Team:CLSB-UK/Project/CmpA

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cmpA gene

The CmpA gene forms the first part of the CmpABCD operon, which codes for a bicarbonate transporter. It encodes the substrate-binding protein of the transporter and plays a crucial role in the carbon-concentrating machinery of synechocystis.

Function

The CmpA protein is used to transport bicarbonate ions (HCO3-) into the carboxysome, an organelle that contains Rubisco. The HCO3- is then converted into CO2. This mechanism allows synechocystis to concentrate CO2 in the carboxysome around Rubisco and exclude O2, for which Rubisco also has a high affinity. The concentration of CO2 inside the carboxysome can be up to 1000 times higher than in the cytoplasm, resulting in a significant increase in photosynthetic efficiency.

Structure

CmpA is an α-ß protein similar in structure to the nitrate transporter NrtA. It consists of two domains arranged in a C-clamp shape around the ligand-binding site. Both of these domains contain a central complex of five-stranded ß-pleated sheets surrounded by α-helices. They are joined to each other by a sheaf of irregularly coiled elements. The entire protein is anchored to the plasma membrane by a lipid; CmpA is a member of the periplasmic-binding protein superfamily.

Mechanism

Bicarbonate ions bind to the ligand-binding site contained within the cleft formed by the two α-ß domains. These two domains then wrap around the ligand. This involves only minor conformational changes; the ligand-binding site is fairly inflexible. It should be noted that the binding of bicarbonate is coupled to the binding of Ca2+. The two ions bind co-operatively with high affinity. It is thought that Ca2+ may act as a cofactor.

References

Koropatkin NM, Koppenaal DW, Pakrasi HB, Smith TJ. 2007. The structure of a cyanobacterial bicarbonate transport protein, CmpA. J. Biol. Chem. 282:2606–2614

Raven, J.A., Cockell, C.S., De La Rocha, C.L., 2008. The evolution of inorganic carbon 69 concentrating mechanisms in photosynthesis. Philos. Trans. R. Soc. B: Biol. Sci. 363, 2641-2650

Maeda S, Price GD, Badger MR, Enomoto C, Omata T. 2000. Bicarbonate binding activity of the CmpA protein of the cyanobacterium Synechococcus sp. strain PCC 7942 involved in active transport of bicarbonate. J. Biol. Chem. 275:20551–20555