An enzyme shows sigmoidal kinetics with Hill coefficient n=3. A homotropic allosteric activator shifts the curve leftward (lower S50). This activator most likely acts by:

• A Increasing kcat by stabilising the enzyme's transition state
• B Stabilising the R (relaxed, high-affinity) state of the allosteric enzyme, increasing cooperative substrate binding ✓
• C Competing with the substrate for the active site at low concentrations
• D Preventing product inhibition by binding to the product release site

Explanation

Allosteric enzymes exist in T (taut, low-affinity) and R (relaxed, high-affinity) conformational states in equilibrium. Homotropic activators (typically the substrate itself acting at regulatory sites) or heterotropic activators shift the T/R equilibrium toward the R state, increasing substrate affinity for all subunits cooperatively. This leftward shift of the sigmoidal V vs. [S] curve reduces S50 (equivalent of Km for sigmoidal enzymes) without necessarily changing Vmax. Sigmoidal kinetics with Hill coefficient n>1 indicate positive cooperativity. The activator does not change the transition state energy (kcat) or compete with substrate. Product inhibition is a separate regulatory mechanism.

Reference: Harper's Illustrated Biochemistry, 32nd ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.