Subunit neighbor interactions in enzyme kinetics: half-of-the-sites reactivity in a dimer.

Abstract

An isologous enzyme dimer is considered in which the subunits, if operating independently, would obey Michaelis-Menten kinetics. Because of neighbor interactions, the rate constants of the kinetic cycle in either subunit depend on the state (E or ES) of the other subunit. The steady-state behavior of this dimer system, with interactions, is investigated. In what is probably the most important special case, ES .cntdot. ES is destabilized considerably by the neighbor interaction compared to E .cntdot. ES. This leads to half-of-the-sites reactivity (1 subunit is in state ES; the other subunit cycles between E and ES), negative cooperativity and a considerable enhancement of enzyme activity relative to the activity of independent subunits.