title: Comparison of coagulation factor Xa and des-(1-44)factor Xa in the assembly of prothrombinase.
abstract: The gamma-carboxyglutamic acid (Gla)-domain region of factor X (residues 1-44 of the light chain) was selectively removed by limited proteolysis with alpha-chymotrypsin. The Gla-domainless factor X was then activated by the factor X coagulant protein of Russell's viper venom. Apparent dissociation constants Kd' values for the interaction of factor Va with either factor Xa or Gla-domainless factor Xa were determined kinetically using prothrombin as the substrate. In the absence of phospholipid, factor Va interacted with Gla-domainless factor Xa with lower affinity (Kd' 4 X 10(-6) M) than with factor Xa (Kd' = 5 X 10(-8) M). At saturating concentrations of factor Va, maximal rates of thrombin formation were similar for either enzyme. The addition of phospholipid increased the affinity of factor Va for factor Xa approximately 75-fold (Kd' = 3.3 X 10(-10) M). In contrast, phospholipid had no effect on the affinity of Gla-domainless factor Xa for factor Va (Kd' = 4 X 10(-6) M). The maximal rate of thrombin formation increased approximately 300-fold with the addition of phospholipid to the factor Xa-factor Va system. Under the same conditions, phospholipid had no effect on the rate of thrombin formation when Gla-domainless factor Xa was the enzymatic moiety. These results demonstrate phospholipid has little or no effect on factor Va function when factor Xa has lost its Gla-mediated Ca2+-binding sites.
mesh_terms: D000818:Animals; D002417:Cattle; D005165:Factor V / Q000378:metabolism*; D005170:Factor X / Q000378:metabolism*; D015951:Factor Xa; D007700:Kinetics; D010446:Peptide Fragments / Q000378:metabolism*; D011485:Protein Binding; D011516:Prothrombin / Q000378:metabolism
chemical_list: C040716:Factor Xa, des(1-44)-; D010446:Peptide Fragments; C022475:prothrombinase complex; D005165:Factor V; D011516:Prothrombin; D005170:Factor X; D015951:Factor Xa