Binding kinetics, structure-activity relationship, and biotransformation of the complement inhibitor compstatin

We have previously identified a 13-residue cyclic peptide, Compstatin, that binds to complement component C3 and inhibits complement activation. Herei n, we describe the binding kinetics, structure-activity relationship, and b iotransformation of Compstatin, Biomolecular interaction analysis using sur face-plasmon resonance showed that Compstatin bound to native C3 and its fr agments C3b and C3c, but not C3d, While binding of Compstatin to native C3 was biphasic, binding to C3b and C3c followed the 1:1 Langmuir binding mode l; the affinities of Compstatin for C3b and C3c were 22- and 74-fold lower, respectively, than that of native C3, Analysis of Compstatin analogs synth esized for structure-function studies indicated that 1) the 11-membered rin g between disulfide-linked Cys(2)-Cys(12) constitutes a minimal structure r equired for optimal activity; 2) retro-inverso isomerization results in los s of inhibitory activity; and 3) some residues of the type I beta -turn seg ment also interact with C3, In vitro studies of Compstatin in human blood i ndicated that a major pathway of biotransformation was the removal of IIe(1 ), which could be blocked by N-acetylation of the peptide. These findings i ndicate that acetylated Compstatin is stable against enzymatic degradation and that the type I beta -turn segment is not only critical for preservatio n of the conformational stability, but also involved in intermolecular reco gnition.