Cytochrome c oxidase contains two heme redox center, a penta-coordinated heme a3 and a hexa-coordinated heme a. Most articles descirebes that inhibitor molecules attac the penta-coordinated heme a3 center, however I coundn't find any information about the ligand binding properties of the hexa-coordinated heme a.

Another heme protein, neuroglobin (Ngb), also contains hexa‑coordinated heme prosthetic group with proximal and distal His ligands. Therefore, ligand binding of Ngb is accompanied by the dissociation of the distal His ligand from the heme center.

Is cytochrome c oxidase also capable of ligand binding in similar manner at the hexa-coordinated heme a redox center?