Disulfide bonds Cys 9–Cys 57, Cys 34–Cys 88 and Cys 38–Cys 90 of the β-subunit of human chorionic gonadotropin are crucial for heterodimer formation with the α-subunit: experimental evidence for the conclusions from the crystal structure of hCG

Abstract

Human chorionic gonadotropin (hCG) is a heterodimeric glycoprotein hormone essential for the establishment and maintenance of pregnancy. The α- and β-subunits of hCG are highly cross-linked internally by disulfide bonds that seem to stabilize the tertiary structures required for the noncovalent association of the subunits to generate hormonal activity. This paper describes the results of our studies on the role of the disulfide bonds of hCG-β in heterodimer formation with the α-subunit. Six disulfide peptides incorporating each of the six disulfide bonds of hCG-β were screened, along with their linear counterparts, for their ability to competitively inhibit the recombination of α- and β-subunits. The disulfide peptides Cys (9–57), Cys (34–88) and Cys (38–90) were found to inhibit the α/β recombination whereas the remaining three disulfide peptides viz. Cys (23–72), Cys (26–110) and Cys (93–100) did not exhibit any inhibition activity. Interestingly, none of the linear peptides could inhibit the α/β recombination. Results clearly demonstrate that the disulfide bonds Cys 9–Cys 57, Cys 34–Cys 88 and Cys 38–Cys 90 of the β-subunit of hCG are crucial for heterodimer formation with the α-subunit thus providing experimental confirmation of the conclusions from the crystal structure of the hormone.