The multiple roles of carbonic anhydrase in calcium carbonate mineralization

Abstract

Carbonic anhydrase (CA), a ubiquitous enzyme that catalyzes the reversible hydration of CO₂, is known to be involved in the formation of CaCO₃ biominerals and is currently used for biomimetic CO₂ mineral sequestration. However, its specific role(s) in calcium carbonate (bio)mineralization is poorly understood. Here we show that CA catalyzes the formation of the reactive precursors (i.e., HCO₃⁻ and CO₃²⁻ ions) required for mineralization, accelerates the precipitation of metastable amorphous calcium carbonates and their solution-mediated conversion into crystalline calcite, which grows via a non-classical nanoparticle aggregation mechanism that facilitates CA occlusion. Ca⁺ and CO₃²⁻ ions promote the partial unfolding and oligomerization of CA, resulting in fibril- and sheet-like supramolecular assemblies that template nanostructured calcium carbonate crystallization. By losing its catalytic activity following the observed conformational changes, CA elicits a mechanism for arresting calcium carbonate mineralization. Our results show that CA can play multiple, until now unrecognized, critical roles, namely, as an enzyme and structural protein in CaCO₃ biomineralization and also helps to explain the observed loss of enzymatic activity during ex situ CO₂ mineral sequestration.