American Society of Hirudotherapy

The ornithodorin-thrombin crystal structure, a key to the TAP enigma?

Research article published in The EMBO journal (1996)

Last Updated: June 18, 2026Reviewed by: ASH Editorial Board
Research article — evidence reviewArticle reference
Evidence: Preclinical (animal)Drug Developmentvan de Locht A et al. · The EMBO journal, 1996

Abstract

Ornithodorin, isolated from the blood sucking soft tick Ornithodoros moubata, is a potent (Ki = 10(-12) M) and highly selective thrombin inhibitor. Internal sequence homology indicates a two domain protein. Each domain resembles the Kunitz inhibitor basic pancreatic trypsin inhibitor (BPTI) and also the tick anticoagulant peptide (TAP) isolated from the same organism. The 3.1 A crystal structure of the ornithodorin-thrombin complex confirms that both domains of ornithodorin exhibit a distorted BPTI-like fold. The N-terminal portion and the C-terminal helix of each domain are structurally very similar to BPTI, whereas the regions corresponding to the binding loop of BPTI adopt different conformations. Neither of the two 'reactive site loops' of ornithodorin contacts the protease in the ornithodorin-thrombin complex. Instead, the N-terminal residues of ornithodorin bind to the active site of thrombin, reminiscent of the thrombin-hirudin interaction. The C-terminal domain binds at the fibrinogen recognition exosite. Molecular recognition of its target protease by this double-headed Kunitz-type inhibitor diverges considerably from other members of this intensely studied superfamily. The complex structure provides a model to explain the perplexing results of mutagenesis studies on the TAP-factor Xa interaction.

Abstract sourced from PubMed (NCBI) for the cited record. See the original publication for the authoritative version.

Publication typeJournal ArticleResearch Support, Non-U.S. Gov't
Indexed MeSH termsAmino Acid SequenceAnimalsAprotininArthropod ProteinsBinding SitesCrystallography, X-RayEnzyme InhibitorsFactor XaIntercellular Signaling Peptides and ProteinsModels, MolecularMolecular Sequence DataPeptides

Summary

Ornithodorin, isolated from the blood sucking soft tick Ornithodoros moubata, is a potent (Ki = 10(-12) M) and highly selective thrombin inhibitor.

Why This Matters for Hirudotherapy

This article describes the crystal structure of ornithodorin, a potent thrombin inhibitor (Ki ~10^-12 M) isolated from the soft tick Ornithodoros moubata, in complex with thrombin. The abstract shows ornithodorin is a two-domain Kunitz-type inhibitor whose N-terminus occupies the thrombin active site in a manner reminiscent of the thrombin-hirudin interaction, while its C-terminal domain binds the fibrinogen recognition exosite. For ASH, this is relevant because it provides a comparative structural framework for understanding convergent mechanisms of potent thrombin inhibition across blood-feeding species, highlighting how hirudin's dual-site binding strategy is mirrored in a tick-derived molecule. The caveat is that this study involves no leeches or leech-derived molecules whatsoever; it is an indirect, comparative structural reference only.

Citation

The ornithodorin-thrombin crystal structure, a key to the TAP enigma?

van de Locht A et al. · The EMBO journal, 1996

DOIPubMed

Added to ASH library: May 27, 2026 · Site last updated: June 18, 2026

This website provides educational information and does not constitute medical advice, diagnosis, or treatment recommendations. Medicinal leech therapy carries clinically meaningful risks and should be performed only by qualified clinicians under institutionally approved protocols. FDA 510(k) clearance for medicinal leeches is limited to specific indications; investigational and off-label discussions are labeled accordingly. For patient-specific guidance, consult a qualified healthcare provider.