Crystal Structure of the Transthyretin–Retinoic‐Acid Complex

Abstract

Retinoids are quite insoluble and chemically unstable compounds in thc aqucous mcdium, such that natural retinoids nccd to be bound to specific retinoid-binding protcins to be protected, solubilized and transported in body fluids. All-tmns retilioic acid exhibits a relatively high affinity for thyroxine-binding transthyretin irz vitm and this protein is a good candidate for the transport of retinoic acid adn~iniskred us pharmacological or antitumor agent. To dcfitic structural features esseniial for the recognition by transthyrctin of a ligand which is structurally unrclatcd to thyroxine, we have cocrysiallized humun transthyrctin with retinoic acid and deleiinincd its structure at 0.18-nrri tesulution. The retinoid fits into thc two chemically identical thyroxine-binding sites, which are located in the ccntral channcl that rutis through the tetranieric transthyretin. The cyclohexene ring of the hound retinoid is innermost, occupying the sarrie position of the phenolic ring of the bound 3,3'-diiorlo-L-tliyrotiitie, whereas the carboxylate group, like the same group of the thyroid hormone, participatcs in an ionic interaction with the LyslS sidc chain at the entrancc of the channel. Despitc the fact that transthyretin was cocrystallized with all-/rrrn,r-retinoic acid, the isoprene chain of the bound retinoid hus bccn found i n a non-exlendcd conforniation. This feature, that allows thc carboxylate to oricnt in a manner suitable for ion-pair association with the LyslS side chain, is attributable to the conversion of all-trans-retinoic acid into cis-isomers or folded cotifortiiers. It is concluded that the prcsence, i n an essentially hydrophobic rnolecular corc of the appropriate s i 7~ 01 a negatively charged group at the correct position is a crucial requirement for Iigand-tratisthyretirl recognition. Whereas the binding of the ligand has no remarkable conscqucnces for the prolein structure, alltmns-retinoic acid undergoes structural changes such as to interact favorably with residues preseni in the thyroxine-binding sites, rcsetiibling roughly the natural ligantl.

Structure of pig plasma retinol-binding protein at 1.65 Å resolution

“…Thus, the retinol hydroxyl group may significantly contribute to the strength of binding of the vitamin to the protein. This is in agreement with the results of competition binding experiments showing a lower binding affinity of retinoids for RBP as compared with retinol (Berni R., unpublished results) and with structural data for the binding of retinol analogs to RBP (Zanotti, Malpeli et aL, 1993;Zanotti et al, 1994). The non-perfect planarity of the rest of the tail, and in particular of the last two C atoms, may be attributed to the constraints imposed by the protein side chains and by the very limited space available in the fl-barrel cavity.…”

Synthesis, Structure, and Activity of Diclofenac Analogues as Transthyretin Amyloid Fibril Formation Inhibitors

“…It has been observed that TTR, like albumin, is quite promiscuous in its binding to small molecules. The range of structures that are competent as amyloid fibril formation inhibitors has been alluded to above; it has also been found in unrelated studies that TTR binds to aurones, 37 bipyridines, 35 retinoic acid (even without the intermediacy of retinol binding protein), 52 and various organic halogen substances (OHS). 34,[53][54][55][56] In fact, it has been suggested that TTR transports polychlorinated biphenyls in vivo.…”

Transthyretin: a review from a structural perspective

“…The authors hypothesize that these aromatic residues, which are absent in transthyretin, could account for the difference in thyroxine binding affinity displayed by the two proteins. In 1995, the laboratory of Zanotti [62] determined the crystal structure of a complex of transthyretin with retinoic acid. The retinoid fits into the two chemically identical thyroxine-binding sites.…”