**6. Conclusion**

Noble gases bind to proteins essentially through non-covalent van der Waals interactions, their binding constant depending on their electronic polarizability. In the three studied enzymes, gas occupancies were in the order of their polarizability, Xe > Kr > Ar, as it was already found for T4 lysozyme (Quillin et al., 2000). The only slight exception was the internal cavity of egg white lysozyme, where its smaller size prevented xenon higher occupancy.

The major physiological targets of gaseous anesthetics are postulated to be neuronal channels receptors, like the NMDA receptor which is inhibited by xenon (Campagna et al., 2003; Franks, 2008), or the GABAA receptor which could be modulated by argon (Abraini et al., 2003). However, at lower concentration, gas would bind mainly to globular targets, amongst them enzymes, whose functions would be modulated by the presence of inert gas.

From the present study, we can infer different mode of inhibition by gas. In urate oxidase, gas inhibited the catalytic reaction through an indirect mechanism; the presence of the gas within the cavity would prevent the cavity contraction, thus modifying the active site flexibility. In elastase, gas inhibited the catalytic reaction through a direct mechanism; the presence of the gas in the active site would prevent the substrate binding. In lysozyme, gas would not inhibit the catalytic reaction, their occupation being too weak.

Protein activity requires some conformational flexibility (Frauenfelder et al., 1991). In enzymes, the balance between conformational flexibility and rigidity is adjusted to optimize the catalytic efficiency for a given condition (Chiuri et al., 2009). Cavities would facilitate conformational changes and are though to play a key role in protein function (Hubbard et al., 1994). Anesthetics have been postulated to act by stabilizing high-energy conformers inducing altered functions (Eckenhoff, 2001; Johansson et al., 2005). High pressure was also postulated to stabilize high-energy conformers with altered functions (Frauenfelder et al., 1990; Akasaka, 2006; Fourme et al., 2006). Urate oxidase is thus a key example which highlights the effect of anesthetic, since this enzyme is both inhibited by gas presence in a hydrophobic cavity (Marassio et al., 2011) and by high pressure (Girard et al., 2010).

Inert gas binds to proteins through very weak non-covalent van der waals interaction. How such weak interactions could generate such high biological effect such as anesthesia ? It was suggested that anesthesia would arise from small effects at many biological targets (Eckenhoff, 2001). The present study would confirm this hypothesis, showing that some enzymes could be stabilized by the presence of gas in hydrophobic cavity (as in urate oxidase), some enzymes could be directly inhibited by gas (as in elastase case), and some enzymes are not affected by gas (as in the case of lysozyme). The mechanisms of neuroprotection and anesthesia by inert gases are thus very complex processes with many biological targets whose function are modulated (inhibited or potentiated) by the presence of gas.
