'It's hollow': the function of pores within myoglobin.

Despite a century of research, the cellular function of myoglobin (Mb), the mechanism regulating oxygen (O(2)) transport in the cell and the structure-function relationship of Mb remain incompletely understood. In particular, the presence and function of pores within Mb have attracted much recent attention. These pores can bind to Xe as well as to other ligands. Indeed, recent cryogenic X-ray crystallographic studies using novel techniques have captured snapshots of carbon monoxide (CO) migrating through these pores. The observed movement of the CO molecule from the heme iron site to the internal cavities and the associated structural changes of the amino acid residues around the cavities confirm the integral role of the pores in forming a ligand migration pathway from the protein surface to the heme. These observations resolve a long-standing controversy - but how these pores affect the physiological function of Mb poses a striking question at the frontier of biology.