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Factor Xa (FXa) is a key protease of the coagulation pathway whose activity is known to be in part modulated by binding to factor Va (FVa) and sodium ions. Previous investigations have established that solvent-exposed, charged residues of the FXa alpha-helix 163-170 (h163-170), Arg(165) and Lys(169), participate in its binding to FVa. In the present study we aimed to investigate the role of the other residues of h163-170 in the catalytic functions of the enzyme. FX derivatives were constructed in which point mutations were made or parts of h163-170 were substituted with the corresponding region of either FVIIa or FIXa. Purified FXa derivatives were compared with wild-type FXa. Kinetic studies in the absence of FVa revealed that, compared with wild-type FXa, key functional parameters (catalytic activity toward prothrombin and tripeptidyl substrates and non-enzymatic interaction of a probe with the S1 site) were diminished by mutations in the NH(2)-terminal portion of h163-170. The defective amidolytic activity of these FXa derivatives appears to result from their impaired interaction with Na(+) because using a higher Na(+) concentration partially restored normal catalytic parameters. Furthermore, kinetic measurements with tripeptidyl substrates or prothrombin indicated that assembly of these FXa derivatives with an excess of FVa in the prothrombinase complex improves their low catalytic efficiency. These data indicate that residues in the NH(2)-terminal portion of the FVa-binding h163-170 are energetically linked to the S1 site and Na(+)-binding site of the protease and that residues Val(163) and Ser(167) play a key role in this interaction.