'Active' conformation of an inactive semi-synthetic ribonuclease-S

We have studied the integrity of folded structure of a fully active semi-synthetic ribonuclease-S which lacks amino acid residues 16 through 20, and an inactive one with the same residues deleted and 4-fluoro-l-histidine substituted for active site histidine 12. Using “Y” form crystals, we obtained X-ray structural data to a resolution of 2·6 A and, incorporating phase information calculated from refined ribonuclease-S coordinates, prepared several types of electron density maps. These showed that the overall backbone structure and active site configuration of both analogues do not differ noticeably from those of the native protein. Structural homology extends to the catalytically relevant side-chain at position 12; 4-F-His† assumes the same position as does His in active ribonuclease-S. This supports the view that the 4-F-Hisl2 analogue is inactive due to a change in histidine 12 imidazole basicity, rather than to any significant conformational distortion within the active site.