At the correlation analysis highlighted that positions are strongly correlated in LPurLs. For sequences where 1181 is a leucine residue, position 1286 AZ 960 905586-69-8 located opposite it, on b42 strand is almost always a leucine or an isoleucine residue. However, in sequence where position 1181 is occupied by a phenylalanine residue, position 1286 is almost always a methionine residue. Besides, it has been previously shown breaking amino acid correlation networks can yield deleterious effects on certain consensus mutations. Moreover, these co-evolving residues could be a part of a large network, thus posing challenges in designing compensatory mutations due to cascading effects. Since SCA on PurLs indicated that L1181 is part of such an evolutionarily link, it is no longer surprising that stand alone mutations at this site perturb the connectivity, thereby resulting in destabilization. On the other hand, evolutionary statistics reveal that presence of tryptophan residue at position 1181 does not break the correlation analysis even though it occurs with low frequency at this position. Although the X-ray structure for this mutant is not yet available, based on the crystal structure of F209W it can be concluded that L1181W mutation is tolerated by adjustment in structure of the nearby surface exposed loop, via perturbations in the surrounding region without affecting structure, stability and function. In order to propose a model for allosteric regulation via Xe1 cavity, a structural superposition performed on various ammonia producing enzymes CPS, IGP synthase and PurF. The structural analyses revealed that this cavity was unique to StPurL, while in other structures this space where xenon was found is filled by none other than a phenylalanine residue. Closer examination revealed that the C-terminal helix of StPurL adjacent to the Xe1 site was pushed back, and as a consequence did not superimpose well with the helices in the other glutaminase structures thereby forming the cavity where xenon was bound. A similar scenario was also observed in thermotoga maritima PurQ where this corresponding helix also superimposed 
well with StPurL and did not align with the rest of the glutaminases. In addition, it was noted that the loop containing the residues of the oxyanion hole that are important for subsequent hydrolysis of the thioester intermediate, while having a good overlap between CPS, IGP synthase and GMP synthase, had a bad overlap with StPurL and TmPurQ. Structures of the StPurL protein and the TmPurQ proteins have been reported in an inactive form, where the thioester intermediate is covalently attached to the active site cysteine residue. However, the other representative structures are either in the apo form or with non-covalent inhibitors bound. Therefore, differences in conformation of the helix among these structures may shed light on some of the movements that take place during allosteric regulation of the protein. Since crystal structure is only a static representation it is possible that the cavity observed in StPurL may provide “breathing space” for these movements to occur and consequently these spaces may be occupied during the course of the reaction. Based on the above evidence we propose that upon activation, a Kinase Inhibitor Library customer reviews forward motion of helix a36 occurs which, perturbs the beta strand, b42, forming the opposite wall of the xenon binding cavity. This strand is directly connected to the oxyanion hole residues and a perturbation in this region initiates the correct positioning of the active site via restructuring of the oxyanion hole. Hence, we suggest that this cavity is transient and in the alternate conformations of the protein this cavity may be occupied and this space cannot be arbitrarily filled.