Structural study on intermediate states of carbonic anhydrase II and its variants

Human carbonic anhydrase II (CA II) is a zinc metalloenzyme that catalyzes the reversible interconversion of carbon dioxide (CO2) and water to bicarbonate (HCO3-) and proton (H+). To understand the catalytic mechanism of CA II, intermediate states captured during catalytic activity are essential. In this study, the crystallographic intermediate structures of native CA II (Zn2+???CA II) and four types of metallovariants (apo, Co2+, Ni2+, and Cu2+???CA II) are presented. The intermediate structures of the native CA II and metallovariants show that the metal ion coordination geometry depends on metal ion (tetrahedral for Zn2+, tetrahedral to octahedral conversion for Co2+, octahedral for Ni2+, and trigonal bipyramidal for Cu2+). Further investigation demonstrates that these characteristic metal ion coordination geometries affect substrate/product binding and directly modulate the catalytic activity (100% for Zn2+, ~50% for Co2+, ~2% for Ni2+, and 0% for Cu2+). Through a comparative study on the intermediate states, we suggest how metal substitutions on CA II influence its catalytic activity beyond their primary chemical properties