Regulatory behavior of monomeric enzymes. 1. The mnemonical enzyme concept.

Abstract

A new concept, that of a mnemonical transition, is proposed to explain the departure from Michaelian behavior of monomeric enzymes following ordered reaction mechanisms. The concept integrates three simple ideas: the free enzyme occurs under two conformational states in equilibrium; the collision of any of these forms with the first substrate induces the same third new configuration required for proper substrate binding; the collision of only one of these enzyme forms with the last product stabilizes that form without any new conformational change. This whole set of definitions is equivalent to assuming that the free enzyme which is released after catalysis, is in a conformation different from the initial one. The enzyme can be said to “recall” for a while the configuration stabilized by the last product before relapsing to the initial conformation. The non‐hyperbolic behavior is thus the consequence of the cooperation of two different conformations of the free enzyme to the overall reaction process.The reciprocal steady‐state rate equations have been established and thoroughly discussed both for one‐substrate, one‐product and two‐substrate, two‐product mnemonical enzymes. The departure from Michaelian behavior does not appear as a consequence of a slow conformational transition, but is defined in a simpler way by the relative values of the activation free energies of conformation changes required for substrate binding on the two enzyme forms.A two‐substrate, two‐product enzyme following an ordered reaction mechanism and exhibiting the mnemonical transition has a very distinctive kinetic behavior. The curvature of the primary plots is observed with regard to the first substrate only, and is independant of the concentration of the second substrate as well as that of the first product. The enzyme is not inhibited by an excess of the substrate and the primary plots are either concave up or down. The slopes and the intercepts of the straight lines obtained in double reciprocal plots with the second substrate should give, when these are replotted against the reciprocal of the first substrate concentration, a straight line and a curve, respectively. The cooperation of the enzyme conformations to the overall reaction process can be either positive or negative. Since the reciprocal plots cannot exhibit an extreme, the extent of that cooperation can be measured by the numerical value of the second derivative of the reciprocal rate equation. The extent of cooperation between the free enzyme forms is highly controlled by the concentration of the last product. If the cooperation was already negative, the product strengthens that cooperation. If, on the other hand, the cooperation was positive, the product decreases or even reverses that cooperation.A very general property of one‐sited mnemonical enzymes is that cooperation between enzyme forms is only kinetic and does not appear in the substrate binding isotherms.