= Antithrombin
<Antithrombin> is a <blood plasma> <serpin> that inhibits coagulation <serine proteases>, including <factor Xa>. Its activity is enhanced by binding an appropriate sequence in <heparin>. The inhibitory <protein> and its activating carbohydrate partner therefore play different molecular roles.<The anticoagulant activation of antithrombin by heparin by Lei Jin and others>[Abstract; plasma serpin and pentasaccharide-dependent activation]{ref}
In a 1983 experiment, investigators synthesized related four- and five-sugar fragments and checked their structures with <nuclear magnetic resonance>. The tested five-sugar fragment bound <antithrombin> and enhanced inhibition of <factor Xa>; the tested four-sugar fragment did neither in that comparison.<Structure-activity relationship in heparin: a synthetic pentasaccharide with high affinity for antithrombin III and eliciting high anti-factor Xa activity by J. Choay and others>[Abstract; synthetic-fragment binding and anti-factor Xa activity]{ref}
A 1997 structural study connected binding to changes in the main <beta sheet> and exposure of the <reactive-center loop>. This supplied a structural explanation for activation rather than merely another measurement of binding.<The anticoagulant activation of antithrombin by heparin by Lei Jin and others>[Abstract; 2.9-angstrom structure and activation-associated changes]{ref} The sequence-specific comparison matters: counting five sugars does not guarantee that an arbitrary five-sugar chain has the same activity.
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