An Odorant-binding Protein in Bovine Colostrum

1. General features of OBP OBP was first found in 1985 in bovine nasal mucosa as an abundant protein, which is capable of binding bell pepper odor, 2-isobutyl-3-methoxy pyridine. This protein has a hydrophobic internal cavity in the β-barrel domain, and it can bind low molecular weight (usually less than 300 Da) hydrophobic molecules in the cavity with broad specificities. Generally its affinities to the ligands are in the micromolar range. OBP has been found in the nasal mucosa of several vertebrates to date, including human beings. OPB is a relatively small globular protein, of which molecular mass is around 19 kDa. This protein has a β-barrel core domain, which consists of 8 anti-parallel β-sheets, and has a loop domain, which consists of one alpha-helix and one beta-sheet. Generally, there are two disulphide bonds observed like this, and one of them forms a bridge between the barrel core and the loop domain. I did not mentioned in this picture, but usually there is a 310-helix exists at the N-terminal region. So far, only the bovine nasal OBP is known to lack the conserved disulphide bond, which enables the protein to form dimeric structure by so-called domain swapping. All other OBPs are supposed to be monomers. OBP is also known to belong to the lipocalin superfamily, of which member proteins share a well-conserved 3D structure but widely distributed on their amino acid sequences and biological functions such as odorant carrier, odorant scavenger, regulator of the olfactory receptor, transducer of the olfactory receptor, and pheromone transporter. There are three conserved motifs of the lipocalins. Among the so-called kernel lipocalins, all these three motifs are conserved. On the other hand, among the outlier lipocalins, including OBPs, just one or two out of these three motifs are conserved.