Allosteric regulation of proteins: what does an allosteric activator do? Choose the most accurate description of how an allosteric activator affects ligand binding and conformational state.

Introduction / Context:
Allosteric regulation is central to metabolism and oxygen transport. Small molecules can bind to sites distinct from the active or primary binding site and alter protein conformation. Understanding how an allosteric activator shifts equilibria between conformational states helps explain sigmoidal binding curves and cooperative behavior.

Given Data / Assumptions:

• Proteins such as hemoglobin exist in T (tense) and R (relaxed) states.
• Allosteric activators often favor the high-affinity R state.
• We discuss binding affinity for the primary ligand (for example, O2 for hemoglobin).

Concept / Approach:
Allosteric activators stabilize conformations with higher affinity for the primary ligand. In the Monod–Wyman–Changeux framework, an activator increases the population of R state relative to T state. This shift raises apparent affinity and can enhance cooperativity depending on the system.

Step-by-Step Solution:
Define allosteric activator: ligand that binds a regulatory site and favors a conformation with increased primary-ligand affinity.Map to conformational states: activator stabilizes R state over T state.Conclude effects: increased affinity plus R-state stabilization.

Verification / Alternative check:
Oxygen binding to hemoglobin itself is homotropic activation, promoting the R state; heterotropic activators in other systems (for example, aspartate activating carbamoyl phosphate synthetase) similarly increase affinity or activity.

Why Other Options Are Wrong:
An activator does not decrease affinity; prevention of substrate binding is antagonistic and characteristic of inhibitors, not activators.

Common Pitfalls:
Confusing allosteric activators with catalytic activators; allostery changes conformational equilibria, not necessarily catalytic mechanism directly.

Final Answer:
Both (a) and (c).