**5. Structure of lysozyme under inert gas pressure**

298 Current Trends in X-Ray Crystallography

**Xe**

**Kr**

**Ar**

Argon occupancy was higher in elastase than in urate oxidase (Table 4), probably because of the smaller size of the gas binding site in elastase which would allow a better binding of

0 10 20 30 40 50 60 70 **Pressure (bar)**

Argon-induced expansion of the S1 pocket was very low and not very significant (less than

Contrary to xenon and krypton cases, when the water molecule W-S1 was present in the native gas-less structure, this water molecule remained visible close to the argon electronic density (Figure 7), with a lower occupation factor. The distance between the argon atom and

Fig. 7. *2Fo-Fc* electron density map of elastase under an argon pressure of 10 bar, contoured

**Ar**

**W-S1** 

In elastase, there is one unique gas binding site, located within the S1 pocket in the active site. Contrary to urate oxidase where the gas binding site was an empty cavity, there might be a water molecule in the moderately hydrophobic S1 pocket. When present, this water

Xenon inhibited directly elastase catalytic activity by taking the place of the substrate, even if its inhibition stayed lower than its occupancy. It is also likely that krypton inhibited elastase catalytic activity. Since krypton occupancy was lower than xenon occupancy, krypton-induced inhibition is expected to be lower than xenon-induced inhibition. In the range 5-10 bar, krypton occupancy ranged between 25 and 35%, inducing probably a rather

molecule was replaced by xenon and krypton, but remained visible close to argon.

**4.5 Conclusion on elastase structures under inert gas pressure** 

Fig. 6. Xenon, krypton and argon occupancies as a function of pressure.

5 %), probably due to the small size of the argon atom.

**Occupancy (%)**

the W-S1 molecule was about 2.8 Å.

argon.

at 1 .
