Kinetic behavior of the general modifier mechanism of Botts and Morales with non-equilibrium binding

TL;DR

This paper analyzes the kinetic behavior of the general modifier mechanism in enzyme regulation, revealing that a modifier's role as activator or inhibitor can change in non-equilibrium conditions, with implications for drug effects.

Contribution

It introduces a non-equilibrium analysis of the modifier mechanism, revealing that the traditional classification of modifiers as activators or inhibitors depends on equilibrium assumptions.

Findings

Switching behavior occurs only in non-equilibrium steady states.

Drugs can switch from activators to inhibitors as concentration increases.

The net flux provides a biophysical measure of enzyme activity in NESS.

Abstract

In this paper, we perform a complete analysis of the kinetic behavior of the general modifier mechanism of Botts and Morales in both equilibrium steady states and non-equilibrium steady states (NESS). Enlightened by the non-equilibrium theory of Markov chains, we introduce the net flux into discussion and acquire an expression of product rate in NESS, which has clear biophysical significance. Up till now, it is a general belief that being an activator or an inhibitor is an intrinsic property of the modifier. However, we reveal that this traditional point of view is based on the equilibrium assumption. A modifier may no longer be an overall activator or inhibitor when the reaction system is not in equilibrium. Based on the regulation of enzyme activity by the modifier concentration, we classify the kinetic behavior of the modifier into three categories, which are named hyperbolic behavior, bell-shaped behavior, and switching behavior, respectively. We show that the switching phenomenon, in which a modifier may convert between an activator and an inhibitor when the modifier concentration varies, occurs only in NESS. Effects of drugs on the Pgp ATPase activity, where drugs may convert from activators to inhibitors with the increase of the drug concentration, are taken as a typical example to demonstrate the occurrence of the switching phenomenon.