Fig. 2.

Trypsin in its apo state can exist in substates S₁ (green) and S₂ (red). The key difference between these two states is in the loop L. In S₁ the loop is as in the X-ray pose and the protein is available for binding. In S₂ the loop has undergone a distortion initiated primarily by glycine–serine residues (S213 and G214) that engage in hydrogen bond interaction with other residues (D216 and Q217). In this state, the protein is temporarily unavailable for binding. See text for further details and mean lifetimes.