Role of Hydrodynamic Shear on Activity and Structure of Proteins
69
is extracellular and hence the protein product will be exposed to the turbulent
conditions existing in the fermenter. Apart from enzymes, there are also several
biologically active proteins which are produced via bioprocesses including
vaccines, monoclonal antibodies and interferons. These proteins have high
molecular weights and complex three dimensional structures. It is therefore
important to know the effect of hydrodynamic forces on these proteins as the
native conformation of these structures is important for their bioactivity. Considering the importance of these biomolecules in the future generation of
therapeutics it is important to establish a direct relationship between hydrodynamic shear and structure of proteins. Hence, future work should include
simultaneous measurement of stresses and protein activity and monitoring the
changes in the structure.
From a review of the literature it can be deduced that although proteins
appear to be more robust than cells (microbial, plant or animal) with respect to
shear damage due to fluid forces, in the presence of air they are rapidly
denatured. This effect can be greatly enhanced under conditions where a gasliquid interface occurs along with agitation. This condition is commonly observed in the aerobic fermentations for the production of extracellular
enzymes and proteins. In the case of gas-liquid dispersions the turbulence
structure is even more complex. Fortunately techniques are now available
(laser Doppler anemometry and its modified versions) for the measurement
of stresses in such complex systems. Future studies in this area should include
a detailed investigation of stress in gas-liquid dispersions. Earlier workers
1-313 have shown that the addition of stabilization agents such as Zonyl,
Triton X-100 and bovine serum albumin can prevent protein deactivation in the
presence of a gas-liquid interface. Measurements of shear stresses are needed
to investigate these systems together with their biological effects to enable us
to develop a rationale for the selection of additives (and their concentration)
for given applications.
A complete understanding of the shear stresses and their effects on the
structure and function of proteins will help develop new designs of equipment
(such as fermentors, cell homogenisers, solid-liquid separation devices and
pumps) which generate stresses within the permissible levels without compromising on the efficiency for the purpose for which they are to be used. For
instance in the case of fermenters, it will be possible to evolve new impeller
designs which generate low levels of stresses and still give fairly high rates
of oxygen dissolution. Similarly, in the case of separation operations, such
as precipitation, there are no models which completely describe the process.
The influence of shear stress, especially during the aging of precipitates, on
the final particle size of precipitate and its subsequent separation by centrifugation also needs to be studied in detail. Such rational designs of equipment for bioprocesses assume greater significance in view of the increased
number of protein products which are being produced commercially with
applications in several areas including food, pharmaceuticals, agriculture
and health care.
69
is extracellular and hence the protein product will be exposed to the turbulent
conditions existing in the fermenter. Apart from enzymes, there are also several
biologically active proteins which are produced via bioprocesses including
vaccines, monoclonal antibodies and interferons. These proteins have high
molecular weights and complex three dimensional structures. It is therefore
important to know the effect of hydrodynamic forces on these proteins as the
native conformation of these structures is important for their bioactivity. Considering the importance of these biomolecules in the future generation of
therapeutics it is important to establish a direct relationship between hydrodynamic shear and structure of proteins. Hence, future work should include
simultaneous measurement of stresses and protein activity and monitoring the
changes in the structure.
From a review of the literature it can be deduced that although proteins
appear to be more robust than cells (microbial, plant or animal) with respect to
shear damage due to fluid forces, in the presence of air they are rapidly
denatured. This effect can be greatly enhanced under conditions where a gasliquid interface occurs along with agitation. This condition is commonly observed in the aerobic fermentations for the production of extracellular
enzymes and proteins. In the case of gas-liquid dispersions the turbulence
structure is even more complex. Fortunately techniques are now available
(laser Doppler anemometry and its modified versions) for the measurement
of stresses in such complex systems. Future studies in this area should include
a detailed investigation of stress in gas-liquid dispersions. Earlier workers
1-313 have shown that the addition of stabilization agents such as Zonyl,
Triton X-100 and bovine serum albumin can prevent protein deactivation in the
presence of a gas-liquid interface. Measurements of shear stresses are needed
to investigate these systems together with their biological effects to enable us
to develop a rationale for the selection of additives (and their concentration)
for given applications.
A complete understanding of the shear stresses and their effects on the
structure and function of proteins will help develop new designs of equipment
(such as fermentors, cell homogenisers, solid-liquid separation devices and
pumps) which generate stresses within the permissible levels without compromising on the efficiency for the purpose for which they are to be used. For
instance in the case of fermenters, it will be possible to evolve new impeller
designs which generate low levels of stresses and still give fairly high rates
of oxygen dissolution. Similarly, in the case of separation operations, such
as precipitation, there are no models which completely describe the process.
The influence of shear stress, especially during the aging of precipitates, on
the final particle size of precipitate and its subsequent separation by centrifugation also needs to be studied in detail. Such rational designs of equipment for bioprocesses assume greater significance in view of the increased
number of protein products which are being produced commercially with
applications in several areas including food, pharmaceuticals, agriculture
and health care.