Reflections on Edsall's carbonic anhydrase: paradoxes of an ultra fast enzyme.

John Edsall's investigations of human erythrocyte carbonic anhydrase, a zinc metalloenzyme that powerfully catalyzes the reversible hydration of carbon dioxide, highlighted a conundrum regarding the correct hydration product. The measured kinetic parameters could not be reconciled with the choice of carbonic acid, since its bimolecular recombination rate with enzyme would exceed the diffusion limit. The alternate choice of bicarbonate obviated the recombination rate problem but required that the active site deprotonation exceed the diffusion-limited maximum rate by an even greater extent. This paradox was resolved in favor of bicarbonate when the unsuspected role of buffer species indirectly deprotonating the enzyme was finally proposed, spurring numerous investigations to verify the hypothesis. Edsall's laboratory also reported the accidental discovery of the first competitive inhibitor, imidazole. This opened new avenues to understanding the binding of the CO(2) substrate and stimulated many investigations on this inhibitor. Paramagnetic NMR and crystallographic studies demonstrated that the only other known competitive inhibitor, phenol, apparently shared this unusual binding site. Despite enormous progress since Edsall's retirement, particularly the use of site-directed mutagenesis approaches, the precise interactions of carbon dioxide and bicarbonate with specific active site moieties remain as elusive today as when Edsall first considered these questions.