Role of Hydrodynamic Shear on Activity and Structure of Proteins
51
where E is the specific activity of the enzyme, kl is the deactivation rate
coefficient and the fractional rate of decrease of enzyme activity [( - 1/E)dE/dt-1
are both constant. This type of first order deactivation may be due to disruption
of either a single bond or sensitive structure [3, 4]. Enzyme inactivation is
characterized by fitting the enzyme activity versus time results to the equation
ln(a/ao) = -- kobst
(2)
where ao is the initial enzyme activity at time t = 0 and kob~ is the observed
inactivation rate constant [-5]. If this semilog plot is linear, it is assumed that the
enzyme inactivation is first order. It might be difficult to visualize that enzyme
molecules which are very complex structures exhibit simple inactivation kinetics. This has been explained on the basis that the disruption of a single bond or
structure might eliminate enzyme activity [-4]. However, there are several
reports in the literature to show that enzyme inactivation follows a complex
process and the simple first order model may not be sufficient to describe the
enzyme inactivation process [-6-9]. Enzymes are highly defined structures and
although certain variations may not affect the catalytic activity of the enzyme,
deviations from the native form are likely to alter the specific activity. Henley
and Sadana [-10] have reviewed the literature on enzyme deactivation curves for
various enzymes and used a series type enzyme deactivation [-6] scheme to
model complex enzyme deactivation. Deactivation was represented as a series of
reactions where E ~ E1 ~ Ed. The enzymatic activity
a = (E + Et)/Eo
(3)
= 1/k2 - kx [k2 exp( - kit) - k~exp( - k2t)]
(4)
and can be expressed as
a = (e + ~lel)/Eo
(5)
where cq = El~E, the ratio of the specific activities of the intermediate and the
initial enzyme states. This model visualizes an active enzyme precursor and
a final enzyme state with possible non-zero activity. The enzyme activity here is
a weighted function of the active enzyme states [-10]. The advantage of this
model is that it also allows for a rise in the activity, if ~ is greater than 1. This
may occur in cases where enzymes are stabilized or protected against deactivation [-11-15]. This mechanism may be extended to include a number of steps
and intermediate forms.
The series deactivation model assumes a single native state of the active
enzyme molecule. However, the possibility of different states of the enzyme
molecule being present initially cannot be ruled out, as in the case of isozymes
which arise due to substitution of amino acid residues and are thus not
interconvertible forms. Each of these may further deactivate by different pathways. The deactivation of these isozymes is described by a parallel mechanism
[16]. The deactivation of each of these active states may follow a different
pathway which in itself could be complex leading to a combination of parallel
51
where E is the specific activity of the enzyme, kl is the deactivation rate
coefficient and the fractional rate of decrease of enzyme activity [( - 1/E)dE/dt-1
are both constant. This type of first order deactivation may be due to disruption
of either a single bond or sensitive structure [3, 4]. Enzyme inactivation is
characterized by fitting the enzyme activity versus time results to the equation
ln(a/ao) = -- kobst
(2)
where ao is the initial enzyme activity at time t = 0 and kob~ is the observed
inactivation rate constant [-5]. If this semilog plot is linear, it is assumed that the
enzyme inactivation is first order. It might be difficult to visualize that enzyme
molecules which are very complex structures exhibit simple inactivation kinetics. This has been explained on the basis that the disruption of a single bond or
structure might eliminate enzyme activity [-4]. However, there are several
reports in the literature to show that enzyme inactivation follows a complex
process and the simple first order model may not be sufficient to describe the
enzyme inactivation process [-6-9]. Enzymes are highly defined structures and
although certain variations may not affect the catalytic activity of the enzyme,
deviations from the native form are likely to alter the specific activity. Henley
and Sadana [-10] have reviewed the literature on enzyme deactivation curves for
various enzymes and used a series type enzyme deactivation [-6] scheme to
model complex enzyme deactivation. Deactivation was represented as a series of
reactions where E ~ E1 ~ Ed. The enzymatic activity
a = (E + Et)/Eo
(3)
= 1/k2 - kx [k2 exp( - kit) - k~exp( - k2t)]
(4)
and can be expressed as
a = (e + ~lel)/Eo
(5)
where cq = El~E, the ratio of the specific activities of the intermediate and the
initial enzyme states. This model visualizes an active enzyme precursor and
a final enzyme state with possible non-zero activity. The enzyme activity here is
a weighted function of the active enzyme states [-10]. The advantage of this
model is that it also allows for a rise in the activity, if ~ is greater than 1. This
may occur in cases where enzymes are stabilized or protected against deactivation [-11-15]. This mechanism may be extended to include a number of steps
and intermediate forms.
The series deactivation model assumes a single native state of the active
enzyme molecule. However, the possibility of different states of the enzyme
molecule being present initially cannot be ruled out, as in the case of isozymes
which arise due to substitution of amino acid residues and are thus not
interconvertible forms. Each of these may further deactivate by different pathways. The deactivation of these isozymes is described by a parallel mechanism
[16]. The deactivation of each of these active states may follow a different
pathway which in itself could be complex leading to a combination of parallel