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Figure 7.
FIGURE 7. Different mechanisms of the polyanion
potentiation of serpins. Serpins are shown as ellipses,
proteinases as circles, and polyanions as lines in this
schematic representation. Charged binding regions on proteins
are marked with black and charge signs. A, the most prominent
type of serpin activation by the "bridging" mechanism is
depicted (19, 20). Antithrombin binds tightly to a specific site
on the heparin chain. Thrombin binds to the same chain, but with
lower affinity. Only thrombin diffuses one-dimensionally toward
antithrombin until the encounter. Even then, they bind to
different segments on heparin. B, the similar "co-occupation"
mechanism proposed for protein C inhibitor is shown (29).
Protein C inhibitor and protein C bind to the same polyanion
chain, but neither binds with high affinity. Both proteins
migrate along the chain until the encounter. In the Michaelis
complex, they bind to the same site on the polyanion. C, the
"sandwich" mechanism is shown. C1-inh binds a short polyanion
with low affinity. Binding neutralizes surface charge at a
specific region. Proteinase is now attracted to this surface,
which happens to be the contact site in the encounter complex.
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