Annexin refers to a group of recently discovered, structurally related family of proteins. They are found in most living things, and in all the kingdoms with the exception of bacteria. The annexin share the common characteristics of binding to phospholipid membranes and to calcium. They are usually found inside the cell but are sometimes also found on the outside. Annexin was first described in the 1980s, and since then, at least ten different kinds of annexin have been identified in the mammalian species.
Research into the biological functions of annexin have barely scratched the surface of its real role in the human body. Of the ten annexins already identified, their biological functions are specific but diverse. As an enzymatic protein, annexin also goes by the name of lipocortin. Lipocortins are responsible for the suppression of enzymatic activities of another protein called phospholipase A2. This mechanism is analogous to what glucocorticoids, a class of steroid hormones, do to inhibit inflammation.
In humans, the annexin are usually found inside the cells, which make up tissues, which in turn make up the organs and organ systems. However, annexins can also be found outside the cell circulating around in blood. Annexin A1, A2, and A5 are three types that have been found in blood, though their means of getting out of the cell are largely enigmatic. Proteins are usually transported out of the cell through signal peptides, but these annexins do not contain signal peptides.
Of the fundamental research that has been done into these kinds of annexin, A1 plays a role in inflammation processes. A2 and A5 both aid in anticoagulant reactions in the body by competing with other proteins for binding sites. Scientists have been utilizing a combination of structural, cell, and molecular biology methods to investigate more into the nature of annexins. Some of these methods include X-ray crystallography, low- and high-resolution electron microscopy, preparation of annexins with their interacting proteins, and also production of vast quantities of the proteins for experimental purposes.