The activity-related ionization in carbonic anhydrase

Area of Science:

• Biochemistry
• Enzymology
• Coordination Chemistry

Background:

• Carbonic anhydrase (EC 4.2.1.1) catalyzes crucial physiological reactions.
• Enzyme activity is closely tied to the ionization state of residues near the active site zinc ion.
• Understanding these ionizations is key to elucidating enzyme mechanisms.

Purpose of the Study:

• To investigate the ionization events critical for carbonic anhydrase's catalytic activity.
• To model the active-site chelate system using various imidazole ligands.
• To determine the pK(a) values of relevant ionizable groups.

Main Methods:

• Complexation of Zn(II) and Co(II) with imidazole-based ligands.
• Study of simple imidazole and N-methylimidazole systems.
• Investigation of bidentate ligands: Zn(II)-4,4'-bis-imidazoylmethane and Co(II)-cyclic-L-histidyl-L-histidine.
• Analysis of metal-bound water and imidazole coordination.

Main Results:

• In metal-bound species with pyridinium-coordinated imidazole, the pyrrole N-H group was the most acidic.
• Using N-methylimidazole, the pK(a) of metal-bound water in a tri-imidazole ligand field was determined to be 9.1.
• A preference for pyrrole hydrogen labilization was observed.

Conclusions:

• The pK(a) of carbonic anhydrase (pK(enz)) is likely associated with N-H ionization of the imidazole ring.
• This contrasts with the traditional view linking pK(enz) to metal-bound water ionization.
• Reevaluation of carbonic anhydrase catalytic mechanisms is proposed based on N-H ionization.