Final answer:
The concept not key in protein-ligand binding is irreversible binding because typical protein-ligand interactions are transient, allowing the ligand to bind and unbind. Specificity and conformational changes are essential aspects of these interactions.
Step-by-step explanation:
The concept that is NOT a key concept in protein-ligand binding is a) Irreversible binding. Protein-ligand interactions typically involve transient interactions where the ligand can bind and unbind to the protein. Specificity is a key aspect because enzymes need to bind to certain substrates with compatible shapes and charges, and conformational change often occurs when an enzyme binds to a substrate or inhibitor; an example is an allosteric inhibitor that binds to a site other than the active site and decreases the enzyme's affinity for the substrate.
Conversely, irreversible binding, where the inhibitor covalently binds and permanently inactivates the enzyme, is not typically a regulatory mechanism in protein-ligand interactions but rather a form of enzyme inhibition. Furthermore, enzymes are not consumed in the reactions they catalyze, they are specific to their substrates, and they lower the activation energy of chemical reactions without increasing the free energy (ΔG) of the reaction.