1nq9
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Crystal Structure of Antithrombin in the Pentasaccharide-Bound Intermediate State
OverviewOverview
Antithrombin is activated as an inhibitor of the coagulation proteases, through its specific interaction with a heparin pentasaccharide. The, binding of heparin induces a global conformational change in antithrombin, which results in the freeing of its reactive center loop for interaction, with target proteases and a 1000-fold increase in heparin affinity. The, allosteric mechanism by which the properties of antithrombin are altered, by its interactions with the specific pentasaccharide sequence of heparin, is of great interest to the medical and protein biochemistry communities., Heparin binding has previously been characterized as a two-step, three-state mechanism where, after an initial weak interaction, antithrombin undergoes a conformational change to its high-affinity state., Although the native and heparin-activated states have been determined, through protein crystallography, the number and magnitude of, conformational changes render problematic the task of determining which, account for the improved heparin affinity and how the heparin binding, region is linked to the expulsion of the reactive center loop. Here we, present the structure of an intermediate pentasaccharide-bound, conformation of antithrombin which has undergone all of the conformational, changes associated with activation except loop expulsion and helix D, elongation. We conclude that the basis of the high-affinity state is not, improved interaction with the pentasaccharide but a lowering of the global, free energy due to conformational changes elsewhere in antithrombin. We, suggest a mechanism in which the role of helix D elongation is to lock, antithrombin in the five-stranded fully activated conformation.
About this StructureAbout this Structure
1NQ9 is a Single protein structure of sequence from Homo sapiens with NAG and NTP as ligands. Full crystallographic information is available from OCA.
ReferenceReference
Crystal structure of antithrombin in a heparin-bound intermediate state., Johnson DJ, Huntington JA, Biochemistry. 2003 Jul 29;42(29):8712-9. PMID:12873131
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