Enzymic Remodelling of the N- and O-Linked Carbohydrate Chains of Human Chorionic Gonadotropin

Abstract

The effects of altered terminal sequences in human chorionic gonadotropin (hCG) N- and O-linked glycans on receptor binding and signal transduction were analyzed using forms of hCG with remodelled carbohydrate chains. hCG derivatives were obtained by enzymic removal of the α3-linked sialic acid residues followed by α6-sialylation, α3-galactosylation or α3-fucosylation of uncovered Galβ14GlcNAc (LacNAc) termini, or α3-sialylation of Galβ13GalNAc sequences. Also a form that carried GalNAcβ14-GlcNAc units, which are typical for pituitary hormone oligosaccharides, was derived by enzymic desialylation and degalactosylation followed by β4-N-acetylgalactosaminylation. The potency to stimulate testosterone production and the binding to the lutotropin/choriogonadotropin receptor of the preparations were compared with those of native and desialylated hCG (as-hCG). The decrease in bioactivity caused by desialylation of hCG was only restored upon α6-sialylation of the Galβ14GlcNAcβ12Manα13Man branch of the N-linked glycans. This was without a major effect on receptor binding. Further α6-sialylation, occurring at the Galβ14GlcNAcβ12Manα16Man branch, resulted in a bioactivity below a level found with as-hCG, concomitant with a decreased receptor binding affinity. Similarly α3-galactosylation of the Galβ14GlcNAcβ12-Manα16Man branch yielded a hCG derivative that showed decreased bioactivity and receptor binding. α3-Fucosylation of native as well as as-hCG also led to a decreased activity. Re-α3-sialylation of the O-linked chains on as-hCG had little effect on the bioactivity and receptor binding. Hormone preparations with GalNAcβ4GlcNAc termini showed lower bioactivity and receptor affinity than as-hCG. It is concluded that the Galβ14GlcNAcβ12Manα13Man- rather than the Galβ1 4GlcNAcβ12-Manα16Man branch of the N-linked glycans on hCG plays an essential role in signal transduction, whereas the latter branch can potentially interfere with receptor binding. Furthermore attachment of sialic acid, but not of other sugars, to the first branch fulfils the requirement for the full expression of bioactivity, while sialylation of the O-linked chains is of minor importance.