The pivotal role of the gonadotropins (GtHs) luteinizing hormone (LH) and follicle stimulating hormone (FSH) in vertebrate reproduction is well documented. LH and FSH bind to the leucine-rich repeats of the extracellular domains of their cognate receptors (R) in the gonads to actuate steroidogenesis and gametogenesis. Though these GtHs specifically bind only to their cognate receptors in mammals, this interaction becomes inconsistent in the case of fish. Whilst in some fish species the gonadotropins show receptor specificity much like their mammalian homologs, FSH and LH have demonstrated mutual and/or singular cross-activation of the FSH and/or LH receptors in several fish species. These complications regarding receptor specificity are further magnified by cross activation by orthologous GtHs from different species. So far no consistent pattern has been established to chart or predict this cross-talk in specific species or higher taxonomic orders. In the current study, we strived to understand the promiscuous nature of FSH and LH through in silico perspectives. While our studies in Russian sturgeon (Acipenser gueldenstaedtii) have shown FSH and LH to exhibit mutual promiscuity but of varying magnitude, in common carp (Cyprinus carpio) only LH showed singular cross-activation of FSHR. In Nile tilapia (Oreochromis niloticus) these gonadotropins exhibited strict specificity towards their cognate receptors similar to humans. Taking these species as representatives of varying degrees of cognate binding shown by the gonadotropins, we have generated docked models of these hormones and the extracellular domain of these receptors using in silico tools. We have further tried to compare these models, their docked poses, and key residues involved in the binding of these hormone-receptor complexes. We have further tried to compare parameters such as phosphorylation sites, N-linked glycosylation motifs, surface charge, etc. of these models to better understand the docking of these complexes. Our results suggest that in cases of cross activation by FSH and/or LH within or across the species, though binding sites are present at the leucine rich repeats on concaved face of the receptor extracellular domain, the docked position and residues involved in binding may vary significantly. Furthermore, though the seatbelt loop and GTHβ loop-2 (gonadotropin hormone subunit- beta) of the gonadotropin hormones act as the key players in binding, the docking of the hormone may vary in conformation. Due the large size and flexibility of these hormones and the receptor binding interface, it seems apparent that there are multiple binding sites that activate the resulting signaling pathway. Our study aspires to provide better insights towards the promiscuous nature of these hormones and attempt to overcome the handicap of lack of individual physical models using in silico tools.
