globin

Globin refers to a widespread family of heme-containing globular proteins characterized by a conserved globin fold. Most globins are about 140–180 amino acids in length and fold into eight alpha-helices surrounding a heme prosthetic group. In the core fold, the iron atom of the heme is coordinated by a proximal histidine, while a distal histidine stabilizes bound ligands such as oxygen and modulates ligand affinity.

In vertebrates, the best-known members are hemoglobins, which transport oxygen in the blood, and myoglobins, which store and facilitate diffusion of oxygen in muscle tissue. Other globins include neuroglobin in the brain and retina, and cytoglobin, found in many tissues and thought to participate in intracellular oxygen delivery and protection against reactive oxygen species. Beyond these, a variety of other globins exist in plants, invertebrates, and microbes, sometimes with functions unrelated to oxygen transport, such as sensing or detoxification.

Globin genes are typically organized in small clusters and have arisen by gene duplication and divergence. The globin superfamily is ancient, with diverse paralogs that have specialized expression patterns and biochemical properties across lineages. Some organisms possess additional globins, including globin X, globin Y, and other family members, illustrating functional diversification of the fold.

Heme iron in the ferrous state binds oxygen reversibly; binding can also involve carbon monoxide or nitric oxide with varying affinity. Globin genes and their regulatory networks contribute to adaptation to hypoxia, metabolism, and protection from oxidative stress.