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.[ref]
The anticoagulant activation of antithrombin by heparin by Lei Jin and others. Abstract; plasma serpin and pentasaccharide-dependent activation
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.[ref]
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
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.[ref]
The anticoagulant activation of antithrombin by heparin by Lei Jin and others. Abstract; 2.9-angstrom structure and activation-associated changes
The sequence-specific comparison matters: counting five sugars does not guarantee that an arbitrary five-sugar chain has the same activity. New to topics? Read the docs here!