Coagulin

Coagulin is a gel-forming protein of hemolymph that hinders the spread of invaders by immobilising them.

The protein contains a single 175- residue polypeptide chain; this is cleaved after Arg-18 and Arg-46 by a clotting enzyme contained in the hemocyte and activated by a bacterial endotoxin (lipopolysaccharide). Cleavage releases two chains of coagulin, A and B, linked by two disulphide bonds, together with the peptide C. Gel formation results from interlinking of coagulin molecules. Secondary structure prediction suggests the C peptide forms an alpha- helix, which is released during the proteolytic conversion of coagulogen to coagulin gel. The beta-sheet structure and 16 half-cystines found in the molecule appear to yield a compact protein stable to acid and heat.