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C.B. Elias and J.B. Joshi
relatively more sensitive to shear deactivation. This was reflected in a rapid
deactivation of cellulase initially after which the deactivation rates became
constant. This shearing effect has been attributed to breakup or disturbance
in the tertiary structure of the cellulase enzyme [80, 81]. These results were
further supported by the studies of Sachse et al. [46] who investigated
shear effects during the production of cellulase using immobilized cells of
Trichoderma reesei. They observed that production of cellulase in a stirred
bioreactor which generated higher shear rates led to a decrease in the
enzyme produced and this was mainly due to damage of the CBH enzyme of
the cellulase complex. Changes in enzyme structure was also deduced by its
reduced adsorbability onto microcrystalline cellulose under higher conditions
of shear, van Tilbeburgh et al. [82], demonstrated that the decrease in the
ability of CBH to adsorb onto insoluble cellulose occurred when the enzyme
molecule was damaged by enzymatic cleavage of the carbohydrate-rich moiety
and this was concurrent with the decrease in enzyme activity towards crystalline
cellulose.
Changes in protein structure have also been observed in structural proteins
in cells. The cytoskeletal proteins show depolymerization and changes in polymer structure when treated to shear stress. This is important when cells of higher
eukaryotes such as plant and animal cells are considered, where proteins play an
important role in maintaining cell shape and hence integrity and function [83]
and has been reported in endothelial and other animal cells [84, 85]. A detailed
understanding of how shear forces modulate protein structure and hence
their stability would be extremely beneficial in designing and optimizing
bioprocesses.
6 Conclusions and Perspectives
Proteins are exposed to an environment of hydrodynamic stress during fermentation, cell homogenisation, membrane filtration, chromatography and during
pipeline transport. On a commercial basis flow is usually turbulent in all these
equipments. Since there is no published work on quantitative measurements of
Reynolds stresses, a very systematic investigation is needed in the future. Studies
relating to shear effects on proteins have been mainly carried out with enzymes
and furthermore, these studies have been restricted only to some industrially
important enzymes such as cellulase, catalase, rennet and alcohol dehydrogenase. Besides these, there are several other enzymes which are being increasingly
used in commercial applications such as proteases and lipases, where studies on
shear stability are not comprehensive. From the literature it can also be seen
that very few studies have been carried out on enzymes under fermentation
conditions. This is especially important in the cases where the enzyme secretion
C.B. Elias and J.B. Joshi
relatively more sensitive to shear deactivation. This was reflected in a rapid
deactivation of cellulase initially after which the deactivation rates became
constant. This shearing effect has been attributed to breakup or disturbance
in the tertiary structure of the cellulase enzyme [80, 81]. These results were
further supported by the studies of Sachse et al. [46] who investigated
shear effects during the production of cellulase using immobilized cells of
Trichoderma reesei. They observed that production of cellulase in a stirred
bioreactor which generated higher shear rates led to a decrease in the
enzyme produced and this was mainly due to damage of the CBH enzyme of
the cellulase complex. Changes in enzyme structure was also deduced by its
reduced adsorbability onto microcrystalline cellulose under higher conditions
of shear, van Tilbeburgh et al. [82], demonstrated that the decrease in the
ability of CBH to adsorb onto insoluble cellulose occurred when the enzyme
molecule was damaged by enzymatic cleavage of the carbohydrate-rich moiety
and this was concurrent with the decrease in enzyme activity towards crystalline
cellulose.
Changes in protein structure have also been observed in structural proteins
in cells. The cytoskeletal proteins show depolymerization and changes in polymer structure when treated to shear stress. This is important when cells of higher
eukaryotes such as plant and animal cells are considered, where proteins play an
important role in maintaining cell shape and hence integrity and function [83]
and has been reported in endothelial and other animal cells [84, 85]. A detailed
understanding of how shear forces modulate protein structure and hence
their stability would be extremely beneficial in designing and optimizing
bioprocesses.
6 Conclusions and Perspectives
Proteins are exposed to an environment of hydrodynamic stress during fermentation, cell homogenisation, membrane filtration, chromatography and during
pipeline transport. On a commercial basis flow is usually turbulent in all these
equipments. Since there is no published work on quantitative measurements of
Reynolds stresses, a very systematic investigation is needed in the future. Studies
relating to shear effects on proteins have been mainly carried out with enzymes
and furthermore, these studies have been restricted only to some industrially
important enzymes such as cellulase, catalase, rennet and alcohol dehydrogenase. Besides these, there are several other enzymes which are being increasingly
used in commercial applications such as proteases and lipases, where studies on
shear stability are not comprehensive. From the literature it can also be seen
that very few studies have been carried out on enzymes under fermentation
conditions. This is especially important in the cases where the enzyme secretion