!
!
Figura 3: Potenciais estresses ambientais enfrentados pela Saccharomyces
cerevisiae durante o processo de produção de etanol. (Fonte: Bai, 2008)
Do ponto de vista molecular, linhagens industriais, quando comparadas às
linhagens de laboratório, são geneticamente mais complexas e não possuem
estabilidade em seu estado haplóide, sendo aneuploidias comuns em leveduras
industriais. Por outro lado, estas linhagens tem evoluído para uma melhor adaptação a
diferentes ambientes ou nichos ecológicos modificados, ou não, pela atividade
humana. Este processo é denominado “domesticação” e pode ser responsável por
certas características genéticas das linhagens industriais como, por exemplo, número
variado de ploidia e polimorfismo cromossômico (Lucena et al., 2007). A
modificação da constituição genética das linhagens industriais para uma melhor
adaptação ao ambiente industrial ocorre por meio de eventos como recombinação
mitótica entre seqüências homólogas, crossing-over mitótico e conversões gênicas.
Estas mudanças tendem a ser fixadas numa linhagem como resultado de uma pressão
seletiva para as características genéticas que melhor satisfazem àquelas condições
ambientais (Leite, 2008). Pretorious (2000) descreveu ainda a presença de
cromossomos extras em linhagens industriais e com ela a superexpressão
concomitante de genes, uma adaptação deste tipo de linhagem que poderia conferir
mais uma vantagem sobre as linhagens de laboratório (Zaldivar, 2002).
Portanto, devido ao longo processo de “domesticação”, as linhagens
industriais tem acumulado uma alta complexidade genômica. Esta complexidade tem
increased osmotic press ure, lower flux of pyruvate due
to the utilization of glycolytic intermediates subsequent
to the step in the pathway producing reduced NAD for
biosynthesis all can stimulate the conversion of dihy-
droxyacetone phosphate to glycerol ( Ingledew, 1999).
Other by-products such as organic acids and higher
alc oho ls are produced at much lower levels. The
production of these by-products as well as the growth
of yeast cells inevitably direct some glycolytic inter-
mediates to the corresponding metabolic pathways,
decreasing the ethanol yield to some extent. In the
industry, the ethanol yield that is calculated based on the
total sugar feeding into the fermentation system without
deduction of the residual sugar can be as high as 90–
93% of its theoretical value of ethanol to glucose
(Ingledew, 1999). Therefore, the residual sugar must be
controlled at a very low level. For example, no more
than 2 g l
−1
and 5 g l
−1
are controlled for the residual
reducing sugar and total sugar, respectively, in the
ethanol product ion from starch materials. Any ethanol
fermentation research which is expected to be practical
needs to bear these criteria.
During ethanol fermentations, yeast cells suffer from
various stresses. Some are environmental such as nu-
trient deficiency, high temperature and contamination,
while the others are from the yeast cell metabolism such
as ethanol accumulation and its corresponding inhibi-
tion on yeast cell grow th and ethanol production, es-
pecially under very high gravity (VHG) conditions that
will be discussed later. Fig. 3 summ arizes some of these
stresses. Many of them are synergistic, affecting yeast
cells more severely than any single one, leading to
reduced yeast viability and vigor as well as lower eth-
anol yield.
2.2. Zymomonas mobilis
Z. mobilis is an anaerobic, gram-negative bacterium
which produces ethanol from glucose via the Entner–
Doudoroff (ED) pathway in conjun ction with the en-
zymes pyruvate decarboxylase (PDC) and alcohol de-
hydrogenase (ADH) (Conway, 1992), as illustrated in
Fig. 4. This microorganism was originally discovered in
fermenting sugar-rich plant saps, e.g. in the traditional
pulque drink of Mexico, palm wines of tropical African,
or ripening honey (Swings and Deley, 1977).
Compared with the EMP pathway of S. cerevisiae,
which involves the cleavage of fructose-1, 6-bispho-
sphate by fructose bisphosphate aldolase to y ield one
molecule each of glyceraldehy des-3- phosphate and di-
hydroxyaceto ne pho sphate, the E D pathway forms
glyceraldehyde-3-phosphate and pyruvate by the
cleavage of 2-keto-3-deoxy -6-phosphogluconate by
2-keto-3-deo xy-gluconate aldolase, yielding only one
molecule ATP per glucose m olecule. As a conse-
quence, Z. mobilis pr oduces less biomass than S.
cerevisiae, and more car bon is funneled to the ethanol
fermentation. It was reporte d that the ethanol yield of
Z. mobilis could be as hig h as 97% of the theoretical
yield of ethanol to glucose (Sprenger, 1996), wh ile
only 90 – 9 3% can be achi eved for S. cerevi siae. Also,
as a con sequ ence of the lo w ATP yield, Z. mob ilis
maintains a higher glucose metabolic flux, and
correspondingly, guarantees its higher ethanol pro-
ductivity, normally 3– 5 folds higher than that of
S. cerevisiae (Sprenger, 1996).
Despite these advant ages, Z. mobilis is not suitable
for the industrial ethanol production. Firstly, this
species has a ve ry spec ific substrate spectrum includ-
ing only three sugars:
D-glucose, D-fructose, and
sucrose. Its growth on sucrose is accompanied by the
extracellular formation of fructose oligomers (levan)
and so rb itol, with a significant decrease in its ethanol
yield (Sp re nger, 1996) , making it u nsuitable for the
ethanol production from molasses. Since it can
effectively ferment only glucose in the hydrolysate of
starch m aterials, not other suga rs such as sucrose,
fructose and malto se form ed in the cooking and
saccharifying, it is also un suitable for the ethanol
pr oduction from sta rch materials. Th e etha nol fe rmen-
tation industry cannot use pure glucose as its raw
material like many re searchers did in their laboratory
studies. Secondly, although Z. mobilis is generally
regard ed as sa fe (GRAS) (Lin and Tanaka, 2006) , its
biomass is not commonly ac ceptable to be used as
animal feed, whic h inevitably generates the problem
for its biomass dispos al if it replaces S. cerevisiae in
Fig. 3. Potential environmental stresses on S. cerevisiae during ethanol
fermentation (Ingledew, 1999).
92 F.W. Bai et al. / Biotechnology Advances 26 (2008) 89–105