# Structural insights into C3 convertase activity of the classical pathway of complement

## TL;DR

This study reveals the structural details of how the classical pathway of the complement system cleaves C3, a key immune process, using cryo-EM.

## Contribution

The paper provides the first cryo-EM structures of classical pathway C3 convertase complexes and explains a charge switch-over mechanism for C3 cleavage.

## Key findings

- C2 and C4b form proconvertases and convertases similar to the alternative pathway.
- C3 binds to C4b through two interfaces, one shared with the SCIN-inhibited C3bBb dimer.
- A charge switch-over mechanism facilitates C3 cleavage and repels the product C3b.

## Abstract

Immune protection by the complement system depends on C3 cleavage by C3 convertases that is critical to all three activation pathways. Structural data on convertase formation in the classical pathway and on C3-substrate binding to convertases is lacking. We present the cryo-EM structures of the proconvertase (C4b2), convertase (C4b2b), and convertase-substrate complex (C4b2b-C3) of the classical pathway. The data show that C2 and C4b form proconvertases and convertases like factor B and C3b of the alternative pathway. Substrate C3 binds C4b of the convertase through two interfaces: one also found in the SCIN-inhibited C3bBb dimer, and another facilitated by conformational changes in C3. Bending of C3 and swinging of the C2 protease bring the C3-scissile loop into the active site. The second, charged, C4b-interaction site favors C3- substrate binding, but upon cleavage repels product C3b. Thus, a charge switch-over mechanism effects the catalytic turnover of the convertases producing opsonin C3b.

The complement system depends on C3 cleavage that is critical to all three activation pathways, but currently structural data is lacking. Here authors present cryo-EM structures revealing a charge switch-over mechanism.