Studies on the relevance of the glycan at Asn-52 of the α-subunit of human chorionic gonadotropin in the αβ dimer

Abstract

Glycosylation of Asn-52 of the α-subunit (αAsn-52) is required for bioactivity of the αβ-dimeric human chorionic gonadotropin (hCG), although at a molecular level the effect of the glycan at αAsn-52 is not yet understood. To study the role of this glycan for heterodimer stability, the β-subunit was recombined in solution with either the α-subunit or the α-subunit enzymically deglycosylated at αAsn-52. Enzymic deglycosylation avoids modification of the glycans at αAsn-78 and disturbing the protein folding. The efficiency of recombination after 16h is 80%, independent of whether αAsn-52 is glycosylated or not. The dissociation constant of the hCG complex, with or without the glycan at αAsn-52, is less than 1×10−5s−1, indicating that the glycan at αAsn-52 does not contribute significantly to the stability of the dimer. CD and NMR spectra indicate a local conformational difference between both αβ-dimeric hCG variants, most probably involving amino acids of the hCG β-subunit close to the glycan at αAsn-52. These data explain the native-like receptor-binding abilities of hCG lacking the glycan at αAsn-52. It is proposed that for bioactivity the glycan at αAsn-52 is necessary for inducing and stabilizing a conformational change in hCG upon binding to the receptor, resulting in activation of the signal-transduction pathway.