Chemical Synthesis of the β-Subunit of Human Luteinizing (hLH) and Chorionic Gonadotropin (hCG) Glycoprotein Hormones

Abstract

Human luteinizing hormone (hLH) and human chorionic gonadotropin (hCG) are human glycoprotein hormones each consisting of two subunits, an identical α-subunit and a unique β-subunit, that form noncovalent heterodimers. Structurally, β-hCG shares a high degree of sequence similarity with β-hLH, including a common N-glycosylation site at the N-terminus but differs mainly in the presence of an extended C-terminal portion incorporating four closely spaced O-linked glycans. These glycoproteins play important roles in reproduction and are used clinically in the treatment of infertility. In addition, the role of hCG as a tumor marker in a variety of cancers has also attracted significant interest for the development of cancer vaccines. In clinical applications, these hormones are administered as mixtures of glycoforms due to limitations of biological methods in producing homogeneous samples of these glycoproteins. Using the powerful tools of chemical synthesis, the work presented herein focuses on the highly convergent syntheses of homogeneous β-hLH and β-hCG bearing model glycans at all native glycosylation sites. Key steps in these syntheses include a successful double Lansbury glycosylation en route to the N-terminal fragment of β-hCG and the sequential installation of four O-linked glycosyl-amino acid cassettes into closely spaced O-glycosylation sites in a single, high-yielding solid-supported synthesis to access the C-terminal portion of the molecule. The final assembly of the individual glycopeptide fragments involved a stepwise native chemical ligation strategy to provide the longest and most complex human glycoprotein hormone (β-hCG) as well as its closely related homologue (β-hLH) as discrete glycoforms.