Page 4 of 18
| An
enzyme catalytic process is a cyclic reaction because the enzyme is recycled
at each turnover. A cyclic process will respond to a periodic driving force
with which the enzyme can interact. As a result of this interaction, the
enzyme will oscillate between its different conformational states. This
phenomenon has been shown to have an implication in cellular membrane processes.
To examine the cyclic behavior of an enzyme, we will consider the simple
Michaelis-Menten mechanism (scheme 1 of Fig. 1). The enzyme bonds to the
substrate to form an enzyme-substrate complex.
p. 321 {Fig. 1. Cyclic enzyme catalytic process. Many membrane processes mediated by receptors and enzymes exhibit kinetic characteristics similar to the {?-missing} of a Michaelis-Menten enzyme. Such an enzyme is susceptible to periodic perturbation. This paper considers how oscillating electric fields can interact with membrane ATPases and in so doing induce enzyme conformational oscillations, thus allowing utilization of the binding energy of ligands for catalyzing endergonic reactions. See text for details.} The
product is then released and the initial enzyme state is regenerated when
the complex dissociates. The driving force of this reaction is the negative
free energy of the S to P conversion. In fact, with a non-reversible step
at the product releasing step, the reaction is implied to proceed to the
left even if the free energy has a positive sign. Enzyme recycling has
a specific rate, given by the turnover rate of the Michaelis-Menten mechanism.
Generally, most investigators agree that there is another state preceding
the formation of the product, namely the enzyme-product complex, as shown
in scheme 2. If the two reversible steps are much faster than the dissociation
of the enzyme- product complex, the kinetics of scheme 2 will be indistinguishable
from that of scheme 1, and scheme 2 is in essence a Michaelis-Menten mechanism.
In the third scheme, we simply rewrite the second scheme in a more consistent
manner. It becomes a cyclic mechanism. Again, the reaction is driven by
the negative free energy of the S to P conversion, although the description
of the process is inherently unidirectional since it is shown to proceed
only in the clockwise direction. To be more precise, the enzyme state which
favors the binding substrate must be different from the state which favors
the binding of the product state. A distinction between E(1) and E(2) is
necessary. The Michaelis-Menten mechanism of scheme 1 is now more generally
written as scheme 4. However, scheme 4 has an inconsistency. We all accept
that without an additional (p. 322) (continued) |
