Structure of cobalt carbonic anhydrase complexed with bicarbonate

Area of Science:

• Biochemistry
• Structural Biology
• Enzymology

Background:

• Human carbonic anhydrase II is a crucial enzyme in biological systems.
• Understanding its catalytic mechanism, particularly substrate binding, is essential.
• Cobalt(II) substitution provides a model for studying active site dynamics.

Purpose of the Study:

• To determine the three-dimensional structure of a complex between cobalt(II)-substituted human carbonic anhydrase II and bicarbonate.
• To elucidate the binding interactions between the enzyme's active site and its substrate.
• To propose a reaction mechanism based on the determined complex structure.

Main Methods:

• X-ray crystallography was employed to determine the structure of the enzyme-substrate complex.
• The resolution of the determined structure was 1.9 Å.

Main Results:

• The cobalt(II) ion is coordinated by three histidine residues, one water molecule, and two bicarbonate oxygen atoms.
• The coordination geometry around the cobalt ion is distorted octahedral, differing from the native enzyme's tetrahedral geometry.
• Bicarbonate binds via its two unprotonized oxygen atoms to a carboxylate binding site, suggesting a water-bicarbonate exchange mechanism.

Conclusions:

• The determined structure represents a key intermediate state in the carbonic anhydrase catalytic cycle.
• The findings provide a structural basis for understanding the enzyme's mechanism of action.
• This model is relevant for both the substituted and native human carbonic anhydrase II.