Coagulin is a gel-forming

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, respon ...

hemolymph Hemolymph, or haemolymph, is a fluid, analogous to the blood in vertebrates, that circulates in the interior of the arthropod (invertebrate) body, remaining in direct contact with the animal's tissues. It is composed of a fluid plasma in which ...

that hinders the spread of invaders by immobilising them. It is produced in the coagulogen form before being cleaved into the active form. In human medicine, coagulation of coagulin is the basis of detection of bacterial endotoxin in the

LAL test Limulus amebocyte lysate (LAL) is an aqueous extract of blood cells (amoebocytes) from the Atlantic horseshoe crab ''Limulus polyphemus''. LAL reacts with bacterial endotoxin lipopolysaccharide (LPS), which is a membrane component of gram-negati ...

for parenteral medications. The protein contains a single 175-residue polypeptide chain that is cleaved after Arg-18 and Arg-46 by a

Limulus clotting enzyme Limulus clotting enzyme (, ''clotting enzyme'') is an enzyme. This enzyme catalyses the following chemical reaction : Selective cleavage of -Arg18- and -Arg47- bonds in coagulogen to form coagulin and fragments This enzyme is present in the hem ...

contained in the hemocyte and activated by a bacterial endotoxin (

lipopolysaccharide Lipopolysaccharides (LPS) are large molecules consisting of a lipid and a polysaccharide that are bacterial toxins. They are composed of an O-antigen, an outer core, and an inner core all joined by a covalent bond, and are found in the outer ...

). Cleavage releases two chains of coagulin, A and B, linked by two disulfide bonds, together with the peptide C. Gel formation results from interlinking of coagulin molecules. The full-length structure of a coagulogen is known (); it shares the same cystine-knot cytokine superfamily (fold) as

neurotrophin Neurotrophins are a family of proteins that induce the survival, development, and function of neurons. They belong to a class of growth factors, secreted proteins that can signal particular cells to survive, differentiate, or grow. Growth facto ...

s, with several cystines conserved. The A-B fold wraps around the helical peptide C, forming a compact structure. In

crustaceans Crustaceans (Crustacea, ) form a large, diverse arthropod taxon which includes such animals as decapods, seed shrimp, branchiopods, fish lice, krill, remipedes, isopods, barnacles, copepods, amphipods and mantis shrimp. The crustacean gr ...

, hemolymph coagulation depends on the transglutaminase-mediated cross-linking of specific plasma-clotting proteins, but without the proteolytic cascade. In horseshoe crabs, the proteolytic coagulation cascade triggered by lipopolysaccharides and beta-1,3-glucans leads to the conversion of coagulogen into coagulin, resulting in noncovalent coagulin homopolymers through head-to-tail interaction. However, horseshoe crab transglutaminase does not cross-link coagulins intermolecularly. Recently, coagulins were discovered to be cross-linked on hemocyte cell surface proteins called proxins. This indicates that a cross-linking reaction at the final stage of hemolymph coagulation is an important innate immune system of horseshoe crabs. In comparison, mammalian

blood coagulation Coagulation, also known as clotting, is the process by which blood changes from a liquid to a gel, forming a blood clot. It potentially results in hemostasis, the cessation of blood loss from a damaged vessel, followed by repair. The mechanism o ...

is based on the proteolytically induced polymerization of

fibrinogen Fibrinogen (factor I) is a glycoprotein complex, produced in the liver, that circulates in the blood of all vertebrates. During tissue and vascular injury, it is converted enzymatically by thrombin to fibrin and then to a fibrin-based blood ...

s. Initially,

fibrin Fibrin (also called Factor Ia) is a fibrous, non-globular protein involved in the clotting of blood. It is formed by the action of the protease thrombin on fibrinogen, which causes it to polymerize. The polymerized fibrin, together with pla ...

monomers noncovalently interact with each other. The resulting homopolymers are further stabilized when the plasma transglutaminase cross-links bonds between molecules.

References

Protein domains {{protein-stub