Structural and Functional Characterization of the Pro64Ser Leptin Mutant: Implications for Congenital Leptin Deficiency.

Congenital leptin deficiency or dysfunction is a form of monogenic childhood obesity. The disease is primarily caused by mutations in the LEP gene, which encodes for the expression of a hormone called leptin. The mutations typically impair leptin synthesis, secretion, or binding to the leptin receptor (LepR). The Pro64Ser mutation in leptin, despite not affecting the protein's stability or its binding affinity to the LepR, completely abolishes the protein's ability to mediate intracellular signaling via the LepR. To elucidate the mechanism underlying this signal inhibition and to further understand the mechanism of leptin-mediated LepR signal transduction, we performed extensive molecular dynamics simulations of both the wild-type (WT) and mutant (MT) leptins. Our simulations reveal that the Pro64Ser mutation increases the rigidity of AB loop N-terminus and thus prevents the loop's conformational changes required for interaction with the LepR immunoglobulin-like domain (IgD). Conversely, the CD loop of the MT exhibits increased flexibility compared to the WT. This elevated flexibility potentially hinders the protein's transition into helical structure and subsequent interaction with the IgD. Given that the interactions between leptin and the LepR IgD are crucial for the formation of higher-order leptin-LepR assembly and the following intracellular signal transduction, the observed changes in the MT leptin loop dynamics provide a mechanistic explanation for the signaling defects.