Title: The Role of Electrostatics in the Function of Homologous Thioester Containing Proteins: Insights into the Evolution of the Complement C3d:Cr2 Interaction
Abstract: Complement fragment C3d, a key component of the complement immune system, is involved in the opsonization of foreign pathogens (as a domain of C3), and is a link between innate and adaptive immunities. The complement immune system is an ancient mechanism for defense, and can be found in species that have been on the earth for the last 600 million years; however, the link between the complement system and adaptive immunity is a much more recent adaptation. Human C3d has high charge content, as do most complement proteins, and is believed to have evolved favoring the role of electrostatics in its function. To investigate the role of electrostatics in the function and evolution of human C3d, we have constructed 23 homology models of C3d homologues from various animal species with sequence similarities in the range of ∼30-80%. Electrostatic potentials of each homologue were calculated and electrostatic similarity methods were employed to identify conserved electrostatic “hotspots”. Electrostatic similarity methods were also utilized to analyze the effects of structural perturbations on the electrostatic character of the C3d homologues. Distributions of electrostatic similarity, based on families of perturbed structures produced either through an MD simulation or theoretical alanine-scan mutagenesis, illustrate that the electrostatic character of the functional sites of human C3d are resistant to change. Electrostatic similarity analysis of complement C3d identifies the binding sites for both host and pathogenic ligands, and such analysis could serve as a guide in computational drug design targeting C3d.