Molecular basis for inhibition of α-thrombin activity by bacterial lipopolysaccharides

Dysregulation of the coagulation pathway is a hallmark of endotoxemia and Gram-negative sepsis, leading to disseminated intravascular coagulation resulting from aberrant thrombin generation. Bacterial components such as lipopolysaccharides (LPS) have been shown to indirectly contribute to thrombin generation by activating or assembling members of the coagulation cascade in an LPS-chemotype dependent manner. Conversely, studies have shown that LPS may directly inhibit thrombin activity through an undefined mechanism. We studied whether the LPS chemotypes from bacteria including Escherichia coli (O111:B4, O26:B6), Klebsiella pneumoniae, and Pseudomonas aeruginosa bind and regulate thrombin activity. We found that these LPS chemotypes bound thrombin exosites via their polysaccharide regions. Only the E.coli O26:B6 and K.pneumoniae LPS chemotypes induced global structural changes in thrombin. Both the monomeric forms of E.coli O26:B6 and K.pneumoniae LPS chemotypes reduced the catalytic efficiency of thrombin and thrombin-dependent fibrin polymerization in purified systems. The E.coli O26:B6 LPS chemotype retained anticoagulant activity in plasma as an Ca²⁺-stabilized aggregate form. Our data suggests that select LPS chemotypes bind and inhibit the proteolytic activity of thrombin in a manner dependent on their supramolecular state. The heterogeneity of physicochemical properties of bacterial envelope components may contribute to the dysregulation of the coagulation pathway during endotoxemia.