Among the proteins detected in 0.5% glucose we identified a
protein involved in the synthesis of amino acids: 5-methyltetrahydropteroyltriglutamate-
homocysteinemethyltransferase. Moreover,
we found the cytoplasmatic and nuclear poly (A)-binding protein
involved in the regulation of peptide formation on ribosomes. In our
study the naturally induced diauxic shift resulted in an induction of
the global protein synthesis. This, in turn, allows the de novo
biosynthesis of functional mitochondria. Glucose restriction also
induces the synthesis of a few proteins necessary for respiratory
growth such as the mitochondrial ATP synthase subunit alpha that
produces mitochondrial ATP. This confirmed the induction of
respiration during the diauxic shift.
In the current study we detected that, during the first hours of
fermentation, the expression of proteins involved in response to
oxidative stress is increased in yeast cells. In particular, we found two
members of the heat shock protein 70 family (hsp70) which are
stress-induced and not detected in 0.5% glucose: ssa2p and ssb1p.
These stress response proteins can indicate a better adaptation during
fermentation and be important for maintaining the viability of cells
during fermentative conditions. Changes in proteins involved in
energetic metabolism and oxidative stress control may reflect a
proteome adaptation as a response to perturbation of protein levels
caused by stimuli of different origin [41].
In our study we see that, in high glucose, the percentage of glucose
used for fermentation is low compared to total glucose consumption
and that the ethanol production is inhibited. We deduced that glucose
was also used to produce other molecules, such as glycerol. Further
evidence was observed, since adh1p, an enzyme related to ethanol
production, was down-regulated. Moreover, we found an increase in
the expression of many proteins involved in glycerol biosynthesis. We
determined that DL-glycerol-3-phosphatase1 (involved in glycerol
biosynthesis) was expressed exclusively in this condition. This is
induced in response to osmotic stress. These results confirmed the
theory according to whom S. cerevisiae accelerates its production of
glycerol and acidic compounds rather than synthesizing ethanol to
minimize the effects of the high osmotic conditions generated by a
high glucose concentration and ethanol production [33]. We can
conclude that yeast cells in high glucose concentration have a
decreased growth rate during the initial phase of fermentation
because part of their metabolism is occupied in the synthesis of
compounds to resist osmotic stress.