This domain is the cadmium carbonic anhydrase repeat unit of the beta-carbonic anhydrase of a marine diatom [1], that uses both zinc and cadmium for catalysis of the reversible hydration of carbon dioxide for use in inorganic carbon acquisition for photosynthesis (thus being a cambialistic enzyme). Compared with alpha- and gamma-carbonic anhydrases that use three histidines to coordinate the zinc-atom, this beta-carbonic anhydrase has two cysteines and one histidine, and rapidly binds cadmium [2].

1: Diversity of the cadmium-containing carbonic anhydrase in marine diatoms and natural waters. Park H, Song B, Morel FM; Environ Microbiol. 2007;9:403-413. PMID:17222138

2: Structure and metal exchange in the cadmium carbonic anhydrase of marine diatoms. Xu Y, Feng L, Jeffrey PD, Shi Y, Morel FM; Nature. 2008;452:56-61. PMID:18322527