KINETICS OF ENZYME REACTIONS WITH INTERACTION BETWEEN A SUBSTRATE AND A (METAL) MODIFIER

This study describes a kinetic model of an enzyme reaction in which a (metal) modifier and a reactant combine with the enzyme and with each other. The prototype reaction requires adenosine triphosphate (reactant) and magnesium ion (modifier), the “active substrate” being the magnesium-adenosine triphosphate complex. Sigmoid velocity curves can occur in this model, which does not assume an enzyme with multiple interacting subunits. Qualitative criteria for distinguishing different effects of the modifier and the reactant are: (1) Is a velocity curve concave or sigmoid ? (2) What is the relative position of velocity curves obtained with higher fixed concentrations of modifier or reactant ? (3) Does a peak velocity occur? (4) What is the position of the peak velocity ? (5) How sigmoid is the velocity curve obtained with equal concentrations of modifier and reactant ? (6) When either modifier or reactant is in excess is the velocity constant ? These criteria help distinguish, for example, inhibition by free reactant (adenosine triphosphate) from activation by free modifier (magnesium). Graphical methods for estimating several kinetic parameters are presented. The methods do not require ths association constant of the modifier-reactant complex (e.g., magnesium-adenosine triphosphate); in some cases that constant can be estimated graphically from the enzyme kinetic data. © 1969, American Chemical Society. All rights reserved.