Three-dimensional structures of gonadotropins

Most secreted proteins are modified post-translationally with the addition of carbohydrate. It has been difficult to use crystallography to solve the structures of these proteins due to the inherent heterogeneity of the carbohydrate. The structure of the chemically deglycosylated form (hydrogen fluoride treated) of human chorionic gonadotropin (hCG) has been solved through crystallographic techniques. Unfortunately this form of hCG is not biologically active, and exhibits immunochemical differences from native hormone. In addition, subunit interactions appear altered after chemical deglycosylation as indicated by the increased thermal stability of the HF-treated hormone. The Asn 52 glycan on the α-subunit of hCG has been identified as being required for biological activity, it is, therefore, of physiological importance to determine the structure of the hormone with its carbohydrate intact. Also, it has not been possible to obtain crystals of the individual glycosylated subunits of hCG. Therefore an alternative method to solve the structure of the biologically active form of the hormone in solution as well as its separated subunits is necessary. Structural information utilizing NMR techniques can be obtained from native hCG subunits in solution if they can be uniformly labeled with ¹³C and ¹⁵N isotopes. We have developed a universal nonradioactive isotope, labeling medium enriched in ¹³C and ¹⁵N which can be used to express uniformly labeled hCG from Chinese hamster ovary cells suitable for solving the structure of the individual subunits and ultimately that of the native, biologically active hormone. The isotopically labeled recombinant hCG and its purified subunits are essentially identical to urinary hCG on comparison by biochemical, immunochemical, biological activity and the ability of the isolated subunits to recombine to form a biologically active dimer. Mass spectrometric analysis and preliminary structural NMR data indicate that the labeling is uniform and there is greater than 90% incorporation, sufficient for complete structural determination studies. This labeled growth medium represents a technological advance which will enable the rapid solution of the structures of the other glycoprotein hormones, as well as other glycoproteins which have proven unsuitable for crystallographic study.