JBC, Vol. 250, Issue 7, 2630-2634, Apr, 1975
Magnetic resonance study of exchangeable protons in human carbonic anhydrases
R. K. Gupta and J. M. Pesando
A titratable exchangeable proton resonance assignable to a histidine
imidazole ring N--H proton is observed approximately minus 15 ppm downfield
from tetramethylsilane. The chemical shift of this resonance is affected by
sulfonamide and anion inhibitors, and by removal of zinc or replacement of
zinc by cobalt, indicating that the proton is located at or near the active
site. The pH dependence of the chemical shift of this resonance, which is
abolished by inhibitors, reflects the titration of a group with a pK-a of
7.3 in human carbonic anhydrase B and smaller than or equal to 7.1 in human
carbonic anhydrase C. These pK-a values are interpreted to be due to the
ionization of a neutral imidazole to form the imidazolate anion coordinated
to zinc. A mechanism for enzymatic catalysis involving reversible
deprotonation and coordination of a histidine to the metal is consistent
with these studies.
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