Figure 6.

ChdC active site gate that may play a role in a CpfC–ChdC protein–protein interaction. Juxtaposed structures of (a) apo-ChdC (tan, Protein Data Bank entry 1T0T) and (b) coproheme-bound ChdC (green, Protein Data Bank entry 5T2K, both structures from Geobacillus thermophilus) reveal a mobile active site loop (amino acids ~110–135) that closes in toward the active site to sequester coproheme (arrows in panel a indicate the direction of the change in the residue position upon coproheme binding). Solvent-exposed areas of the active site are colored gray. Point mutations were generated at the positions shown. The mutant proteins and the phenotype of a complemented ΔchdC strain were characterized. Note that the residue at position 129 is a lysine in the S. aureus homologue. The locations of residues 114–119 could not be crystallographically mapped and are missing in the 5T2K structure (b).¹²