Thesis (MSc)--Stellenbosch University, 2013. / ENGLISH ABSTRACT: Humanity is currently dependant on fossil fuels as an energy source. Increasing
economic development and industrialization is, however, raising the demand for this
unsustainable energy source. This increased pressure on dwindling reserves and
growing concern over detrimental environmental effects associated with the use of
these fuels have sparked great interest in the development of alternative sources.
Bioethanol has surfaced as a good alternative to fossil fuels, as it can be produced
from cheap, abundant, renewable, non-food sources. Bioethanol is also carbonneutral,
i.e. utilisation thereof leaves the net level of carbon dioxide in the
atmosphere unperturbed. Lignocellulose, more specifically its cellulose fraction, has
been identified as a possible feedstock for the production of bioethanol. The use of
lignocellulose as feedstock will allow for a more sustainable supply and much
needed energy security. Lignocellulosic feedstocks can be divided into two main categories, i.e. wastes from
processes other than fuel production and crops grown specifically for fuel production.
Cereal crops such as triticale have been identified as good industrial crops for the
production of energy. Triticale’s higher biomass yield, moderate water and nutrient
requirements, steadily increasing area of cultivation and main use as an animal feed
and not a human food source, makes it attractive as feedstock for the production of
bioethanol.
The combined activity of endoglucanases, exoglucanases and β-glucosidases is
needed to hydrolyse crystalline cellulose to fermentable sugars. The high cost of
these enzymes is, however, the most significant barrier to the economical production
of bioethanol from cellulosic biomass. A promising strategy for a reduction in costs
is the production of these cellulolytic enzymes, hydrolysis of biomass and
fermentation of the resulting sugars to bioethanol in a single process step via a
cellulolytic microorganism. The development of such a consolidated bioprocessing
(CBP) organism can be achieved by the introduction of cellulolytic activity into a
noncellulolytic microorganism that is able to ferment glucose to ethanol.
Saccharomyces cerevisiae is a good host candidate for CBP as this yeast’s high
tolerance towards ethanol and its use in industrial applications has been established.
The enzymatic activities of endoglucanases and exoglucanases are, however, inhibited by the build-up of cellobiose during the hydrolysis of cellulose. This effect
may be alleviated with the introduction of a better functioning β-glucosidase into the
system. β-Glucosidases hydrolyse cellobiose to glucose, alleviating the inhibition on
the enzymatic activities of endoglucanases and exoglucanases.
Despite advances in enzyme production systems and engineering enzymes currently
in use for higher stability and activity, there is still a demand to expand the current
collection of enzymes. Bioprospecting for novel cellulolytic enzymes focuses on
specific environment, with high turnover rates of cellulosic material or extreme
conditions, such as the composting process. These enzymes are becoming more
attractive compared to their mesophillic counterparts due to their potential industrial
applications and the fact that they represent the lower natural limits of protein
stability. / AFRIKAANSE OPSOMMING: Die mensdom is hoofsaaklik van fossielbrandstowwe as 'n energiebron afhanklik.
Toenemende ekonomiese ontwikkeling en industrialisasie verhoog egter die
aanvraag na hierdie onvolhoubare energiebron. Druk op kwynende reserwes en
groeiende kommer oor die nadelige gevolge vir die omgewing wat met die gebruik
van hierdie brandstowwe gepaard gaan, het tot groot belangstelling in die
ontwikkeling van alternatiewe bronne gelei. Bio-etanol is 'n goeie alternatief vir
fossielbrandstowwe, want dit kan van goedkoop, vollop, hernubare nievoedselbronne
geproduseer word. Bio-etanol is ook koolstof-neutraal; die gebruik
daarvan laat die netto vlak van koolstofdioksied in die atmosfeer onverstoord.
Lignosellulose, en meer spesifiek die sellulose fraksie, is as moontlike grondstof vir
die vervaardiging van bio-etanol geïdentifiseer. Die gebruik van lignosellulose as
grondstof sal meer volhoubare voorsiening en broodnodige energie-sekuriteit
verseker. Sellulose grondstowwe kan in twee hoof kategorieë verdeel word, nl. Newe
produkteafval van prosesse anders as brandstofproduksie en gewasse wat spesifiek
vir brandstofproduksie gekweek word. Graangewasse soos korog is geïdentifiseer
as 'n goeie industriële gewas vir die produksie van energie. Korog se hoër biomassa
opbrengs, matige water en voedingstofvereistes, groeiende bewerkingsgebied en die
gebruik as 'n veevoergewas eerder as 'n menslike voedselbron, maak dit aantreklik
as 'n grondstof vir die vervaardiging van bio-etanol.
Die gesamentlike aktiwiteit van endoglukanases, eksoglukanases en β-glukosidases
is nodig om kristallyne sellulose tot fermenteerbare suikers te hidroliseer. Die hoë
koste van hierdie ensieme is egter die grootste hindernis vir die ekonomiese
produksie van bio-etanol vanaf sellulosiese biomassa. 'n Belowende koste
verminderingstrategie is die produksie van hierdie sellulolitiese ensieme, die
hidrolise van biomassa, en die fermentasie van die suikers na bio-etanol in 'n
enkelstap-proses via 'n sellulolitiese mikro-organisme. Die ontwikkeling van so 'n
gekonsolideerde bioprosesserings (CBP) organisme kan deur die uitdrukking van
sellulolitiese aktiwiteite in 'n nie-sellulolitiese mikro-organisme wat wel in staat is om
glukose na etanol om te fermenteer, gerealiseer word. Saccharomyces cerevisiae is 'n goeie kandidaat gasheer vir CBP, omdat hierdie gis
‘n hoë verdraagsaamheid teenoor etanol toon en sy gebruik in industriële
toepassings gevestig is. Die ensiematiese aktiwiteite van endoglukanases en
eksoglukanases word egter deur die ophoop van sellobiose gedurende die hidrolise
van sellulose geïnhibeer. Hierdie effek kan met die byvoeging van meer effektiewe
β-glukosidases verlig word. β-Glukosidases hidroliseer sellobiose na glukose en
verlig dus die inhibisie op die endoglukanase en eksoglukanase ensiematiese
aktiwiteite.
Ten spyte van vooruitgang in ensiemproduksie stelsels en ensiemmodifiserings
strategieë wat tans vir hoër stabiliteit en aktiwiteit in gebruik is, bestaan daar steeds
'n behoefte om die bestaande versameling van ensieme uit te brei. Bioprospektering
vir nuwe sellulolitiese ensieme fokus op spesifieke omgewings, met hoë
omsetkoerse van sellulose materiaal of omgewings met uiterste toestande, soos die
komposterings-proses. Hierdie ensieme is besig om meer aantreklik in vergelyking
met hul mesofieliese eweknieë te raak as gevolg van hul potensiele industriële
toepassings en die feit dat hulle die laer natuurlike grense van proteïen-stabiliteit
verteenwoordig. / Stellenbosch University and the Technology Innovation Agency for financial support
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/79985 |
| Date | 03 1900 |
| Creators | Nel, De Wet Andries |
| Contributors | Volschenk, Heinrich, Stellenbosch University. Faculty of Science. Dept. of Microbiology. |
| Publisher | Stellenbosch : Stellenbosch University |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | English |
| Type | Thesis |
| Format | x, 81 p. : ill. |
| Rights | Stellenbosch University |
Page generated in 0.0024 seconds