Insertion loop 256-268 in coagulation factor IX restricts enzymatic activity in the absence but not in the presence of factor VIII

Insertion loop 256-268 in coagulation factor IX is discussed. It restricts enzymatic activity where factor VIII is absent. a study has shown that the loop restricts the activity of factor IXa toward synthetic and natural substrates. Complex formation of factor VIIIa cuts back on the inhibitory effect of the insertion loop on activation of FX.