The refined atomic structure of carbonic anhydrase II at 1.05 Å resolution:: implications of chemical rescue of proton transfer

Using synchrotron radiation and a CCD detector, X-ray data have been collected at 100 K for the His64Ala mutant of human carbonic anhydrase II complexed with 4-methylimidazole (4-MI) to a maximal 1.05 Angstrom resolution, allowing full anisotropic least-squares refinement. The refined model has a conventional R factor of 15.7% for all reflections. The C-alpha coordinates of the model presented here have an r.m.s. deviation of 0.10 Angstrom relative to the previously determined structure at 1.6 Angstrom resolution. Several amino-acid residues (six of the 255 observed) have been identified with multiple rotamer side-chain conformations. C, N and O atoms can be differentiated with selective electron-density map contouring. The estimated standard deviations for all main-chain non-H atom bond lengths and angles are 0.013 and 0.030 Angstrom, respectively, based on unrestrained full-matrix least-squares refinement. This structure gives detailed information about the tetrahedrally arranged zinc ion coordinated by three histidine N atoms (His94 N-epsilon2, His96 N-epsilon2 and His119 N-delta1) and a water/hydroxide, the multiple binding sites of the proton chemical rescue molecule 4-MI and the solvent networks linking the zinc-bound water/hydroxide and 4-MI molecules. This structure presents the highest resolution structure of a carbonic anhydrase isozyme so far determined and adds to the understanding of proton-transfer processes.