RCSB PDB - 1FQL: X-RAY CRYSTAL STRUCTURE OF ZINC-BOUND F95M/W97V CARBONIC ANHYDRASE (CAII) VARIANT (original) (raw)
Structural influence of hydrophobic core residues on metal binding and specificity in carbonic anhydrase II.
[Cox, J.D.](/search?q=citation.rcsb%5Fauthors:Cox, J.D.), [Hunt, J.A.](/search?q=citation.rcsb%5Fauthors:Hunt, J.A.), [Compher, K.M.](/search?q=citation.rcsb%5Fauthors:Compher, K.M.), [Fierke, C.A.](/search?q=citation.rcsb%5Fauthors:Fierke, C.A.), [Christianson, D.W.](/search?q=citation.rcsb%5Fauthors:Christianson, D.W.)
(2000) Biochemistry 39: 13687-13694
PubMed: 11076507 Search on PubMed
PubMed Abstract:
Aromatic residues in the hydrophobic core of human carbonic anhydrase II (CAII) influence metal ion binding in the active site. Residues F93, F95, and W97 are contained in a beta-strand that also contains two zinc ligands, H94 and H96. The aromatic amino acids contribute to the high zinc affinity and slow zinc dissociation rate constant of CAII [Hunt, J. A., and Fierke, C. A. (1997) J. Biol. Chem. 272, 20364-20372]. Substitution of these aromatic amino acids with smaller side chains enhances Cu(2+) affinity while decreasing Co(2+) and Zn(2+) affinity [Hunt, J. A., Mahiuddin, A., & Fierke, C. A. (1999) Biochemistry 38, 9054-9062]. Here, X-ray crystal structures of zinc-bound F93I/F95M/W97V and F93S/F95L/W97M CAIIs reveal the introduction of new cavities in the hydrophobic core, compensatory movements of surrounding side chains, and the incorporation of buried water molecules; nevertheless, the enzyme maintains tetrahedral zinc coordination geometry. However, a conformational change of direct metal ligand H94 as well as indirect (i.e., "second-shell") ligand Q92 accompanies metal release in both F93I/F95M/W97V and F93S/F95L/W97M CAIIs, thereby eliminating preorientation of the histidine ligands with tetrahedral geometry in the apoenzyme. Only one cobalt-bound variant, F93I/F95M/W97V CAII, maintains tetrahedral metal coordination geometry; F93S/F95L/W97M CAII binds Co(2+) with trigonal bipyramidal coordination geometry due to the addition of azide anion to the metal coordination polyhedron. The copper-bound variants exhibit either square pyramidal or trigonal bipyramidal metal coordination geometry due to the addition of a second solvent molecule to the metal coordination polyhedron. The key finding of this work is that aromatic core residues serve as anchors that help to preorient direct and second-shell ligands to optimize zinc binding geometry and destabilize alternative geometries. These geometrical constraints are likely a main determinant of the enhanced zinc/copper specificity of CAII as compared to small molecule chelators.
Organizational Affiliation:
Roy and Diana Vagelos Laboratories, Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6323, USA.