Pcy. Liaw et al., Comparison of heparin- and dermatan sulfate-mediated catalysis of thrombininactivation by heparin cofactor II, J BIOL CHEM, 274(39), 1999, pp. 27597-27604
Heparin and dermatan sulfate activate heparin cofactor II (HCII) comparably
, presumably by liberating the amino terminus of HCII to bind to exosite I
of thrombin, To explore this model of activation, we systematically substit
uted basic residues in the glycosaminoglycan-binding domain of HCII with ne
utral amino acids and measured the rates of thrombin inactivation by the mu
tants. Mutant D, with changes at Arg(184), Lys(185), Arg(189), Arg(192), Ap
g(193) demonstrated a similar to 130-fold increased rate of thrombin inacti
vation that was unaffected by the presence of glycosaminoglycans. The incre
ased rate reflects displacement of the amino terminus of mutant D because (
a) mutant D inactivates gamma-thrombin at a 65-fold slower rate than alpha-
thrombin, (b) hirudin-(54-65) decreases the rate of thrombin inactivation,
and (c) deletion of the amino terminus of mutant D reduces the rate of thro
mbin inactivation similar to 100-fold, We also examined the contribution of
glycosaminoglyean-mediated bridging of thrombin to HCII to the inhibitory
process. Whereas activation of HCII by heparin was chain-length dependent,
stimulation by dermatan sulfate was not, suggesting that dermatan sulfate d
oes not utilize a template mechanism to accelerate the inhibitory process.
Fluorescence spectroscopy revealed that dermatan sulfate evokes greater con
formational changes in HCII than heparin, suggesting that dermatan sulfate
stimulates HCII by producing more effective displacement of the amino termi
nus.