Molecular basis of lateral force spectroscopy nano-diagnostics: Computational unbinding of autism related chemokine MCP-1 from IgG antibody

Abstract

Monocyte-chemoattractant protein-1 (MCP-1), also known as CCL2, is a potent chemoattractant of T cells and monocytes, involved in inflammatory and angio-proliferative brain and retinal diseases. Higher expression of MCP-1 is observed in metastatic tumors. Unusual levels of MCP-1 in the brain may be correlated with autism. Immunochemistry where atomic force microscope (AFM) tips functionalized with appropriate antibodies against MCP-1 are used could in principle support medical diagnostics. Useful signals from single molecule experiments may be generated if interaction forces are large enough. The chemokine-antibody unbinding force depends on a relative motion of the interacting fragments of the complex. In this paper the stability of the medically important MCP-1- immunoglobulin G antibody Fab fragment complex has been studied using steered molecular dynamics (SMD) computer simulations with the aim to model possible arrangements of nano-diagnostics experiments. Using SMD we confirm that molecular recognition in MCP1-IgG is based mainly on six pairs of residues: Glu39A - Arg98H, Lys56A - Asp52H, Asp65A - Arg32L, Asp68A - Arg32L, Thr32A - Glu55L, Gln61A - Tyr33H. The minimum external force required for mechanical dissociation of the complex depends on a direction of the force. The pulling of the MCP-1 antigen in the directions parallel to the antigen-antibody contact plane requires forces about 20 %-40 % lower than in the perpendicular one. Fortunately, these values are large enough that the fast lateral force spectroscopy may be used for effective nano-diagnostics purposes. We show that molecular modeling is a useful tool in planning AFM force spectroscopy experiments.