Sugar moiety has a direct influence on the secondary structure and properties of sturgeon gonadotropin

Sturgeon Acipenser güldenstädti Br. gonadotropic hormone (GTH) β-subunit (β-GTH) was selectively chemically deglycosylated (dg) by anhydrous hydrogen fluoride and trifluoromethane sulfonic acid. Two dgβ-GTH molecular forms retaining 35% (dgβ-GTH1) and 13% (dgβ-GTH2) of their initial carbohydrates were obtained. Investigation of the reassociated α-β hybrid dimers (recombinants) α-GTH+dgβ-GTH1 and α-GTH+dgβ-GTH2 showed that the immunoreactivity with antiserum raised against standard GTH dropped by 22.5%. Hybrid dimers were recognised by the standard GTH antibodies, which indicated that the sugar moiety was not required for the counterpart subunit interaction. Hormonal activity of the α-GTH+dgβ-GTH1 dropped twofold while the hybrid dimer α-GTH+dgβ-GTH2 completely lost its activity in in vitro tests on the frog oocytes. CD-spectra indicated considerable changes in the secondary structure of the α-GTH+dgβ-GTH2: content of the β-structures decreased fourfold, but disordered structure content increased by 52% in comparison with that of the standard α-GTH+β-GTH. However, the individual dgβ-GTH1 and dgβ-GTH2 did not show any considerable differences in their secondary structure in comparison with that of the standard subunit β-GTH. It was evident that the slightly changed secondary structure of the dgβ-GTH2 as such might not be able to produce drastic distortions in the hybrid dimer structure. A conclusion is made that the carbohydrate moiety of the β-GTH has a predominant role as a direct stabiliser of the functionally effective secondary structure of the whole sturgeon GTH heterodimeric α-β molecule.