Structural changes in the TED (a) and CUB (b) domains between C3 and the contradicting crystal structures of C3b. (A) Significant conformational changes between the TED domains of C3 (grey) and C3b (pale blue) are only observed for the C3b structure by Janssen et al. (2I07; left) but not for the alternative structure by Abdul Ajees et al. (2HR0, right). In the case of peptide 1037–1044 (yellow and orange in C3 and C3b, respectively), the observed changes in HDX are therefore in closer agreement with structure 2I07. (B) While the CUB domain remains in a compact state in 2I07, the same domain appears rather unfolded and elongated in 2HR0. In the HDX analysis, the majority of the CUB-derived peptides show no significant change in deuteration (yellow), which would again support the 2I07 structure. In agreement with HDX data, peptide 957–968 (red) gets more solvent-exposed in both C3b variants.