Basic introduction to enzyme-substrate binding.

Distinguishes between the induced fit model and the lock-and-key model.

This is a good place to involve the discussion a bit with substrate-ligand binding beyond the context of enzyme activity. With enzymes, you have catalytic turnover, so there are two ways for enzyme-substrate complex to dissociate (unbinding or turnover) not just unbinding. On the contrary, though, a more simple binding-unbinding model is relevant with many proteins and their ligands. The link above is a subsection of a broader article on dissociation constant, relevant to simple protein-ligand binding, K_d. You don't want to go into the MCAT without being familiar with K_d and its reciprocal, association constant, K_a. In the context of enzymes, though, it's more appropriate to conceptualize binding affinity in terms of Michaelis constant, K_M which takes catalytic turnover into account.