Mechanisms of Antithrombin Polymerisation and Heparin Activation Probed by the Insertion of Synthetic Reactive Loop Peptides

Incubation of antithrombin with a series of synthetic reactive loop peptides showed that 6-mer and 7-mer peptides, P₁₄-P₉ and P₁₄-P₈ of antithrombin respectively, induced loop-sheet polymerisation and binary complex formation. These peptides are likely to anneal to the upper part of the dominant A-sheet, favouring sheet opening and allowing insertion of a second reactive loop in the lower part of the A-sheet to form polymers. The insertion of longer peptides filled the A-sheet beyond the P₇ position and prevented polymerisation. Heparinised antithrombin was more resistant to polymerisation and peptide insertion, indicating that heparin induces a conformational change that closes the A-sheet and expels the reactive loop.