Active site of bovine factor XI (plasma thromboplastin antecedent)

Abstract

FACTOR XI (plasma thromboplastin antecedent) is a plasma glycoprotein which participates in the early stages of blood coagulation¹. A deficiency of this protein leads to a haemorrhagic disease in which the intrinsic pathway of blood coagulation is partially or completely blocked². During normal coagulation, factor XI is converted to a serine protease (factor XI_a)³, and this enzyme in turn activates factor IX (Christmas factor) by limited proteolysis⁴. Factor XI_a is inhibited by serine protease inhibitors such as diisopropyl phosphorofluoridate, and this inhibitor is bound to a serine residue in the active site of the enzyme³. We recently purified bovine factor XI approximately 28,000-fold in 30% yield. The final preparation was homogeneous as measured by sodium dodecyl sulphate polyacrylamide gel electrophoresis and immunoelectrophoresis⁵. It has a molecular weight of 124,000 as determined by sedimentation equilibrium and is composed of two similar or identical polypeptide chains held together by a disulphide bond(s). We present here data showing that bovine factor XI contains an amino acid sequence which is very similar to the active site region of many other serine proteases involved in the coagulation process.