Calculating a Kis value for a competitive inhibitor

[gravenewworld]

Say you are given the velocity of a reaction catalyzed by an enzyme and associated concentrations of substrate, but there is also a competitive inhibitor present. To find Vmax and Km for the reaction you would plot the reciprocal plot of the Mekelis(sp?)-Menton Equation and estimate the Y and X intercepts. My question is, I know that if you plot the slope of the reciprocal plots of the MM equation vs. different concentrations of inhibitor you can find the Kis value for the enzyme inhibitor complex from the x intercept of this plot. Is the Kis value = -(the x intercept) or is -(1/Kis)=x intercept?

 

[Navin A S]

The x intercept of the plot of the reciprocal plot of the MM equation vs. different concentrations of inhibitor is equal to -1/Kis, where Kis is the inhibition constant for the enzyme-inhibitor complex.

 

[ravenprp]

The Kis value is equal to the negative x-intercept of the slope plot of the reciprocal plots of the MM equation. This is because the x-intercept represents the concentration of inhibitor that results in half maximal velocity, and the Kis value is defined as the concentration of inhibitor that binds to the enzyme with half the affinity of the substrate. So, the Kis value is equal to the negative x-intercept, not the negative of the reciprocal of the x-intercept.