Constitutive expression of enzymes in Pichia pastoris for use in lignin valorisation

Conacher, Cleo Gertrud (2018-03)

Thesis (MEng)--Stellenbosch University, 2018.

Thesis

ENGLISH SUMMARY: Lignin valorisation is a key breakthrough in developing efficient integrated biorefineries. Enzymatic modification of lignin wastes offers an environmentally benign, energy efficient avenue for lignin upgrading. However, for enzymatic lignin valorisation to be a viable option, large quantities of the selected enzymes are required at minimal cost. Heterologous enzyme production using recombinant P. pastoris is a well-studied method for the successful large-scale production of recombinant eukaryotic proteins. Considering the disadvantages associated with methanol-induction, the use of the constitutive PGAP promoter for heterologous protein production in P. pastoris is the preferred expression strategy. Three enzymes of interest for lignin degradation and modification were selected in this study for recombinant enzyme production: glucuronoyl esterase (GE), cellobiose dehydrogenase (CDH) and laccase (LCC). These enzymes were selected based on novelty and their broad range of functions in the lignocellulose biodegradation process. These enzymes are representative of carbohydrate degradation (GE, CDH: Fenton reaction) and lignin degradation and/or modification (LCC). The aim was to create, and provide fundamental data of, the capabilities of a constitutive P. pastoris expression system to produce three enzymes (CDH, LCC, and GE), that are associated with lignin valorisation. This study is rooted in aspects of molecular biology as well as bioprocess engineering. The DNA 2.0 (USA) pJexpress expression system, which is free of any proprietary restrictions, was used to create the expression constructs. The created recombinant P. pastoris strains were screened at shake-flask level, and based on these results, one strain for each enzyme was selected for further studies. The enzyme production process was scaled up to a 14 L bioreactor. A two-stage fermentation strategy, consisting of a batch phase, followed by a constant glycerol fed-batch stage was implemented. The fermentation culture was harvested and concentrated through a two-stage tangential filtration process, after which freezing was evaluated as a possible storage strategy. Glycerol was evaluated as a possible cryoprotectant. The constant glycerol feed strategy was shown to be effective, returning high biomass and protein yields. Results of the bioreactor fermentations showed similar biomass growth kinetics (μmax = 0.15 – 0.17 h-1) and biomass yields (119.54-136.47 gdcw/L) throughout the fermentation process for the three recombinant enzymes. High titres of recombinant protein were obtained, with the highest being glucuronoyl esterase, at 2778.01 mg/L, followed by cellobiose dehydrogenase, at 1489.3 mg/L, and lastly laccase, at 778.54 mg/L. The first incidence of constitutive expression of H. jecorina glucuronoyl esterase and N. crassa cellobiose dehydrogenase is reported here. In addition, the highest yield of constitutively expressed T. versicolor laccase lcc2 is also reported. Although the laccase fermentation returned the lowest productivity, preliminary experiments showed that lowering the fermentation temperature may improve this value by aiding in secretion and/or preventing temperature-related laccase degradation at 30 °C. No glycerol was accumulated during the glycerol fed-batch stage. A decline in growth rate was observed during the fed-batch stage due to constant feeding rate during biomass growth. However, since constitutive expression is largely growth associated, it may be beneficial to increase the glycerol feed rate in order to maintain the growth rate at values nearer to the maximum growth rate of the recombinant strains (0.15 – 0.17 h-1). Tangential flow filtration was successfully used to concentrate the cell-free enzyme extracts, where 5.20 – 6.01-fold increases in volumetric activity were obtained, with final volumetric activity yields of 49.57-60.04 %. It was found that glucuronoyl esterase was the most sensitive to volumetric activity loss after refrigeration and freezing, with a decrease of 10.71 % in volumetric activity after freezing at -20 °C. Cellobiose dehydrogenase showed no sensitivity to the freezing process, and laccase activity was successfully preserved with the use of 10 % glycerol as a cryoprotectant. This study has successfully reported the baseline capabilities of a constitutive P. pastoris expression system to produce three enzymes, namely cellobiose dehydrogenase, laccase, and glucuronoyl esterase, associated with lignin valorisation, using a bioreactor fermentation bioprocess. Even though an identical molecular and bioprocess strategy was used, there were significant differences in protein secretion of each enzyme, emphasising the effect of the selected gene on secretion levels in P. pastoris. The yields obtained in the current study may be improved at the microbiological level by improving secretion with the use of alternative secretion factors or gene coding sequences. The bioprocess may be improved by optimisation of the glycerol feed, to maintain the growth rate at a value nearer to the maximum growth rate of each strain. Further, preliminary data suggests that lowering the fermentation temperature may be beneficial for laccase production.

