Calcium ion binding to human and bovine factor X.

Area of Science:

• Biochemistry
• Molecular Biology
• Protein Chemistry

Background:

• Human and bovine factor X possess glutamyl residues in their N-terminal regions, modified to gamma-carboxyglutamyl (Gla) residues.
• These Gla residues are crucial for calcium ion binding, a critical step in the coagulation cascade.

Purpose of the Study:

• To characterize calcium ion binding to human factor X for the first time.
• To compare calcium ion binding mechanisms between human and bovine factor X.
• To investigate the role of the Gla domain in cooperative calcium binding in bovine factor X.

Main Methods:

• Equilibrium dialysis was employed to measure calcium ion binding to human and bovine factor X.
• Data analysis involved various models, including those accounting for multiple binding sites and cooperativity.
• Calcium binding to isolated Gla regions and Gla-domainless factor X was also assessed.

Main Results:

• Human factor X exhibited two classes of calcium binding sites: one high-affinity site and 19 low-affinity, non-interacting sites, with no cooperativity observed.
• Bovine factor X displayed cooperative calcium binding, best explained by a model with one tight site, four cooperative sites, and 18 low-affinity sites.
• Gla-domainless factor X showed two classes of low-affinity binding sites, indicating the Gla domain's importance for high-affinity and cooperative binding.

Conclusions:

• Calcium binding to human factor X is non-cooperative and involves distinct high- and low-affinity sites.
• Cooperative calcium binding in bovine factor X is mediated by its Gla domain.
• Understanding these species-specific differences in calcium binding is vital for comprehending factor X function in hemostasis.