myoglobins

Myoglobins are a family of globular, heme-containing proteins that store and facilitate diffusion of oxygen within muscle tissue. In vertebrates, myoglobin is a single-chain protein of about 153 amino acids, with a molecular weight of approximately 17 kilodaltons. It binds a heme group in which iron is coordinated by a proximal histidine; a distal histidine stabilizes bound oxygen. The globin fold gives the protein a compact, strictly monomeric structure, in contrast to the oligomeric hemoglobins that function in blood.

In muscles, myoglobin increases the rate at which oxygen diffuses from the blood to mitochondria, particularly during sustained contraction or high metabolic demand. It binds oxygen released from hemoglobin, acting as an oxygen reserve when arterial PO2 falls. The oxygen affinity of myoglobin is high, resulting in a hyperbolic oxygen-binding curve and a low P50 value.

Myoglobins are widespread in vertebrate muscles, with the MB gene encoding the canonical muscle form in mammals. Some species possess multiple myoglobin genes or harbor myoglobin-like globins, including divergent forms in fish or invertebrates, illustrating an ancient and diverse globin family. In addition to myoglobin, related globins such as neuroglobin and cytoglobin share structural similarity but have distinct expression patterns and functions.

Clinically, myoglobin levels in blood rise after muscle injury or myocardial ischemia, serving as an early diagnostic marker before other cardiac enzymes. Research on myoglobins informs understanding of oxygen storage in tissues, adaptation to hypoxia, and the evolution of globin proteins.