Inhibition of human choriotropin binding to receptor by human choriotropin alpha peptides. A comprehensive synthetic approach

Synthetic overlapping peptides of the alpha-subunit of human chorionic gonadotropin (hCG) were made by solid-phase peptide synthesis employing a comprehensive synthetic approach. The entire primary structure of the alpha-subunit was synthesized as a series of nine consecutive peptides, each 15 residues in length, and overlapping with its two adjacent neighbors by 5 residues on each side. Receptor binding activity of each synthetic peptide was measured by the inhibition of binding of 125I-labeled hCG to rat ovarian receptor. Peptides alpha 21-35, alpha 31-45, alpha 71-85, and alpha 81-92 were shown to compete for binding with native hCG, thus demonstrating that at least two regions on the alpha-subunit may be part of the binding site(s) of the hormone. The low affinity of the peptides (10(-5)-10(-6) M) compared to native hormone (10(-10) M) for receptor is not unexpected due to the probability of discontinuous and multiple sites involved in receptor binding. An ultrapure preparation of hCG alpha-subunit also had low affinity (10(-5), suggesting that conformational changes upon combination with beta-subunit to form dimer or changes in conformation after binding are necessary for high affinity interaction. These results correlate with previous predictions of binding sites based on studies employing chemical and enzymatic modifications of intact hormone and show that synthetic peptide strategies are helpful in the elucidation of protein structure and function.