Crystallization, data collection, and model refinement statistics.

X-ray diffraction data sets were obtained from 354 lysozyme crystals soaked with N-acetyl glucosamine and from 103 thermolysin crystals soaked with asparagine. Each X-ray data set was used to estimate the refined occupancy of the ligand (Orefine) and a least squares procedure was used to fit Eq. 1 to these occupancies. The two fitted parameters were the occupancy at infinite time (Omax, from which an intra-crystalline dissociation constant Kdcryst can be calculated using Eq. 2) and a fitting parameter related to diffusion speed (τ). For each measured value “x”, both the mean “x¯” and the population standard deviation “σ(x)” are listed as x¯ ± σ(x). The population standard deviation was calculated using the formula σ(x) = (Σ (x−x¯) 2/n) ½, where “n” is the number of measurements (always equal to 354 for lysozyme data and 103 for thermolysin data).