Role of Hydrodynamic Shear on Activity and Structure of Proteins
1 Introduction
49
Proteins constitute one of the largest and most important class of products of
the biotechnological industry. Any parameter which affects either the yield or
recovery of protein thus becomes the focus of vast research. Proteins which are
composed of amino acids have a characteristic three dimensional configuration
due to their specific tertiary or quarternary structure. The activity of the protein
is closely related to its proper configuration. Any change in the physical
structure of the protein may lead to its denaturation and deactivation. Proteins
are produced by cells either intracellularly or they may be extracellular and are
secreted into the medium, they may also occur as complexes associated with the
cell membrane.
During the process of production and recovery proteins are subjected to
fluid forces which arise due to operations such as stirring, pumping and centrifugation. The hydrodynamic shear forces resulting due to fluid motion cause
damage to the large molecular weight proteins, resulting in denaturation and
inactivation of the protein [1]. This is a major concern as it affects the overall
efficiency of protein recovery and final yield of the product. A considerable
amount of research has been devoted to studying the effects of hydrodynamic
shear stress on proteins, especially with respect to the enzymes [2]. Enzymes are
subjected to shear stresses during their production in fermentors, during isolation and purification steps in downstream operations and also during their use
in enzyme reactors, especially if stirred reactors are employed to perform
enzyme catalysed reactions.
Although a large amount of literature exists pertaining to enzyme deactivation which deals with different parameters such as pH and temperature,
relatively little is known about the inactivation of enzymes due to hydrodynamic
shear stress. It is likely that enzymes are inactivated when exposed to an
air-liquid interface and that this effect is enhanced under conditions where
agitation is also present. A variety of proteins have been studied with respect to
their sensitivity to shear. Table 1 summarizes the different proteins and systems
used for subjecting proteins to shear. From a review of the literature it can be
seen that different types of proteins such as enzymes, antibodies, plasma proteins, milk and soya protein have been examined using diverse systems ranging
from simple shaking flasks, viscometers, capillary devices to stirred tank reactors. Furthermore it is clear that enzymes are the proteins of choice when
conducting experiments with shear stress, as the loss in catalytic activity resulting due to any changes in protein structure can be easily monitored. The other
important area studied has been the precipitation of proteins. Soya protein
precipitation has been the most commonly employed as it is easily available.
The present review discusses the effects of fluid shear stress on proteins
including enzymes. A brief description on deactivation has been included in
order to understand the effect of shear on the deactivation kinetics of proteins.
The model systems used to subject proteins to shear and some unit operations
1 Introduction
49
Proteins constitute one of the largest and most important class of products of
the biotechnological industry. Any parameter which affects either the yield or
recovery of protein thus becomes the focus of vast research. Proteins which are
composed of amino acids have a characteristic three dimensional configuration
due to their specific tertiary or quarternary structure. The activity of the protein
is closely related to its proper configuration. Any change in the physical
structure of the protein may lead to its denaturation and deactivation. Proteins
are produced by cells either intracellularly or they may be extracellular and are
secreted into the medium, they may also occur as complexes associated with the
cell membrane.
During the process of production and recovery proteins are subjected to
fluid forces which arise due to operations such as stirring, pumping and centrifugation. The hydrodynamic shear forces resulting due to fluid motion cause
damage to the large molecular weight proteins, resulting in denaturation and
inactivation of the protein [1]. This is a major concern as it affects the overall
efficiency of protein recovery and final yield of the product. A considerable
amount of research has been devoted to studying the effects of hydrodynamic
shear stress on proteins, especially with respect to the enzymes [2]. Enzymes are
subjected to shear stresses during their production in fermentors, during isolation and purification steps in downstream operations and also during their use
in enzyme reactors, especially if stirred reactors are employed to perform
enzyme catalysed reactions.
Although a large amount of literature exists pertaining to enzyme deactivation which deals with different parameters such as pH and temperature,
relatively little is known about the inactivation of enzymes due to hydrodynamic
shear stress. It is likely that enzymes are inactivated when exposed to an
air-liquid interface and that this effect is enhanced under conditions where
agitation is also present. A variety of proteins have been studied with respect to
their sensitivity to shear. Table 1 summarizes the different proteins and systems
used for subjecting proteins to shear. From a review of the literature it can be
seen that different types of proteins such as enzymes, antibodies, plasma proteins, milk and soya protein have been examined using diverse systems ranging
from simple shaking flasks, viscometers, capillary devices to stirred tank reactors. Furthermore it is clear that enzymes are the proteins of choice when
conducting experiments with shear stress, as the loss in catalytic activity resulting due to any changes in protein structure can be easily monitored. The other
important area studied has been the precipitation of proteins. Soya protein
precipitation has been the most commonly employed as it is easily available.
The present review discusses the effects of fluid shear stress on proteins
including enzymes. A brief description on deactivation has been included in
order to understand the effect of shear on the deactivation kinetics of proteins.
The model systems used to subject proteins to shear and some unit operations