AFRIKAANSE OPSOMMING : Waardetoevoeging van lignien is a belangrike stap tot die ontwikkeling van doeltreffende geïntergreerde bio- raffinaderye. ‘n Moontlike omgewingsbewus, energie doeltreffende oplosing is die ensiematiese degradasie en/of modifikasie van lignienverrykte afval. Vir die ensiematiese omskakling van lignien om ‘n realistiese opsie te wees, groot skaal produksie moet ekonomies haalbaar wees in terme van kostes. Heteroloë produksie van ensieme in rekombinante P. pastoris is ‘n bekende metode vir die suksesvolle grootskaalse produksie van rekombinante eukariotiese proteïene. As die nadele wat verband hou met metanol-induseerbare promotors in ag geneem word, is die konstitutiewe PGAP promotor vir heteroloë produksie van proteïen in P. pastoris ‘n verkieslike uitdrukking strategie. Drie ensieme vir lignien afbraak en modifikasie was gekies vir rekombinante ensiem produksie: glucuronoyl esterase (GE), sellobiose dehidrogenase (SDH) en laccase (LCC). Die ensieme was gekies gebaseer op nuwigheid, asook hul wye verskeidenheid van funksies in terme van lignosellulose biodegradasie. Hierdie ensieme verteenwoordig beide koolhidraat degradasie (GE, CDH: Fenton reaksie) en lignien degradasie en/of modifikasie (LCC). Die doel van die studie was om fundamentele data te skep van P. pastoris se vermoë om die drie begenoemde ensieme uit te druk onder beheer van die konstitutiewe PGAP promotor, met die oog op lignien waardetoevoeging. Hierdie studie bevat aspekte van molekulêre biologie asook bioproses ingenieurswese. Die DNA 2.0 (VSA) pJexpress uitdrukking stelsel, wat vry van enige eiendomsregtelike beperkings is, was gebruik om die uitdrukkingskonstrukte te skep. Die rekombinante P. pastoris stamme was gekeur op skud-fles vlak, en een gisstam vir produksie van elke ensiem was gekies gebaseer op sekresieproduksie van die ensieme. Ensiem produksie was opgeskaal na 14 L bioreaktors toe, met ‘n twee-stadium fermentasie strategie, wat bestaan uit 'n enkellading fase, gevolg deur 'n konstante gliserolvoerfase. Die fermentasie kultuur is geoes en die supernatant gekonsentreer deur 'n twee-stadium filtrasie proses, waarna vries as 'n moontlike berging strategie geëvalueer was. Gliserol was ook as n moontlike kriobeskermingsmiddel geëvalueer. Die konstante gliserolvoerfase was doeltreffend, met hoë opbrengste in terme van biomassa en proteïenproduksie. Die bioreaktor fermentasies het soortgelyke biomassa groei-kinetika (μmax = 0.15 – 0.17 h-1) en -opbrengste (119.54-136.47 gdcw/L) getoon regdeur die fermentasie prosese. Hoë vlakke van rekombinante proteïen was verkry, GE het die hoogste vlakke gehad met 2778.01 mg/L, gevolg deur SDH met 1489.3 mg/L, en laastens LLC met 778.54 mg/L. Hierdie is die eerste studie wat na die konstitutiewe uitdrukking van H. jecorina GE en N. crassa SDH kyk, asook die hoogste gerapoteerde vlakke van konstitutiewe uitgedrukte T. versicolor laccase lcc2. Alhoewel die laccase fermentasie die laagste produktiwiteit teruggekeer het, dui voorlopige eksperimente doorop dat verlaging van die fermentasie temperatuur hierdie waarde kan verbeter, deur temperatuur-verwante degradasie van LCC te verhoed. Geen gliserol was versamel gedurende die gliserolvoerfase nie. Daar was ’n afname in groeitempo tydens die gliserolvoerfase as gevolg van konstante voedings tempo gedurende biomassa groei. Egter, sedert konstitutiewe uitdrukking grootliks groei geassosieer is, mag dit voordelig wees om die gliserol voedings tempo te verhoog, om die groeitempo so na as moontlik aan die maksimum groeitempo van die rekombinante stamme te hou (0.15 – 0.17 h-1). Tangensiale vloei filtrasie is suksesvol gebruik om die sel-vrye ensiem ekstrakte te konsentreer, waar 5.20 – 6.01-voud stygings in volumetriese aktiwiteit verkry is, met finale volumetriese aktiwiteit opbrengste van 49.57-60.04 %. Die sensitiefste ensiem na verkoeling en vries was GE met 'n afname van 10.71 % in volumetriese aktiwiteit na bevriesing by -20 ° C. Sellobiose dehidrogenase het geen sensitiwiteit getoon na die vries proses nie, en LCC se aktiwiteit was suksesvol behou met die gebruik van 10% gliserol as 'n kriopreserveermiddel. Hierdie studie rapporteer op 'n P. pastoris uitdrukking stelsel, gebaseer op die konstitutiewe PGAP promotor, en die stelsel se produsksie vermoë in ‘n bioreaktor fermentasie proses. Die vermoë van die stelsel was getoets met die uitdrukking van drie lignien waardetoevoegende ensieme, glucuronoyl esterase, sellobiose dehidrogenase en laccase. Ahoewel 'n identiese molekulêre en bioproses strategie gebruik was, was daar beduidende verskille in proteïen afskeiding van elke ensiem. Hierdie waarneming lê klem op die invloed van die geselekteerde gene op afskeiding vlakke in P. pastoris. Die proteïen opbrengste verkry in die huidige studie kan verbeter word op mikrobiologiese vlak deur afskeiding te verbeter met behulp van alternatiewe afskeiding faktore of gene manipulasie. Die bioproses kan verder verbeter word deur optimalisering van die gliserolvoerfase, en so die groeitempo nader aan die maksimum groeitempo van elke stam te hou. Voorlopige data dui ook daarop aan dat fermentasie temperatuur verlaging dalk voordelig vir laccase produksie sal wees.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/103618
This item appears in the following collections: