The biological action of choriogonadotropin is not dependent on the complete native quaternary interactions between the subunits

Human CO (hCG) is a member of the glycoprotein hormone family characterized by a heterodimeric structure consisting of a common alpha-subunit noncoval ently bound to a hormone-specific beta-subunit, The two subunits are highly intertwined and only the heterodimer is functional, implying that the quat ernary structure is critical for biological activity. To assess the depende nce of the bioactivity of hCG on the heterodimeric interactions, alpha- and beta-subunits bearing mutations that prevent assembly were covalently link ed to form a single chain hCG. Receptor binding and signal transduction of these analogs were tested and their structural integrity analyzed using a p anel of monoclonal antibodies (mAbs). These included dimer-specific mAbs, w hich react with at least four different epitope sites on the hormone, and s ome that react only with the free beta-subunit. We showed that there was si gnificant loss of quaternary and tertiary structure in several regions of t he molecule. This was most pronounced in single chains that had one of the disulfide bonds of the cystine knot disrupted in either the alpha- or beta- subunit. Despite these structural changes, the in vitro receptor binding an d signal transduction of the single chain analogs were comparable to those of the nonmutated single chain, demonstrating that not all of the quaternar y configuration of the hormone is necessary for biological activity.