Altering Heparin Cofactor II at VAL439 (P6) either Impairs Inhibition of Thrombin or Confers Elastase Resistance*

Heparin cofactor II (HCII) is plasma glycoprotein and thrombin inhibitor of the serpin type previously shown to inhibit thrombin in the absence of its N-terminal 74 amino acids, and to be cleaved by neutrophil elastase (NE) at two sites: I66-F67 and V439-G440, the P6-P5 bond of the reactive center loop. We examined the contribution of Val439 to the reaction of HCII with thrombin and NE. Hexahistidine-tagged HCII proteins lacking residues 1-66 …