Ca2+-dependent structural changes in bovine blood coagulation factor Va and its subunits.

Area of Science:

• Biochemistry
• Structural Biology
• Protein Interactions

Background:

• Bovine blood coagulation factor Va is essential for hemostasis.
• Factor Va is composed of heavy (Vh) and light (Vl) subunits.
• Calcium ions (Ca2+) play a critical role in factor Va assembly and function.

Purpose of the Study:

• To spectroscopically characterize conformational changes during factor Va formation.
• To elucidate the role of calcium in subunit association and structural rearrangements.
• To determine the calcium dependence and binding kinetics of factor Va assembly.

Main Methods:

• Circular dichroism (CD) spectroscopy to assess secondary structure.
• Intrinsic fluorescence spectroscopy to monitor tertiary structural changes.
• Extrinsic fluorescence spectroscopy using 8-anilino-1-naphthalenesulfonate (ANS).
• Functional assays to measure cofactor activity recovery.

Main Results:

• No significant change in secondary structure (beta-sheet, alpha-helix) was observed during factor Va formation.
• Calcium binding induced a small increase in intrinsic fluorescence and a blue shift, indicating tertiary structural changes.
• ANS fluorescence decreased, suggesting hydrophobic regions became inaccessible upon subunit association.
• Calcium-dependent spectral changes correlated with functional cofactor activity recovery, with a Kd of approximately 50 microM.

Conclusions:

• Calcium binding to factor Va subunits does not alter secondary structure.
• The formation of factor Va involves calcium-mediated tertiary structural changes and hydrophobic interactions between subunits.
• The apparent dissociation constant for calcium in factor Va assembly is approximately 50 microM.