Structural Features of Grayanotoxins II and III and Their Affinity for Human Carbonic Anhydrase I: A Molecular Docking Study

Grayanotoxins are tetracyclic diterpenoid compounds of plants of the family Ericaceae, including the genus Rhododendron. Grayanotoxins are traditionally believed to target voltage-gated sodium channels, but an extract of honey containing Rhododendron ponticum pollen was recently shown to inhibit metal-dependent carbonic anhydrase, an effect attributed to grayanotoxin III (GRTIII). Since such honey may contain other grayanotoxin forms as well, a comparative study of their binding to human carbonic anhydrase is of interest. The aim of this work was to perform molecular docking of grayanotoxin II (GRTII) and grayanotoxin III (GRTIII) into the active site of human carbonic anhydrase I (hCA I) and to identify differences in their binding mechanisms as forms with different toxicity. The geometries of GRTII and GRTIII were optimized in Gaussian 9.0 with reference to X-ray diffraction data (GRTIICCDC:1170390, GRTIIICCDC:1246563); docking was carried out using the CB-DOCK2 method against the hCA I structure (RCSBPDB2FW4). Structurally, GRTII and GRTIII differ in the environment of the tenth carbon atom of the framework: in GRTIII it bears a hydroxyl group and a methyl radical, whereas in GRTII it is involved in an exocyclic double bond with a methylene fragment. According to the docking results, these groups at C10 of the seven-membered ring of GRTIII provide additional binding to hCA I: the methyl radical interacts with tyrosine (Tyr7), and the hydroxyl group interacts with serine (Ser231) and histidine (His64). For GRTII, no binding through this fragment is observed. The stronger binding of GRTIII to the active sites of hCA I was shown to be due to the presence of a hydroxyl group and a methyl radical in the seven-membered ring of the toxin; the results are consistent with published data on the relative toxicity of GRTII and GRTIII.