:Limulus clotting factor B

{{Infobox enzyme

| Name = Limulus clotting factor B

| EC_number = 3.4.21.85

| CAS_number = 848851-53-6

| GO_code =

| image =

| width =

| caption =

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Limulus clotting factor B ({{EC number|3.4.21.85}}) is an enzyme.{{cite journal | vauthors = Nakamura T, Horiuchi T, Morita T, Iwanaga S | title = Purification and properties of intracellular clotting factor, factor B, from horseshoe crab (Tachypleus tridentatus) hemocytes | journal = Journal of Biochemistry | volume = 99 | issue = 3 | pages = 847–57 | date = March 1986 | doi = 10.1093/oxfordjournals.jbchem.a135545 | pmid = 3519594 }} This enzyme catalyses the following chemical reaction

: Selective cleavage of -Arg98-Ile- bond in limulus proclotting enzyme to form active clotting enzyme

From the hemocyte granules of the horseshoe crabs Limulus and Tachypleus. This enzyme is downstream of Limulus clotting factor C, but upstream of Limulus clotting enzyme.{{cite journal |last1=Iwanaga |first1=S |title=Biochemical principle of Limulus test for detecting bacterial endotoxins. |journal=Proceedings of the Japan Academy. Series B, Physical and Biological Sciences |date=May 2007 |volume=83 |issue=4 |pages=110–9 |doi=10.2183/pjab.83.110 |pmid=24019589 |pmc=3756735|bibcode=2007PJAB...83..110I }}