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

Abstract

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. � 2021 The Authors. Published by American Chemical Society