Fig. 3. Myoglobin consists of a backbone and heme-binding domain. (A) Myoglobin was the first protein to be subjected to X-ray crystallography. The backbone of myoglobin consists of eight -helices (blue) that wrap around a central pocket containing a heme group (red), which is capable of binding various ligands including oxygen, carbon monoxide and nitric oxide. (B) The protoheme group is bracketed or stabilized by histidine residues above (His64) and below (His93).