NMR studies of retinoid-protein interactions: the conformation of [13C]-beta-ionones bound to beta-lactoglobulin B.
OBJECTIVE
Vitamin A (retinol) and its metabolites comprise the natural retinoids. While the biological action of these molecules are thought to be primarily mediated by ca. 55 kDa nuclear retinoic acid receptors, a number of structurally similar 15-20 kDa proteins are involved in the transport, and possibly metabolism, of these compounds. The milk protein beta-lactoglobulin B (beta-LG) is an 18 kDa protein which binds retinol and may be involved in oral delivery of retinol to neonates. beta-LG also binds drugs and other natural products and is of potential interest as a protective delivery vehicle.


METHODS
To examine the conformation of the model retinoid beta-ionone both in solution and when bound to beta-LG, NMR and computational methods have been employed.


RESULTS
Taken together, NMR studies of beta-ionone in solution measuring scalar and dipolar coupling, as well as CHARMm calculations, suggest beta-ionone prefers a slightly twisted 6-s-cis conformation. Isotope-edited NMR studies of 13C-labeled beta-ionones bound to beta-LG, primarily employing the HMQC-NOE experiment, suggest beta-ionone also binds to beta-LG in its 6-s-cis conformation.


CONCLUSIONS
The methods employed here allow estimates of protein-bound ligand conformation. However, additional sites of ligand labeling will be necessary to aid in binding site localization.
Curley
R W
RW
Division of Medicinal Chemistry and Pharmacognosy, College of Pharmacy, The Ohio State University, Columbus 43210, USA. curley.1@osu.edu
Sundaram
A K
AK
Fowble
J W
JW
Abildgaard
F
F
Westler
W M
WM
Markley
J L
JL
D001665:Binding Sites / Q000502:physiology; D002250:Carbon Radioisotopes / Q000737:chemistry; D003198:Computer Simulation; D003512:Cyclohexanones / Q000138:chemical synthesis; D007782:Lactoglobulins / Q000138:chemical synthesis / Q000378:metabolism*; D008024:Ligands; D009682:Magnetic Resonance Spectroscopy; D008956:Models, Chemical; D045792:Norisoprenoids; D011487:Protein Conformation; D012176:Retinoids / Q000737:chemistry* / Q000378:metabolism*; D013729:Terpenes / Q000138:chemical synthesis / Q000378:metabolism*
D002250:Carbon Radioisotopes; D003512:Cyclohexanones; D007782:Lactoglobulins; D008024:Ligands; D045792:Norisoprenoids; D012176:Retinoids; D013729:Terpenes; C008157:beta-ionone