FH19-20 residues involved in binding to C3b and sialic acid. Shown are mesh-and-sphere representations of the FH19-20 structure (Protein Data Bank [PDB] ID code 2G7I)¹⁴  from two different angles. (A) Binding sites for C3b/C3d and sialic acid. The main interface residues forming the C3b binding site (N1117, Q1139, Y1142, P1166, K1188, and Y1190)^21,22  are shown in green. The residues likely to have an impact on sialic acid binding (I1169, R1182, T1184, L1189, S1191, G1194, V1197, E1198, F1199, and R1215)¹⁷  are shown in dark yellow. Residues of specific relevance to this study (T1184, L1189 and E1198) have been highlighted with orange. (B) Location of the residues that were mutated in the control constructs. Whereas the aHUS-associated mutations T1184R, L1189R, and E1198A lead to increased FH19-20 affinity for heparin, other mutations have been reported to either decrease (R1182A, K1186A, and R1215Q) or have no effect (W1157L, W1183L) on the interaction.^12,13  Residues R1182, W1183, K1186, and R1215 are depicted in blue. Residue W1157 is invisible because it is buried within the domain 19.