The C-terminal substrate binding domain of LysR-type transcriptional regulator HcaR in involved in 3-phenylpropionic acid catabolism, contains the type2 periplasmic binding fold.HcaR, a member of the LysR family of transcriptional regulators, controls the expression of the hcA1, A2, B, C, and D operon, encoding for the 3-phenylpropionate dioxygenase complex and 3-phenylpropionate-2',3'-dihydrodiol dehydrogenase, that oxidizes 3-phenylpropionate to 3-(2,3-dihydroxyphenyl) propionate. Dioxygenases play an important role in protecting the cell against the toxic effects of dioxygen. The expression of hcaR is negatively auto-regulated, as for other members of the LysR family, and is strongly repressed in the presence of glucose. This substrate-binding domain shows significant homology to 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.