Effects of Ca²⁺ ions on the horseshoe crab coagulation cascade triggered by lipopolysaccharide.

This article focuses on the role of calcium ions (Ca²⁺) in modulating the lipopolysaccharide (LPS)-triggered coagulation cascade of horseshoe crab hemolymph, which involves three serine protease zymogens: prochelicerase C (proC), prochelicerase B (proB), and the proclotting enzyme (proCE). The cascade was divided into three stages: (1) autocatalytic activation of proC on LPS into α-chelicerase C, (2) activation of proB by α-chelicerase C on LPS into chelicerase B, and (3) activation of proCE by chelicerase B into the clotting enzyme (CE). The study found that Ca²⁺ ions specifically enhance Stage 2, accelerating the conversion of proB to chelicerase B, while having no enhancing effect on Stages 1 and 3 or on the enzymatic activities of the fully activated proteases. Isothermal titration calorimetry revealed that Ca²⁺ ions bind directly to LPS but not to the recombinant zymogens, suggesting that Ca²⁺-bound LPS acts as a more efficient scaffold facilitating the proteolytic cascade. These findings provide insight into the molecular mechanism by which Ca²⁺ ions amplify the horseshoe crab coagulation response to bacterial endotoxins.