Determination of true Ki values in enzyme dead-end inhibition kinetic studies

The true K i value for a dead-end inhibitor in multisubstrate reactions can not always be derived directly from various kinetic plots. The reaction mechanism must be known before the appropriate plot can be selected. In ordered sequential mechanisms, the true K i value can be derived from Dixon plots or secondary plots of slopes only if the inhibitor is competitive for the first substrate. In rapid equilibrium random mechanisms, no type of plot gives the true K i value unless K i = K i′ . In that case the true K i value can also be derived directly from Dixon plots or secondary plots of slopes in the competitive case. In the Ping Pong mechanisms the K i value can be derived directly from the Dixon plot or secondary plot of slopes only when the inhibitor is fully competitive for the first substrate. Secondary plots of intercepts and plots of S/ v vs [I] never give true K i values directly, but the K i values can be calculated if the substrate kinetic constants are known.