Gamma Carbonic Anhydrase

Introduction/Background

Gamma carbonic anhydrase is an enzyme that is produced by the methanoarchaeon Methanosarcina thermophila ^[1]. Gamma carbonic anhydrase, often referred to as Cam (for carbonic anhydrase of M. thermophila) is part of a class of enzymes known as the carbonic anhydrases, which catalyze the reversible hydration of carbon dioxide (CO₂) to the bicarbonate ion (HCO₃^-). There are three distinct classes of carbonic anhydrases, the alpha (α) form found in mammals, the beta (β) form found in bacteria and higher order photosynthetic plants, and the gamma (γ) form found in archaea. Evolutionary studies show that the three classes of carbonic anhydrases do not have common ancestry, however additional studies into the catalytic mechanism of γ-carbonic anhydrase indicate that α and γ carbonic anhydrases similar use of a zinc hydroxide mechanism to oxidize CO₂.^[2]

Function of Gamma Carbonic Anhydrase

The main biological function of γ-carbonic anhydrase in M. thermophila is to catalyze the hydration of CO₂ to HCO₃^- ion, in order to reduce the concentration of CO₂ produced in acetate metabolism. M. thermophila are methanogenic anaerobes which produce methane by reducing the methyl groups of a variety of organic molecules such as acetate, methanol and methylamines ^[3]. When high concentrations of acetate are available, M. thermophila is able to switch its metabolism to use acetate to produce methane from reduced the methyl group and carbon dioxide from the oxidized carbonyl group ^{[1] [3]}. The role of γ-carbonic anhydrase in acetate metabolism is drive forward the reaction of acetate to methane and CO₂ by removing the CO₂ concentration by converting it to HCO₃^- outside the cell ^{[1] [3]}.

Structure

According to the Structural Classification of Proteins (SCOP), γ-carbonic anhydrase is part of the Trimeric LpxA Enzyme superfamily, which is characterized by single-stranded polypeptides with left-handed beta-helix fold. The overall structure of γ-carbonic anhydrase is a trimer complex composed of left-handed beta-helix monomers. The beta-helix consists of three untwisted, parallel sheets that are connected by left-handed crossovers ^[1]. Like all the carbonic anhydrase proteins, γ-carbonic anhydrase contains a metal ion ligand in its active site. The unique feature of γ-carbonic anhydrase is its ability to utilize two metal ions equivalently for its active site, the zinc ion, Zn²⁺ and the cobalt ion, Co²⁺. Studies have shown that the enzyme is able to use these two ions interchangeable depending on their availability ^[2]. While studies indicate that there are no significant differences between zinc-bound and cobalt-bound γ-carbonic anhydrase structures, the catalytic mechanism for binding of carbon dioxide is different between the three classes of carbonic anhydrases. In both zinc-bound and cobalt-bound γ-carbonic anhydrases there are three histidine residues (His81, His117 and His122 residues) that coordinate the ion with the active site ^[1]. In addition to the three histidines, there are several other residues which have been identified to play an important role in the active site catalytic mechanism. For example the glutamine residue Gln75 and the asparagine residue Asn73 have been found to help orient the cobalt ion for attack on the carbon dioxide, and the asparagine residue Asn202 prepares the carbon dioxide, by polarization, for attack by the cobalt ion ^[4].