The 99 and 170 loop-modified factor VIIa mutants show enhanced catalytic activity without tissue factor

Area of Science:

• Biochemistry
• Enzymology
• Protease structure-function relationships

Background:

• Coagulation factor VIIa (VIIa) is a serine protease with critical roles in hemostasis.
• Surface loops, specifically the 99 and 170 loops, are implicated in modulating VIIa activity.
• Understanding loop function is key to comprehending VIIa's catalytic mechanisms.

Purpose of the Study:

• To investigate the functional roles of the 99 and 170 surface loops of VIIa.
• To determine how mutations in these loops affect catalytic efficiency (kcat/Km) and substrate binding (Km).
• To assess the impact of loop modifications on proteolytic activity towards different substrates.

Main Methods:

• Site-directed mutagenesis to create VIIa mutants (VII-30, VII-39) by replacing loop residues with those from trypsin.
• Enzymatic assays using chromogenic substrate S-2288 to measure kinetic parameters (kcat, Km, kcat/Km).
• Evaluation of proteolytic activity against macromolecular substrates (S-alkylated lysozyme) and factor X activation.

Main Results:

• VIIa-30 mutant showed a 3-fold increase in kcat/Km for S-2288 hydrolysis, primarily due to decreased Km.
• VIIa-39 mutant exhibited an 18-fold enhancement in kcat/Km in the absence of soluble tissue factor (sTF), attributed to both decreased Km and increased kcat.
• The 99 loop primarily influences Km, while the 170 loop significantly affects kcat, suggesting independent functional roles.

Conclusions:

• The 99 and 170 loops of VIIa play distinct roles in modulating enzyme kinetics.
• Modifications in these loops can independently alter substrate binding and catalytic turnover rates.
• These findings provide insights into the structure-function relationships governing VIIa proteolytic activity.