The C-terminal substrate binding of LysR-type transcriptional regulator (CbbR) of RubisCO operon, which is involved in the carbon dioxide fixation, contains the type 2 periplasmic binding fold.
CbbR, a LysR-type transcriptional regulator, is required to activate expression of RubisCO, one of two unique enzymes in the Calvin-Benson-Bassham (CBB) cycle pathway. All plants, cyanobacteria, and many autotrophic bacteria use the CBB cycle to fix carbon dioxide. Thus, this cycle plays an essential role in assimilating CO2 into organic carbon on earth. The key CBB cycle enzyme is ribulose 1,5-bisphosphate carboxylase/oxygenase (RubisCO), which catalyzes the actual CO2 fixation reaction. The CO2 concentration affects the expression of RubisCO genes. It has also shown that NADPH enhances the DNA-binding ability of the CbbR. RubisCO is composed of eight large (CbbL) and eight small subunits (CbbS). The topology of this substrate-binding domain is most similar to that of the type 2 periplasmic binding proteins (PBP2), which are responsible for the uptake of a variety of substrates such as phosphate, sulfate, polysaccharides, lysine/arginine/ornithine, and histidine. The PBP2 bind their ligand in the cleft between these domains in a manner resembling a Venus flytrap. After binding their specific ligand with high affinity, they can interact with a cognate membrane transport complex comprised of two integral membrane domains and two cytoplasmically located ATPase domains. This interaction triggers the ligand translocation across the cytoplasmic membrane energized by ATP hydrolysis.