Specific Structural-Functional Role of the Pronounced Negative Molecular Charge of the Sturgeon Gonadotropin β-Subunit

Free negatively charged carboxyl groups (COOH) were selectively modified (neutralised) in sturgeon (Acipenser güldenstädti Br) gonadotropic hormone (GTH) β-subunit (βGTH). Eight COOH groups were neutralised by reaction with glycine ethyl ester. Investigation of modified βGTH showed that specific immunoactivity with antiserum raised against standard βGTH was completely lost. Analysis of CD spectra and the content of secondary structure elements of standard and modified βGTH did not indicate any considerable differences. Investigation of reassociated α-β dimer comprising standard αGTH and modified βGTH showed that hormonal activity was completely lost while immunoreactivity was lowered by about 30% in comparison with that of the standard subunit α-β dimer. It was concluded that eight free COOH groups located on the surface of βGTH are the main structural components of the species-specific antigenic determinant groups of the subunit, but are not directly involved in either maintaining the conformation of the subunit nor in its ability to interact with the native counterpart subunit. COOH groups, as bearers of the negative charge located on the surface of βGTH, play a decisive role in the specific hormonal activity of GTH determined by oocyte maturation tests. Free COOH groups are included in the effector zones (active sites) of GTH dimer molecule responsible for the stereospecific interaction with the test-oocyte membrane hormone-sensitive binding (receptor) sites to induce the biological effects.