Myoglobin (oxy-myoglobin at 1.0 Angstrom resolution: PDB 1A6M. Sperm whale myoglobin at 1.7 Angstrom resolution: PDB 1VXH) is a single-chain protein of 153 amino acids, containing an iron porphyrin group in the center and being the primary oxygen-carrying pigment of muscle tissues. Unlike hemoglobin, to which it is structurally related, this protein does not exhibit cooperative binding of oxygen. Instead, the binding of oxygen by myoglobin is unaffected by the oxygen tension in the surrounding tissue. In 1957, John Kendrew and associates successfully determined the structure of myoglobin by high-resolution X-ray crystallography.