title: The refined atomic structure of carbonic anhydrase II at 1.05 A resolution: implications of chemical rescue of proton transfer.
abstract: 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 A 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 A relative to the previously determined structure at 1.6 A 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 A, 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(epsilon 2), 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.
authors.affiliation: Department of Biochemistry and Molecular Biology, University of Florida, Gainesville, FL 32610, USA.
authors.forename: David
authors.initials: D
authors.lastname: Duda
authors.forename: Lakshmanan
authors.initials: L
authors.lastname: Govindasamy
authors.forename: Mavis
authors.initials: M
authors.lastname: Agbandje-McKenna
authors.forename: Chingkuang
authors.initials: C
authors.lastname: Tu
authors.forename: David N
authors.initials: DN
authors.lastname: Silverman
authors.forename: Robert
authors.initials: R
authors.lastname: McKenna
mesh_terms: D019943:Amino Acid Substitution; D001665:Binding Sites; D024402:Carbonic Anhydrase II / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D018360:Crystallography, X-Ray / Q000379:methods; D017357:Cyclotrons; D006639:Histidine / Q000235:genetics / Q000378:metabolism; D006801:Humans; D006878:Hydroxides / Q000737:chemistry / Q000378:metabolism; D008628:Mercury / Q000737:chemistry / Q000378:metabolism; D008958:Models, Molecular; D011487:Protein Conformation; D011522:Protons; D014867:Water / Q000737:chemistry; D015032:Zinc / Q000378:metabolism
chemical_list: D006878:Hydroxides; D011522:Protons; D014867:Water; D006639:Histidine; C031356:hydroxide ion; D024402:Carbonic Anhydrase II; D008628:Mercury; D015032:Zinc