The following graph represents the enzyme kinetics observed for an enzyme (E) in the presence of two different inhibitors, P and Q, each at two different concentrations (x-axis: −1/v; y-axis: −1/[S]).

Based on the graph, what is the substrate concentration (in μM) at which both inhibitors achieve 50% inhibition?

Correct option is C

Correct Answer:
(c) 1.0 μM
Explanation:
Fifty percent inhibition corresponds to a condition where the reaction velocity is half of the uninhibited velocity at the same substrate concentration.
On a Lineweaver–Burk plot, this is identified by comparing inhibitor lines with the control line at a common 1/[S] value where the increase in 1/v reflects 50% loss of activity.
From the graph, both inhibitors P and Q show their 50% inhibitory effect at 1/[S] = 1, corresponding to [S] = 1.0 μM.
Information Booster :
· Lineweaver–Burk plots help visualize inhibitor effects on Km and Vmax.
· A 50% reduction in velocity is a standard reference point for comparing inhibitor strength.
· Identical inhibition at a given [S] implies comparable functional inhibition under those conditions.
· Reciprocal plots allow easy substrate concentration estimation from straight-line intersections.
Additional Information (Incorrect Options):
0.333 μM and 0.5 μM: These correspond to higher 1/[S] values where inhibition differs between P and Q.
2.0 μM: At higher substrate concentrations, inhibition is less pronounced and does not reflect 50% inhibition for both inhibitors simultaneously.