Carbon Dioxide "Trapped" in a β-Carbonic Anhydrase

Area of Science:

• Biochemistry
• Structural Biology
• Enzymology

Background:

• Carbonic anhydrases (CAs) are crucial enzymes catalyzing CO2 hydration/dehydration at near-diffusion limits.
• This enzyme family exhibits diverse protein folds while maintaining high catalytic efficiency.
• Beta-carbonic anhydrases (β-CAs) typically function as dimers or higher-order oligomers.

Purpose of the Study:

• To elucidate the structure of a β-CA (psCA3) from Pseudomonas aeruginosa in complex with CO2.
• To compare the CO2 binding site of psCA3 with that of human α-CA II (hCA II).
• To investigate potential functional roles of additional CO2 binding sites in β-CAs.

Main Methods:

• Pressurized cryo-cooled crystallography was employed to determine the structure of psCA3.
• The structure was refined to 1.6 Å resolution.
• Comparative analysis with the known structure of human CA II was performed.

Main Results:

• The structure of psCA3 complexed with CO2 was determined at high resolution.
• CO2 binding orientation in psCA3 is identical to that in hCA II, despite differing protein folds.
• A novel CO2 binding site was identified at the dimer interface of psCA3.

Conclusions:

• The conserved CO2 binding orientation highlights a fundamental aspect of carbonic anhydrase function across different folds.
• The CO2 binding site at the dimer interface may play a role in the allosteric regulation of β-CAs.
• This interface site could serve as an alternative binding pocket for various ligands, as previously observed.