Fig. 2: Structural changes in the HisFH complex upon allosteric activation.

a Crystal structure of the HisFH complex (hC84A) in the inactive conformation (HisF in gray, HisH in yellow; PDBID: 7AC8, chains C/D) bound to Gln (left) and in the active conformation (HisF in marine, HisH in orange, chains E/F) bound to ProFAR and Gln (right). The largest structural differences between the two conformations are observed in HisF (red arrows). The reorientation of HisH relative to HisF is indicated by a cyan arrow and leads to a reduction of the angle between fF120, hW123 and hG52 (Cα atoms) from 25° to 10°. The major structural changes between the inactive and active conformations are indicated as cartoons. b The local backbone RMSD between the inactive and active conformations (PDBID 7AC8, chains C/D and E/F), after individual alignment of the proteins, mapped onto the structure of the active conformation. c Closeup of the HisF active site: Comparison between the inactive (left; PDBID 7AC8; chain A) and the active conformation (right; PDBID 7AC8, chain E). Important hydrogen bond interactions are shown in green. fL1 is shown in red. The compaction of the HisF active site in the active conformation is indicated by red arrows. d Oxyanion hole formation in the active conformation of HisH: The oxyanion hole is not formed in the inactive conformation (left; PDBID 7AC8, chain D). Red arrows indicate important conformational changes upon formation of the active conformation. The oxyanion hole (right; PDBID 7AC8, chain F) is formed by the rotation of the hG50/hV51 amide group. The side chain of the substrate Gln rotates into the oxyanion hole and forms hydrogen bonds to hG50 and hL85 (cyan). Other hydrogen bonds are shown in green. e The rearrangement of the HisF–HisH interface moves fα3/α4 and fD98 closer to the active site (red arrows). Upon transition from the inactive (left; PDBID 7AC8, chains C/D) into the active conformation (right; PDBID 7AC8, chains E/F) fD98 moves by 3.6 Å and interacts with hN12. Additional backbone hydrogen bonds are formed that connect hL4 and hα1 in HisH with fα4 and fα3 in HisF.