Position 216 holds a tiny Glycine — an amino acid with no side chain. The active site pocket is wide open and accessible.
Drugs like Nafamostat slide directly into the S1 pocket, form a salt bridge with the active site, and inhibit the enzyme effectively.
The key fits the lock.
Position 216 holds Aspartate (D216) — a charged amino acid whose carboxylate side chain projects directly into the active site pocket.
The D216 carboxylate creates electrostatic repulsion that traps the activation loop in a kinked "hook" conformation — physically blocking the same pocket that drugs must enter.
The medicine cannot enter.
The D216 anchor is pH-sensitive. Under normal conditions (pH 7.4), the electrostatic repulsion of the D216 carboxylate holds the activation loop in its anchored, partially inactive state.
Under acidic conditions — fever, metabolic stress, infection, certain medications — the conformational equilibrium shifts. The anchor partially releases. Proteolytic activity surges.