Carbon Dioxide “Trapped” in a β‑Carbonic Anhydrase

• Published in: Biochemistry (Easton) 2015-11, Vol.54 (43), p.6631-6638
• Main Authors: Aggarwal, Mayank, Chua, Teck Khiang, Pinard, Melissa A, Szebenyi, Doletha M, McKenna, Robert
• Format: Article
• Language: English
• Subjects: Bacterial Proteins - chemistry; Bacterial Proteins - genetics; Bacterial Proteins - metabolism; BASIC BIOLOGICAL SCIENCES; Carbon Dioxide - metabolism; Carbonic Anhydrase II - chemistry; Carbonic Anhydrase II - metabolism; Carbonic Anhydrases - chemistry; Carbonic Anhydrases - genetics; Carbonic Anhydrases - metabolism; Catalytic Domain; CO2 binding; Crystallography, X-Ray; Humans; Ligands; Models, Molecular; Protein Conformation; Protein Structure, Quaternary; Pseudomonas aeruginosa; Pseudomonas aeruginosa - enzymology; Pseudomonas aeruginosa - genetics; Recombinant Proteins - chemistry; Recombinant Proteins - genetics; Recombinant Proteins - metabolism; Species Specificity; β-carbonic anhydrase
• ISSN: 0006-2960; 1520-4995
• DOI: 10.1021/acs.biochem.5b00987

Carbonic anhydrases (CAs) are enzymes that catalyze the hydration/dehydration of CO2/HCO3 – with rates approaching diffusion-controlled limits (k cat/K M ∼ 108 M–1 s–1). This family of enzymes has evolved disparate protein folds that all perform the same reaction at near catalytic perfection. Presented here is a structural study of a β-CA (psCA3) expressed in Pseudomonas aeruginosa, in complex with CO2, using pressurized cryo-cooled crystallography. The structure has been refined to 1.6 Å resolution with R cryst and R free values of 17.3 and 19.9%, respectively, and is compared with the α-CA, human CA isoform II (hCA II), the only other CA to have CO2 captured in its active site. Despite the lack of structural similarity between psCA3 and hCA II, the CO2 binding orientation relative to the zinc-bound solvent is identical. In addition, a second CO2 binding site was located at the dimer interface of psCA3. Interestingly, all β-CAs function as dimers or higher-order oligomeric states, and the CO2 bound at the interface may contribute to the allosteric nature of this family of enzymes or may be a convenient alternative binding site as this pocket has been previously shown to be a promiscuous site for a variety of ligands, including bicarbonate, sulfate, and phosphate ions.