PHARM/PCS V5 · Procedure 19
Enzyme Kinetics II: Competitive Inhibition
Compare an inhibited and an uninhibited Lineweaver-Burk line to get KI for an inhibitor that competes with substrate at the active site.
What it computes
A competitive inhibitor binds reversibly to the enzyme's active site, preventing substrate binding. On a Lineweaver-Burk plot (1/v vs. 1/[S]), the inhibited line has a steeper slope but the same y-intercept as the uninhibited line (both intersect at 1/Vmax). From the two slopes and the inhibitor concentration [I], the dissociation constant KI is computed.
Inputs and outputs
You provide
- Uninhibited data: substrate concentration and velocity, one pair per line
- A second block of the same measurements with inhibitor present
- The inhibitor concentration [I]
PHARM/PCS reports
- Slope, Vmax, and KM for each line
- KI, from the ratio of the two slopes
- Both y-intercepts, so you can judge whether the competitive model fits
- The Lineweaver-Burk plot with the inhibited and uninhibited lines
When to use it
When you suspect an inhibitor competes with substrate at the active site and want its KI. Confirm the pattern against the baseline fit from Procedure 18, and compare with the noncompetitive case in Procedure 20 if the lines don't behave as expected.
Reading the results
The signature is a shared y-intercept (unchanged Vmax) with a steeper slope. If the inhibited and uninhibited lines don't meet on the y-axis, the inhibition probably isn't purely competitive and this KI won't be valid; try Procedure 20 instead.
Check the model first
For true competitive inhibition the two lines share a y-intercept (1/Vmax). Real data scatter, so they rarely match exactly: in the book example they read 2.361 and 2.455. The procedure reports both, so you can decide whether the competitive model holds before trusting KI.
This page describes the procedure. Calculations run inside the licensed PHARM/PCS V5 application. PHARM/PCS is a trademark of MCS Scientific LLC.