Interaction of Human C5a with the Major Peptide Fragments of C5aR1: Direct Evidence in Support of “Two-Site” Binding Paradigm

[Image: see text] The C5a receptor’s (C5aR1) physiological function in various tissues depends on its high-affinity binding to the cationic proinflammatory glycoprotein C5a, produced during the activation of the complement system. However, an overstimulated complement can quickly alter the C5a–C5aR1 function from physiological to pathological, as has been noted in the case of several chronic inflammation-induced diseases like asthma, lung injury, multiorgan failure, sepsis, and now COVID-19. In the absence of the structural data, the current study provides the confirmatory biophysical validation of the hypothesized “two-site” binding interactions of C5a, involving (i) the N-terminus (NT) peptide (“Site1”) and (ii) the extracellular loop 2 (ECL2) peptide of the extracellular surface (ECS) of the C5aR1 (“Site2”), as illustrated earlier in the reported model structural complex of C5a–C5aR1. The biophysical and computational data elaborated in the study provides an improved understanding of the C5a–C5aR1 interaction at an atomistic resolution, highlighting the energetic importance of the aspartic acids on the NT-peptide of C5aR1 toward binding of C5a. The current study can potentially advance the search and optimization of new-generation alternative “antibodies” as well as “neutraligands” targeting the C5a to modulate its interaction with C5aR1.