The enzyme carbonic anhydrase as an integral component of biogenic Ca-carbonate formation in sponge spicules

• Carbonic anhydrase (CA) catalyzes the reversible interconversion of CO2 and H2O to HCO3 (and protons).
• This enzyme is a major factor controlling and accelerating CaCO3 deposition in vitro.
• We examined the role of CA in calcareous spicule formation in the sponge Sycon raphanus
• We determined the parameters for enzymatically controlled CA-driven CaCO3 deposition.
• We outline the reactions in calcareous spicule formation, from abiogenic to biogenic CaCO3 synthesis.

The inorganic scaffold of the spicules, the skeletal elements of the calcareous sponges, is formed of calcium carbonate (CaCO3). The growth of the approximately 300-μm large spicules, such as those of the calcareous sponge Sycon raphanus used in the present study, is a rapid process with a rate of about 65 μm/h. The formation of CaCO3 is predominantly carried out by the enzyme carbonic anhydrase (CA). The enzyme from the sponge S. raphanus was isolated and prepared by recombination. The CA-driven deposition of CaCO3 crystallites is dependent on temperature (optimal at 52 °C), the pH value of the reaction assay (7.5/8.0), and the substrate concentration (CO2 and Ca2+). During the initial phase of crystallite formation, ≈40 μm large round-shaped deposits are formed that remodel to larger prisms. These crystal-like prisms associate to each other and form either rope-/bundle-like aggregates or arrange perfectly with their smaller planes along opposing surfaces of the sponge spicule rays. The CA-dependent CaCO3 deposition can be inhibited by the CA-specific inhibitor acetazolamide. The Michaelis–Menten constant for the CA-driven mineralization has been determined to be around 8 mM with respect to CaCO3. The deposits formed have a Martens hardness of ≈5 GPa. The data presented here highlights for the first time that calcite deposition in the sponge system is decisively controlled enzymatically. This data will contribute to the development of new strategies applicable for the fabrication of novel biomaterials.

Bio-calcite: We learnt from nature (calcareous sponges) how large calcite structures are fabricated under physiological conditions. Here we outline that it is the enzyme carbonic anhydrase that has a crucial role on the bio-synthesis of Ca-carbonate. The kinetic parameters are given for the in vitro synthesis of Ca-carbonate.Figure optionsDownload as PowerPoint slide