Structural study on the role of metal ions in carbonic anhydrase II catalysis

Abstract

Human carbonic anhydrase II (CA II) is a zinc metalloenzyme that catalyzes the reversible interconversion of carbon dioxide and water to bicarbonate and proton. The zinc ion in the active site of CA II can be substituted by other physiologically relevant transition metal ions, which results in drastic changes in the catalytic activity. Until now, however, no clear evidence has been proposed to explain the relationship between metal substitution and the catalytic activity of CA II. In this study, we collected the crystallographic intermediate structures of native CA II (Zn2+–CA II) and four types of metal-substituted CA IIs (apo, Co2+, Ni2+, and Cu2+–CA II). Through a comparative analysis of these structures, we demonstrate that the characteristic metal ion coordination geometries (tetrahedral for Zn2+, tetrahedral to octahedral conversion for Co2+, octahedral for Ni2+, and trigonal bipyramidal for Cu2+) affect substrate/product binding and directly modulate the catalytic activity (100% for Zn2+, ~50% for Co2+, ~2% for Ni2+, and 0% for Cu2+). Our study provides evidence that the metal ions in metalloenzymes have a crucial impact on the catalytic mechanism beyond their primary chemical properties.