shape complementarity

Shape complementarity is the lock-and-key idea applied to fit: a good ligand fills its binding pocket the way a key fills a lock or a hand fills a glove, with bumps matching hollows and few gaps left over. The closer the match, the more atoms touch and the more weak contacts add up, so getting the shape right is one of the first goals of structure-based design.

Filling the pocket well does two things at once. It maximizes the many short-range van der Waals contacts and the hydrophobic burial that come from atoms in close packing, and it squeezes out poorly held water. Conversely, leaving an empty void or, worse, forcing two atoms to overlap (a steric clash) costs energy and weakens or destroys binding.

The honest qualification is that good shape complementarity is necessary but not sufficient. A ligand can fit snugly yet bind weakly if it has to twist into a strained, high-energy conformation to do so, or if it buries polar groups without satisfying their hydrogen bonds. And because proteins are flexible, the 'shape' of the pocket is not fixed — it can mold itself to the ligand through induced fit.