4.5.2.2 Biphasic Kinetics (Nonasymptotic) For the purposes of this discus-
sion, a biphasic kinetic profile is defined as one in which the kinetic profile does
not follow saturation kinetics and has two distinct phases (Fig. 4.6). Note that
sigmoidal kinetics may also be biphasic but exhibits saturation.
At lower substrate concentrations the kinetic profile exhibits curvature
similar to a hyperbolic profile, however, as the substrate reaches higher
concentrations the profile increases linearly (instead of becoming asymptotic)
with no evidence of saturation. In multienzyme systems, this type of profile
may be observed when one of the operable enzymes exhibits a hyperbolic
profile while another enzyme’s actions result in a linear profile. However, when
this type of profile occurs using single enzyme systems (e.g., expressed enzyme
preps) the substrate is either binding in multiple productive orientations within
the enzyme active site, or there are two substrate molecules bound within the
enzyme active site. In this case, one of the binding orientations results in a low
Km, lowVmaxcomponent (responsible for the semihyperbolic nature of the
profile) while the other binding orientation produces the highKm–highVmax
component that results in the linear portion of the profile. As with modeling of
sigmoidal kinetics, one must use an equation that describes the kinetics of
multiple molecules of the same substrate interacting with multiple binding
regions within the enzyme active site, such as Equation 4.7.
v¼ðVm1½SÞþðCLintÞ½S^2
ðKm1þ½SÞð 4 : 7 ÞIn this case the parametersKm1andVm1represent the initial curved portion
of the plot that occurs at lower substrate concentrations (i.e., the lowKmand
lowVmportions of the profile). The CLintterm is used to describe the linear
FIGURE 4.6 Representative plot depicting biphasic kinetics (two phases with one not
achieving saturation).
ATYPICAL KINETICSALLOSTERIC EFFECTS 97