A Plant-Type (β-Class) Carbonic Anhydrase in the Thermophilic Methanoarchaeon Methanobacterium thermoautotrophicum

摘要

Carbonic anhydrase, a zinc enzyme catalyzing the interconversion of carbon dioxide and bicarbonate, is nearly ubiquitous in the tissues of highly evolved eukaryotes. Here we report on the first known plant-type (β-class) carbonic anhydrase in the archaea. TheMethanobacterium thermoautotrophicum ΔH cabgene was hyperexpressed in Escherichia coli, and the heterologously produced protein was purified 13-fold to apparent homogeneity. The enzyme, designated Cab, is thermostable at temperatures up to 75°C. No esterase activity was detected withp-phenylacetate as the substrate. The enzyme is an apparent tetramer containing approximately one zinc per subunit, as determined by plasma emission spectroscopy. Cab has a CO₂ hydration activity with a k _cat of 1.7 × 104 s?1 and K_m for CO₂ of 2.9 mM at pH 8.5 and 25°C. Western blot analysis indicates that Cab (β class) is expressed in M. thermoautotrophicum; moreover, a protein cross-reacting to antiserum raised against the γ carbonic anhydrase fromMethanosarcina thermophila was detected. These results show that β-class carbonic anhydrases extend not only into theArchaea domain but also into the thermophilic prokaryotes.