Final answer:
In allosteric regulation, effector molecules usually bind to an enzyme's allosteric site reversibly and noncovalently, affecting the enzyme's activity by modifying its structure and the affinity of its active site for the substrate. Therefore, the correct answer is 3) reversibly and noncovalently.
Step-by-step explanation:
In allosteric regulation, effector molecules typically bind to the enzyme's allosteric site in a manner that is both reversible and non-covalent.
The regulatory effect of these effector molecules can enhance or diminish the enzyme's activity by inducing a conformational change that affects the active site.
Allosteric enzymes have a distinct allosteric site that is separate from the active site where substrate molecules bind. When allosteric effectors such as ATP, ADP, AMP, NAD+, and NADH attach to the allosteric site, they do so through noncovalent interactions and trigger a structural change in the enzyme.
This conformational change either increases or decreases the affinity of the enzyme's active site for the substrate, thereby modulating the reaction rate.
The effects of allosteric control are reversible; once the allosteric effector's concentration decreases, it will detach from the enzyme, returning the enzyme to its original state.
Therefore, the correct answer is 3) reversibly and noncovalently.