Page 127 - Lignocellulosic Biomass to Liquid Biofuels

Bioconversion of lignocellulosic biomass to bioethanol and biobutanol 101

charged anions and protons, due to their higher intracellular pH (typically
well above the pK a of organic acids). The protons release in cytoplasm
causes the acidification intracellular [257,258]. In order to keep constant
intracellular pH (pHi), the excessive proton accumulation is pumped out
of the cells by various mechanisms, including proton translocation with
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the plasma membrane H -ATPase mediated by ATP hydrolysis. It means
a depletion of ATP levels that could be used to form biomass and, conse-
quently, inhibit a cellular growth [254]. The increase of the PHi acidity
affects the purine bases integrity, causing DNA and RNA damage and
denaturation of essential enzymes inside the cell, such as glycolysis
enzymes and, in particular, the phosphofructokinase that is sensitive to
low pH in vitro [259,260]. Although HS of weak acids is lipophilic, the
polar anionic forms of weak acids are lipophobic and cannot readily across
the lipid bilayer of membrane plasma. On the anion accumulation theory,
anionic forms of the weak acids are captured within the cells where can
reach toxic concentrations [254]. The buildup of anions formed by disso-
ciation of weak acids results in growth of potassium ions transport within
the cell and, consequently, in a turgor pressure increased. For the purpose
of maintaining a constant osmotic pressure and cell volume, more gluta-
mate is translocated out of the cell in the opposite direction to the potas-
sium, disrupting the cytoplasm osmolarity, impairing important metabolic
processes and reducing the cell’s growth potential and viability [261]. The
pHi acidification and the anions accumulation could also affect different
enzymes involved in protein synthesis and decrease the aromatic amino
acid uptake from culture medium through inhibition of amino acid per-
mease [262]. However, alterations of the structure of the plasma mem-
brane or cell wall and challenges to cellular energy balance while
maintaining the pHi could also result in the cellular growth inhibition by
weak organic acids [263].
Similar to furfural toxicity, low concentrations of weak acids
(,100 mmol/L) were shown to enhance ethanol yield to pH value of
around 5.5, whereas higher concentrations of weak acids are correlated
with strong inhibitory effects on cell growth and fermentation process
[262]. The weak acids toxicity depend also on their structure; for instance,
the toxicity of two weak acids, with equal pK a values, increases with their
carbon chain length, affecting lipophilicity. The exact mechanisms that yeast
uses to adapt to weak acids are still unknown. S. cerevisiae utilizes the plasma
1
membrane H -ATPase Pma1p to move protons out of the cell, increasing
the proton pumping capacity of the cell and induces the ATP-binding