Peptide walking experiments identified four regions in hCG as components of its LH receptor-binding site: portions of aL2 and aL3 in the a subunit and PL2 and pL3 in the P subunit. Crystallographic studies on hCG demonstrated that these four regions formed a contiguous surface. Studies on hFSH have implicated three regions in FSH receptor binding, consisting of portions of a-subunit loops aL2 and aL3 along with a different portion of p-subunit loop pL3. The major LH receptor-binding specificity site in hCGp is the determinant loop, which consists of residues 93-100 in the seatbelt loop. The FSH receptor-binding region contains an adjacent peptide segment consisting of residues 101-109 (using hCGp residue numbering). buy ortho tri-cyclen online
In the model for hCG, residues 101— 109 are located slightly further away from the haAsn52 oligosaccharide than the determinant loop. Moyle and colleagues demonstrated that a bifunctional analogue of hCG could be created by substituting the FSHp determinant for the corresponding region of hCGp. These studies did not completely account for the intrinsic FSH activity of eLH and eCG, because residues 101-109 in both of these P subunits resemble LH/CGp more than FSHp. The determinant loop region of both equine gonadotropins possesses unique structure . Substitutions of eLH/CGp determinant loop residues into chimeric hCGp derivatives facilitated binding to the FSH receptor.