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These are two crystal structures of HisJ as determined by x-ray crystallography.  The left structure is that of the open, unliganded form of HisJ.  The ligand or substrate-binding pocket is located along the dotted-line on the right side of the protein.  The right structure is that of the closed, liganded form of HisJ.  In this structure, the substrate, L-histidine, is trapped in the ligand-binding pocket.  Although both forms of HisJ are able to interact with the HisQMP2 complex, the closed, liganded form of HisJ maximally stimulates the HisQMP2 complex.