Myoglobin (Mb) is a small, globular protein primarily responsible for oxygen storage in cardiac and skeletal muscle. Mb is involved in the regulation of cellular oxygen tension in respiring tissues, and regulating the bioavailability of the signaling molecule, nitrous oxide (NO) (Wittenberg and Wittenberg 2003). It contains a single heme molecule and has a molecular weight of approximately 17 kDa.
In 1897 Mörner described a red protein in muscle, which he called “myochrome”.
Early studies of myoglobin are considered to be where the science of protein structure began. John Kendrew and his coworkers determined the atomic structure of sperm whale myoglobin, making it the first protein to have its three dimensional structure revealed by X-ray crystallography. This feat earned Kendrew the Nobel Prize in 1962, shared with Max Perutz who revealed the structure of hemoglobin (Kendrew et al. 1958, Perutz et al. 1960, and Watson and Kendrew 1969).
Recent research involving myoglobin has included the development of assays for serum myoglobin detection (Osman et al. 2013, and Padmanaban et al. 2014), as well as the development of additives to bind myoglobin to preserve red coloring of fresh meat (Miura et al. 2014, and Suman et al. 2014).
The bovine mb gene is located on chromosome 5. It is conserved in human, chimpanzee, Rhesus monkey, dog, mouse, rat, chicken, and zebrafish. It appears to be deleted in amphibians and a few species of Notothenioid icefish of the Antarctic Ocean (Fuchs et al. 2006, and Helbo et al. 2013). In addition to heart and skeletal muscle, Mb has been identified in vascular smooth muscle cells, where it may contribute to vasodilation under hypoxic conditions by converting nitrite to NO (Totzeck et al. 2012). Mb has also been identified in non-muscle tissues of fish species and mammalian cancer cells (Fraser et al. 2006, Roesner et al. 2008, Cossins et al. 2009, Flonta et al. 2009, and Gorr et al. 2011). Mb transcription upregulation is a complex process involving hypoxia in conjunction with intracellular calcium release, which activates calcineurin signaling pathways, transcription factors, and transcriptional co-activators (Kanatous et al. 2009, Kanatous and Mammen 2010, and Helbo et al. 2013). In skeletal muscle, expression increases with exposure to hypoxic stress together with muscle contraction (Kanatous et al. 2009).
Myoglobin contains one protein chain, a heme group with water bound to the iron, and a sulfate ion. The protein chain consists of 8 spring-shaped alpha helices, linked together by short loops and designated as A-H. The heme prosthetic group is sandwiched between helices E and F. This single heme group is noncovalently bound within a deep cleft of the protein, and is composed of a porphyrin ring and a bound iron. Myoglobin can bind a single oxygen molecule between the distal histidine and the heme iron atom. Hydrogen bonding of the distal histidine and the oxygen crowds the area enough to cause the oxygen to bind at an angle with respect to the plane of the heme group (Pratt and Cornely 2004).
Protein Accession Number: P02192
Molecular Weight: 16.9 kDa (Theoretical)
CATH Classification (v. 4.0):
Class: Mainly Alpha
Architecture: Orthogonal Bundle
Isoelectric Point: 6.97 (Theoretical)