Limulus clotting factor C

Limulus clotting factor overbar C (EC 3.4.21.84, factor C, limulus factor C) is an enzyme. This enzyme catalyses the following chemical reaction

Selective cleavage of -Arg¹⁰³-Ser- and -Ile¹²⁴-Ile- bonds in limulus clotting factor B to form factor overbar B.

Cleavage of -Pro-Arg- bonds in synthetic substrates

This enzyme is isolated from the hemocyte granules of the horseshoe crabs Limulus and Tachypleus, where it serves as a LPS endotoxin-sensitive trypsin type serine protease to protect the organism from bacterial infection, initiating a cascade leading to coagulin formation. From the N-terminus to the C-terminus, the domains are:

• EGF-like domain
• 3 Sushi domains
• one LCCL domain and one C-type lectin domain
• 2 more Sushi domains
• a trypsin domain

This enzyme is useful in Limulus amebocyte lysate as the endotoxin-detecting element. It can be produced recombinantly.