CcaA is a β-carbonic anhydrase (CA) that is a component of the carboxysomes of a subset of β-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-CA 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 with the dimer-of-dimers organization of most bacterial β-CAs, 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 probably 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-CAs characterized to date, with activity comparable with the γ-CA, 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.
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The simultaneous binding of two antibodies to the same antigen (antibodies synergy or co-cooperativity) that elicit complement-mediated bactericidal activity is visualized with a model of a complex between Neisseria meningitides factor H binging protein (red cartoon) and monoclonal antibodies 12C1 (blue surface) and JAR5 (green surface). Monoclonal antibodies are schematically depicted with bars (colored light brown and yellow for heavy and light chains, respectively) in the background. Please see pp. 4699–4713 for more information. Picture generated and provided by Enrico Malito.
The structure, kinetics and interactions of the β-carboxysomal β-carbonic anhydrase, CcaA
Leah D. McGurn, Maryam Moazami-Goudarzi, Sean A. White, Tannu Suwal, Beant Brar, Jason Q. Tang, George S. Espie, Matthew S. Kimber; The structure, kinetics and interactions of the β-carboxysomal β-carbonic anhydrase, CcaA. Biochem J 15 December 2016; 473 (24): 4559–4572. doi: https://doi.org/10.1042/BCJ20160773
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