sporopollenin, the main component of a exine layer of the pollen grain. The last part of the thesis focus on glycosyltransferses (GTs), a group of enzymes that catalyze the addition of a sugar moiety to the plant secondary metabolites, including the products of the gerbera type III PKSs.Sekundaarimetaboliitit ovat kasveissa tavattavia yhdisteitä, jotka toimivat pigmentteinä, UV-säteilyn suojana sekä taistelussa mikrobeja vastaan. Myös ihminen on kiinnostunut yhdisteistä, sillä kasvien sekundaarimetabolian tuotteita tai niiden johdoksia käytetään mm. antibiootteina, home- ja hyönteismyrkkyinä sekä lääkkeinä. Tyypin III polyketidisyntaasit ovat kooltaan suhteellisen pieniä (40-45 kDa), rakenteeltaan homodimeerisiä proteiineja. Entsyymit toimivat katalyytteinä biosynteesireaktioissa, jotka muistuttavat etäisesti rasvahappojen biosynteesiä. Tyypin III polyketidisyntaasien reaktiomekanismi tunnetaan suhteellisen hyvin. Reaktio käynnistyy lähtöaineen latauksella, malonyyli-KoA:n dekarboksylaatiolla sekä sen liittämisellä pitenevään polyketidiketjuun. Väitöskirjassani tutkin gerberassa tavattavia tyypin III polyketidisyntaasi-entsyymejä, jotka tuottavat polyketideiksi luokiteltuja sekundaarimetabolian tuotteita. Ensimmäinen julkaisuni koskee gerberan polyketidisyntaaseja, jotka ovat vastuussa antimikrobiaalisen yhdisteen biosynteesistä (4-hydroksi-5-metyylikumariini). Toisessa julkaisussani määritin kaksi muuta gerberan tyypin III polyketidisyntaasia, jotka osallistuvat siitepölyhiukkasen soluseinän biosynteesiin. Kolmas artikkeli koskee glukosyylitransferaaseja, entsyymejä jotka liittävät sokerimolekyylejä kasvien sekundaarimetaboliitteihin.
Subject: biotekniikka
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