Cadmium-containing carbonic anhydrase CDCA1 in marine diatom Thalassiosira weissflogii

Abstract

The Carbon Concentration Mechanism (CCM) allows phytoplakton species to accumulate the dissolved inorganic carbon (DIC) necessary for an efficient photosynthesis even under carbon dioxide limitation. In this mechanism of primary importance for diatoms, a key role is played by carbonic anhydrase (CA) enzymes which catalyze the reversible hydration of CO2, thus taking part in the acquisition of inorganic carbon for photosynthesis. A novel CA, named CDCA1, has been recently discovered in the marine diatom Thalassiosira weissflogii. CDCA1 is a cambialistic enzyme since it naturally uses Cd2+ as catalytic metal ion, but if necessary can spontaneously exchange Cd2+ to Zn2+. Here, the biochemical and structural features of CDCA1 enzyme will be presented together with its putative biotechnological applications for the detection of metal ions in seawaters.

Figure 1

Multiple sequence alignment of CDCA1 single repeats. Conserved residues are indicated with an asterisk (*), while (:) and (.) indicate conservative and semi-conservative substitutions, respectively. Residues involved in metal ion coordination are reported as underlined letters. The alignment was generated with ClustalW, version 1.83.

Figure 2

(A) CDCA1-R1 overall fold. β-strands and α-helices are shown in cartoon representation and named as reported by [3]. Cd²⁺ is also depicted as purple sphere. The CDCA1 two-lobe architecture is highlighted by different colors: lobe 1 in blue and lobe 2 in green; (B) Enlarged view of the CDCA1-R1 active site. Cd²⁺ coordination sphere is shown.

Figure 3

Structural comparison between the Cd-bound (magenta) and metal-free (blue) forms of CDCA1-R1. Cd²⁺ coordinating residues are represented in ball-and-sticks and their displacements are indicated by arrows. The region encompassing the 107–115 sequence is also highlighted.

Figure 4

Model of the CDCA1 full length. R1 (cyan), R2 (blue) and R3 (yellow) are shown in cartoon representation. Interface regions are displayed in orange (R1-R2), green (R2-R3) and magenta (R1-R3). Non conserved residues are displayed in ball-and-stick and the three metal ions are reported as pink spheres.