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.
