Structural Contour Map Of The Iota Carbonic Anhydrase From The Diatom Thalassiosira Pseudonana Using A Multiprong Approach

Carbonic anhydrases (CAs) are a family of ubiquitous enzymes that catalyze the interconversion of CO2 and HCO3-. The "iota" class (iota-CA) was first found in the marine diatom Thalassiosira pseudonana (tp iota-CA) and is widespread among photosynthetic microalgae and prokaryotes. The iota-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 iota-CA from a cyanobacterium and a chlorarachniophyte has been recently determined. However, the three-dimensional organization of the four domain-containing tp iota-CA is unknown. Using biophysical techniques and 3-D modeling, we show that the homotetrameric tp iota-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 tp iota-CA containing fewer domain repeats using experimental data and discuss the implications of this atypical shape on the activity and metal coordination of the iota-CA.