Proteolytic activity of contact factor zymogens

Area of Science:

• Biochemistry
• Hematology
• Molecular Biology

Background:

• Contact activation is initiated by plasma zymogens factor XII (FXII) and prekallikrein converting to active proteases FXIIa and kallikrein upon surface exposure.
• FXIIa is crucial for blood coagulation, activating factor XI (FXI) to FXIa.
• The precise mechanisms sustaining reciprocal activation of FXII and prekallikrein, and triggering contact activation, remain debated.

Purpose of the Study:

• To investigate the inherent enzymatic activities of single-chain factor XII (FXII) and prekallikrein zymogens.
• To explore the potential for surface-independent and surface-dependent autoactivation and cross-activation pathways.
• To elucidate the structural basis for potential zymogen activity through molecular modeling.

Main Methods:

• Assays to detect enzymatic activity of single-chain FXII and prekallikrein in solution.
• Investigation of surface-dependent activation of prekallikrein, FXI, and FXII.
• Molecular modeling to analyze the active site conformation and potential stabilizing interactions.

Main Results:

• Single-chain FXII exhibits low-level activity, catalyzing prekallikrein activation in solution and surface-dependent activation of prekallikrein, FXI, and FXII (autoactivation).
• Prekallikrein also demonstrates activity, cleaving kininogen to release bradykinin and mediating surface-dependent FXII activation.
• Modeling indicates a glutamine residue at position 156 stabilizes an open active site conformation in both FXII and prekallikrein.

Conclusions:

• The inherent activities of FXII and prekallikrein zymogens provide a mechanism for sustained reciprocal activation.
• These intrinsic enzymatic properties can initiate contact activation pathways, challenging the notion of complete zymogen inactivity.
• The findings support the premise that zymogens of certain trypsin-like enzymes possess inherent catalytic activity.