CcaA is a b-carbonic anhydrase that is a component of the carboxysomes of a subset of b-cyanobacteria. This protein, which has a characteristic C-terminal extension of unknown function, is recruited to the carboxysome via interactions with CcmM, which is itself a γ-carbonic anhydrase homolog with enzymatic activity in many, but not all cyanobacteria. We have determined the structure of CcaA from Synechocystis sp. PCC 6803 at 1.45 Å. In contrast to the dimer of dimers organization of most bacterial β-carbonic anhydrases, or the loose dimer-of-dimers-of-dimers organization found in the plant enzymes, CcaA shows a well-packed trimer-of-dimers organization. The proximal part of the characteristic C-terminal extension is ordered by binding at a site that passes through the two-fold symmetry axis shared with an adjacent dimer; as a result, only one of a pair of converging termini can be ordered at any given time. Docking in Rosetta failed to find well-packed solutions, indicating that formation of the CcaA / CcmM complex likely requires significant backbone movements in at least one of the binding partners. Surface plasmon resonance experiments showed that CcaA forms a complex with CcmM with sub-picomolar affinity, with contributions from residues in CcmM's αA helix, and CcaA's C-terminal tail. Catalytic characterization showed CcaA to be among the least active β-carbonic anhydrases characterized to date, with activity comparable to the γ-carbonic anhydrase, CcmM, it either complements or replaces. Intriguingly, the C-terminal tail appears to partly inhibit activity, possibly indicating a role in minimizing the activity of unencapsulated enzyme.
- bacterial microcompartments
- carbonic anhydrase
- carbon dioxide concentrating mechanism
- ©2016 The Author(s)
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