Know the kinetic mechanism of your modifier
Before starting with properly designed measurements aimed at gathering new data, one should be prepared to master not only expected but also apparently odd results. As an example, inhibition at low substrate concentration can first disappear and then turn into net activation just by increasing the substrate concentration while the modifier concentration remains constant.
Systematics = Taxonomy + Nomenclature.
The two subsections herein will describe the properties of the basic modifier mechanisms
but they will be maintained consistent throughout. The table below lists the symbols, their meaning and the default units.
|app||as superscript to a parameter = apparent value in the presence of modifier||-|
|[E], [S], [X]||Free concentrations of enzyme, substrate and modifier, respectively||μM|
|KX||Inhibition or activation dissociation constant of the modifier||μM|
|Km/kcat||Reciprocal specificity constant||µM s|
|v0||Initial velocity in the absence of modifier||µM s−1|
|vX||Initial velocity in the presence of modifier||µM s−1|
|v||Generic initial velocity||µM s−1|
|v/[E]t||Initial velocity divided by the total, titrated enzyme active site concentration||s−1|
|σ = [S]/Km||Normalized substrate concentration||pure number|
|σ/(1+σ)||Specific velocity||pure number|
|α||Reciprocal allosteric coupling constant||pure number|
|β||Factor by which the modifier affects the catalytic constant||pure number|