Role of Hydrodynamic Shear on Activity and Structure of Proteins
53
associated monomers. The interactions of these monomers have a strong influence on enzymatic activity. Irreversible mechanisms occurring at higher protein
concentrations involve intermolecular interactions. These interactions occur via
formation of covalent, hydrogen and ionic bonds. They may on one hand
stabilize the structure of the active enzyme while their dissociation may expose
the hydrophobic amino acid residues which subsequently interact irreversibly to
form inactive aggregates [26].
A general model for enzyme deactivation would therefore include all possible inactivation reactions such as decomposition, dissociation, denaturation
and aggregation. It is evident that this would result in an extremely complex
situation. However, in most of the cases only one or two of the possible reactions
dominate under the real conditions leading to simple behaviour (i.e. real or
apparent first order kinetics). It should be noted that the slope of a first order
plot can have a wide variety of phenomenological meanings [20].
3 Deactivation in Model Contactors
Proteins are subjected to shear during different steps of their production and
recovery. These may occur in process equipment of varied geometric configuration. However, for carrying out studies on the effects of shear, proteins have been
treated in model systems. These include cylindrical tubes and coaxial cylinder
viscometers. Flow through cylindrical tubes is often encountered in protein
isolation and purification operations, such as flow through pumps and ultrafiltration modules. The coaxial cylinder viscometer has been widely used to
study the effect of shear on proteins as well as cells because it is possible to
generate defined shear stresses in the laminar or turbulent region. These model
contactors will be discussed in detail below.
3.1 Cylindrical Tubes
Fully developed laminar flow in a cylindrical tube is one dimensional and has
a parabolic velocity profile. The shear stress in the tube varies linearly with the
radial distance r. The shear stress is zero at the center of the tube and is
maximum along the tube wall and is given as:
z = r6P/2L
(7)
where r is the radial distance from the tube axis, 6P is the pressure drop between
two points situated at distance L. The flow in the tube is turbulent above the
critical Reynolds number of about 2100.
Protein solutions have been subjected to shear by passing them through
capillary tubes of varying diameter under different conditions. Charm and
Wong [27] investigated shear damage in the case of three different enzymes viz.
53
associated monomers. The interactions of these monomers have a strong influence on enzymatic activity. Irreversible mechanisms occurring at higher protein
concentrations involve intermolecular interactions. These interactions occur via
formation of covalent, hydrogen and ionic bonds. They may on one hand
stabilize the structure of the active enzyme while their dissociation may expose
the hydrophobic amino acid residues which subsequently interact irreversibly to
form inactive aggregates [26].
A general model for enzyme deactivation would therefore include all possible inactivation reactions such as decomposition, dissociation, denaturation
and aggregation. It is evident that this would result in an extremely complex
situation. However, in most of the cases only one or two of the possible reactions
dominate under the real conditions leading to simple behaviour (i.e. real or
apparent first order kinetics). It should be noted that the slope of a first order
plot can have a wide variety of phenomenological meanings [20].
3 Deactivation in Model Contactors
Proteins are subjected to shear during different steps of their production and
recovery. These may occur in process equipment of varied geometric configuration. However, for carrying out studies on the effects of shear, proteins have been
treated in model systems. These include cylindrical tubes and coaxial cylinder
viscometers. Flow through cylindrical tubes is often encountered in protein
isolation and purification operations, such as flow through pumps and ultrafiltration modules. The coaxial cylinder viscometer has been widely used to
study the effect of shear on proteins as well as cells because it is possible to
generate defined shear stresses in the laminar or turbulent region. These model
contactors will be discussed in detail below.
3.1 Cylindrical Tubes
Fully developed laminar flow in a cylindrical tube is one dimensional and has
a parabolic velocity profile. The shear stress in the tube varies linearly with the
radial distance r. The shear stress is zero at the center of the tube and is
maximum along the tube wall and is given as:
z = r6P/2L
(7)
where r is the radial distance from the tube axis, 6P is the pressure drop between
two points situated at distance L. The flow in the tube is turbulent above the
critical Reynolds number of about 2100.
Protein solutions have been subjected to shear by passing them through
capillary tubes of varying diameter under different conditions. Charm and
Wong [27] investigated shear damage in the case of three different enzymes viz.