Structural features of mammalian gonadotropins

Abstract

There are two species for which both pituitary and placental gonadotropins are readily available, humans and horses. The human gonadotropins are better characterized than equine gonadotropins. Nevertheless, the latter are very interesting because they provide exceptions to some of the general structure-function principles derived from studies on human and other mammalian gonadotropins. For example, separate genes encode the hLHβ and hCGβ subunits while a single gene encodes eLHβ and eCGβ. Thus, eCG and eLH differ only in their oligosaccharide moieties and eLH is the only LH that possesses the O-glycosylated C-terminal extension previously believed to be restricted to chorionic gonadotropins. Truncation experiments involving eLHβ and hCGβ have suggested the C-terminal extension has no effect on receptor binding. However, the largest of three eCG forms which differ only in the extent of O-glycosylation possessed reduced affinity for LH and FSH receptors. This result suggested that effects of O-glycosylation need to be considered when examining the glycosylation differences between eLH and eCG responsible for the 10-fold lower eCG receptor binding affinity compared with that of eLH. Contribution of αAsn 56 N-linked oligosaccharides to the different biological activities of eLH and eCG has been evaluated following selective removal using peptide- N-glycanase digestion of native equine α-subunit preparations. Hormone-specific patterns of glycosylation were observed on αAsn 56 of eLH, eFSH, and eCG. Removal of αAsn 56 oligosaccharides increased the rate of subunit association, the extent of association, and receptor binding activity. Some unassociated α-subunit oligosaccharides were identified which may interfere with subunit association because they were more abundant in unassociated subunit oligosaccharide maps than in a total oligosaccharide map. This was most striking in the case of eCGα in which two minor peaks became the major oligosaccharide peaks detectable in the unassociated eCGα fraction following association with eLHβ and eFSHβ. The biological activities exhibited by hybrid hormones, eLHα reassociated with oLHβ and pLHβ, found to be greater than those of oLH and pLH provided an interesting exception to the general rule that the β-subunit determines the potency of the heterodimer. LH receptor binding activities of eLHβ-chimeric ovine/equine α-subunits suggested that the equine α-subunit N-terminal domain may be responsible for this effect. Equine FSH has higher FSH receptor binding activity than human, ovine, and porcine FSH preparations. This probably results from two factors. First, the presence of the equine α-subunit promotes receptor binding as noted above. Second, the overall −2 charge of the eFSHβ determinant loop, which is less negative that the −3 observed in other species, results from the presence of an Asn residue at position 88 instead of Asp. This apparently facilitates binding to the FSH receptor.