Human apolipoprotein D (ApoD) is a small glycoprotein associated with high density lipoproteins (HDL) in plasma. It appears promiscuous since it can bind hydrophobic ligands belonging to different lipid groups, with different shapes and biochemical properties; however, it exhibits specificity between very similar lipidic species. Some ligands, such as progesterone and arachidonic acid, bind to the ligand-binding pocket with high affinity, while others may interact with ApoD via its region of surface hydrophobicity. This hydrophobic surface cluster may facilitate its association with HDL particles and facilitate its insertion into cellular lipid membranes. Drosophila NLaz and Schistocerca Laz belong to this group, and share functional properties with human ApoD, including regulation of lifespan, lipid and carbohydrate metabolism control, and protection against oxidative stress or starvation. This group also includes Sandercyanin, a blue protein secreted in the skin mucus of blue forms of walleye, Sander vitreus. Walleye is an important golden yellow commercial and sport fish; the findings of blue walleye are recent. 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.
Comment:hydrophobic cavity binds different hydrophobic ligands; ligands are bound within the beta-barrel in a central internal water-filled cavity lined with polar and hydrophobic amino acids
Jiyao W et al.(2023). "The conserved domain database in 2023.",