Ex-FABP (also known as siderocalin, lipocalin Q83 or protein Ch21) displays a dual ligand binding mode as it can bind siderophore and fatty acids simultaneously. ExFABP has a cavity which extends through the protein and has two separate ligand specificities, one for bacterial siderophores at one end, and other specifically binding co-purified lysophosphatidic acid (LPA), a potent cell signaling molecule, at the other end. As well as acting as an LPA "sensor", Ex-FABP is bacteriostatic, and tightly binds the 2,3-catechol-type ferric siderophores enterobactin, bacillibactin, and parabactin, associated with enteric bacteria and Gram-positive bacilli. This group belongs to the lipocalin/cytosolic fatty-acid binding protein family which have a large beta-barrel ligand-binding cavity. Lipocalins are mainly low molecular weight extracellular proteins that bind principally small hydrophobic ligands, and form covalent or non-covalent complexes with soluble macromolecules, as well as membrane bound-receptors. They participate in processes such as ligand transport, modulation of cell growth and metabolism, regulation of immune response, smell reception, tissue development and animal behavior. Cytosolic fatty-acid binding proteins, also bind hydrophobic ligands in a non-covalent, reversible manner, and have been implicated in intracellular uptake, transport and storage of hydrophobic ligands, regulation of lipid metabolism and sequestration of excess toxic fatty acids, as well as in signaling, gene expression, inflammation, cell growth and proliferation, and cancer development.
Jiyao W et al.(2023). "The conserved domain database in 2023.",