*Article* **Structural Contour Map of the Iota Carbonic Anhydrase from the Diatom** *Thalassiosira pseudonana* **Using a Multiprong Approach**

**Erik L. Jensen 1,† , Véronique Receveur-Brechot 1, Mohand Hachemane 1, Laura Wils <sup>1</sup> , Pascale Barbier 2, Goetz Parsiegla 1, Brigitte Gontero 1,\* and Hélène Launay 1,\***


**Citation:** Jensen, E.L.;

Receveur-Brechot, V.; Hachemane, M.; Wils, L.; Barbier, P.; Parsiegla, G.; Gontero, B.; Launay, H. Structural Contour Map of the Iota Carbonic Anhydrase from the Diatom *Thalassiosira pseudonana* Using a Multiprong Approach. *Int. J. Mol. Sci.* **2021**, *22*, 8723. https://doi.org/ 10.3390/ijms22168723

Academic Editors: Istvan Simon and Csaba Magyar

Received: 21 July 2021 Accepted: 11 August 2021 Published: 13 August 2021

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**Abstract:** Carbonic anhydrases (CAs) are a family of ubiquitous enzymes that catalyze the interconversion of CO2 and HCO3 <sup>−</sup>. The "iota" class (ι-CA) was first found in the marine diatom *Thalassiosira pseudonana* (tpι-CA) and is widespread among photosynthetic microalgae and prokaryotes. The ι-CA has a domain COG4875 (or COG4337) that can be repeated from one to several times and resembles a calcium–calmodulin protein kinase II association domain (CaMKII-AD). The crystal structure of this domain in the ι-CA from a cyanobacterium and a chlorarachniophyte has been recently determined. However, the three-dimensional organization of the four domain-containing tpι-CA is unknown. Using biophysical techniques and 3-D modeling, we show that the homotetrameric tpι-CA in solution has a flat "drone-like" shape with a core formed by the association of the first two domains of each monomer, and four protruding arms formed by domains 3 and 4. We also observe that the short linker between domains 3 and 4 in each monomer confers high flexibility, allowing for different conformations to be adopted. We propose the possible 3-D structure of a truncated tpι-CA containing fewer domain repeats using experimental data and discuss the implications of this atypical shape on the activity and metal coordination of the ι-CA.

**Keywords:** analytical ultracentrifugation; CO2 concentrating mechanism; diffusion-ordered NMR spectroscopy; electrospray ionization mass spectrometry; homotetramer; manganese; metalloprotein; photosynthesis; small-angle X-ray scattering
