Synergy of Putative Binding Modes in the Factor H (CCP 19-20) and C3d Complex

Abstract

Within innate immunity, the complement system surveils biological milieu and acts as a first line of defense against invading pathogens. Of the three pathways that comprise the complement system, the alternative pathway (AP) is critical for propagation of an immune response. The AP attacks foreign and native surfaces alike through feed-forward amplification, attaching complement C3b (C3b) to surfaces where a response will be generated. For this reason, regulation is required to prevent a state of autoimmunity. Complement factor H (FH) fulfills this role, inhibiting the AP on surfaces of self while allowing complement response on surfaces of non-self. FH comprises 20 complement-control protein (CCP) modules that vary in function while maintaining structural homology. Of these modules, CCP 19-20 are responsible for recognition and binding of complement C3d, a subdomain of C3b where surface binding occurs. While there exists two crystallographic structures of FH CCP 19-20 in complex with C3d, the mechanism of interaction has been controversial. Herein we approach computational characterization of interactions between FH CCP 19-20 and C3d using biophysical principles. Namely, we assess each binding mode in terms of electrostatic character, structural stability, dissociative properties, associative properties, and predicted free energy of binding. Through these investigations, we found two of the three binding modes to be similarly stable though each mode is stabilized through distinct physicochemical mechanisms. These modes of binding are likely physiologically relevant, and we propose thermodynamically stable binding with modules 19 and 20, the latter module driven by electrostatics, acting in a synergistic manner to increase the apparent affinity of FH for native surfaces.