Background/Objectives: Follitropin delta is an approved recombinant follicle stimulating hormone (rFSH) expressed in a human cell line. The FSH protein is a heterodimer containing α and β subunits. The FSH α-subunit (FSHα) contains five intramolecular disulfide bonds: C7‑C31, C10‑C60, C28‑C82, C32‑C84 and C59‑C87. The α subunit has four closely spaced cysteine residues that form disulfide bonds. These clustered sites resist protease cleavage, making it difficult to accurately characterize the disulfide bond pairing in this type of protein therapeutic. Methods: The study uses an X-ray crystallography method to determine a high-resolution three-dimensional model structure of the recombinant FSH α-subunit. Results: The crystal structure of FSHα at 2.29 Å resolution, containing the disulfide bonds, is presented. This high-resolution structure provides definitive structural evidence that the disulfide bonds in rFSH are consistent with the expected native structural conformation. Conclusions: The rFSH structure validates the expected cysteine connectivity, overcoming limitations of prior mass spectrometry-based disulfide mapping attempts.

Background/Objectives: Follitropin delta is an approved recombinant follicle-stimulating hormone (rFSH) expressed in a human cell line. Correct disulfide connectivity is a critical quality attribute for rFSH, a heterodimeric glycoprotein composed of noncovalently associated α and β subunits and stabilized by an extensive network of intramolecular disulfide bonds. Disulfide characterization is typically performed by mass spectrometry (MS). However, the closely spaced disulfide bonds within the FSH α-subunit are particularly resistant to proteolytic cleavage, complicating conventional MS-based disulfide mapping. Methods: To overcome limitations of MS-based methods, an X-ray crystallography strategy was employed using a ternary complex of the recombinant FSH heterodimer with an anti-FSHα Fab and a stabilizing anti-kappa VHH. Crystals of the desialylated rFSH/Fab/VHH complex were obtained and diffraction data were collected. Results: The structure of recombinant FSH was determined at 2.29 Å resolution. Electron density surrounding cysteine residues in both the α and β subunits was well defined, allowing unambiguous assignment of all intramolecular disulfide bonds in the crystallized protein. The observed cysteine connectivity is fully consistent with the disulfide architecture of FSH from other sources and supports correct folding of the recombinant Follitropin delta.