Over the past decade it has become apparent that a series of proteins exists with tertiary structure probably close to that of b-lactoglobulin. This family, the lipocalin or lipocalycin family, includes retinol-binding protein (RBP), apolipoprotein D, purpurin, BG protein from olfatory epithelium, bilin-binding protein, among others.
Although the level of sequence homology might seem marginally significant, only 25-30% of common residues, all of these proteins have two streches of similar sequence located in the same region, which has been proposed to be part of the ligand-binding site.
Recently, the fatty acid-binding protein (FABP) family is another identified group of small proteins that bind hydrophobic molecules. These proteins have a similar structural framework but involving a barrel with ten strands instead eight. Structural analysis of both families has shown that large parts of the lipocalin and FABP structures can be quantitatively equivalenced indicanting a very close structural relationship and suggesting that together both form a "structural superfamily" sharing essentially the same fold. It would be logical to refer to these two classes of small proteins as the octinins (8-stranded) and decinins (10-stranded) because both can reasonably be referred to as lipocalycins.
The common property of all these proteins seems to be the ability to bind small hydrophobic molecules. Ligands are bound within the central cavity of the b-barrel, which is lined with hydrophobic residues. It is quite possible that the changes to the amino acid residues in the central pocket could result in different ligand specificity.