Proton transfer from exogenous donors in catalysis by human carbonic anhydrase II

Arch. Biochem. Biophys. 2005, 437, 106-114.

Ileana Elder,a Chingkuang Tu,a Li-June Mig,b Robert McKenna,c and David N. Silvermana
aDepartment of Pharmacology and Therapeutics, University of Florida College of Medicine, Gainesville, FL 32610-0267, USA
bDepartment of Chemistry and Institute for Biomolecular Science, University of South Florida, 4202 Fowler Avenue, Tampa, FL 33620-5250, USA
cDepartment of Biochemistry and Molecular Biology, University of Florida College of Medicine, Gainesville, FL 32610-0267, USA

Received 9 February 2005;  revised 9 February 2005.  Available online 17 March 2005

In the site-specific mutant of human carbonic anhydrase in which the proton shuttle His64 is replaced with alanine, H64A HCA II, catalysis can be activated in a saturable manner by the proton donor 4-methylimidazole (4-MI). From 1H NMR relaxivities, we found 4-MI bound as a second-shell ligand of the tetrahedrally coordinated cobalt in Co(II)-substituted H64A HCA II, with 4-MI located about 4.5 Angstrom capital A, ring from the metal. Binding constants of 4-MI to H64A HCA II were estimated from: (1) NMR relaxation of the protons of 4-MI by Co(II)-H64A HCA II, (2) the visible absorption spectrum of Co(II)-H64A HCA II in the presence of 4-MI, (3) the inhibition by 4-MI of the catalytic hydration of CO2, and (4) from the catalyzed exchange of 18O between CO2 and water. These experiments along with previously reported crystallographic and catalytic data help identify a range of distances at which proton transfer is efficient in carbonic anhydrase II. 

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