Retinol binding protein
Homo sapiens (human)
Retinol or vitamin A is an essential nutrient. The body converts it to retinal which is used in the pigments in the eyes to capture light. Retinol is provided through food. It can come from sources like butter, fish liver or eggs. Some vegetables contain a compound beta-carotene, for instance carrots, hence the name. The liver converts beta-carotene to vitamin A. The liver also stores retinol that is not immediately needed. Too little vitamin A causes night-blindness, making it difficult to see at night. Too much of it is too much of a good thing: excess amounts of vitamin A are toxic, so eating too many carrots is not good for your health. From the liver to the eye is obviously quite a distance. The body must therefore provide a way to get it from the place where it is stored to the place where it is needed. The bus on which retinol rides to the eye, is a protein, retinol-binding protein. Proteins are used to transport small molecules like retinol, using the blood vessels as their highway, floating along in the blood. Small molecules, being small, would get lost, diffusing through membranes, especially somewhat hydrophobic molecules like retinol.
There are many such molecules needing transport through the blood and there is a whole class of proteins providing this transport. They all share a similar structure, like the one seen here: the core of the protein is formed by an 8-stranded anti-parallel beta-sheet which arrange to the shape of a barrel, called the beta-barrel. At the end of the polypeptide is a long alpha-helix, running in approximately the same direction as the barrel axis, connected to the barrel by a disulfide bond. The binding site for retinol is buried inside the center of the barrel. A related family exists: avidin and streptavidin, both binding vitamin H or biotin. They differ from the retinol-binding family by the lack of the terminal alpha-helix and of the disulfide. They are remarkable because they are very very specific for biotin, with the tightest non- covalent binding known in nature.
Protein Data Bank (PDB)
Zanotti, G. Ottonello, S. Berni, R. Monaco, H.L.; "Crystal structure of the trigonal form of human plasma retinol-binding protein at 2.5 A resolution."; J. Mol. Biol.; (1993) 230:613-624 PubMed:8464067.
author: Arno Paehler