globins

Globins are a widespread family of heme-containing proteins defined by a conserved globin fold that forms a compact, predominantly alpha-helical domain. The core feature is a heme prosthetic group in which an iron atom binds small diatomic gases such as oxygen, carbon monoxide, and nitric oxide in a reversible manner. Ligand binding is modulated by specific distal and proximal histidines that help stabilize the bound ligand and tune affinity.

The best-known globins are hemoglobin and myoglobin. Hemoglobin is a multi-subunit oxygen transport protein in red blood cells, delivering O2 from the lungs to tissues throughout the body. Myoglobin is a smaller, monomeric globin that stores and facilitates diffusion of oxygen within muscle tissue. Beyond these, vertebrates possess neuroglobin and cytoglobin, whose precise roles are still being investigated but are thought to relate to cellular oxygen sensing, protection against hypoxia, and reactive species scavenging. Globins also occur in plants and microbes, including leghemoglobins in nitrogen-fixing plant nodules that buffer oxygen to protect nitrogenase.

Globins display extensive evolutionary diversification, ranging from single-domain proteins of about 150 amino acids to multi-domain architectures and globin-containing proteins with additional functional extensions. They are found across bacteria, archaea, plants, and animals, illustrating their ecological and physiological versatility. Despite sequence variation, the heme pocket geometry and the proximal histidine coordinating iron are central to globin function, enabling controlled gas binding and release that supports respiration, metabolism, and cellular protection in diverse biological contexts.