American Society of Hirudotherapy

Two heads are better than one: crystal structure of the insect derived double domain Kazal inhibitor rhodniin in complex with thrombin

Research article published in The EMBO journal (1995)

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, 1995

Abstract

Rhodniin is a highly specific inhibitor of thrombin isolated from the assassin bug Rhodnius prolixus. The 2.6 Angstrum crystal structure of the non-covalent complex between recombinant rhodniin and bovine alpha-thrombin reveals that the two Kazal-type domains of rhodniin bind to different sites of thrombin. The amino-terminal domain binds in a substrate-like manner to the narrow active-site cleft of thrombin; the imidazole group of the P1 His residue extends into the S1 pocket to form favourable hydrogen/ionic bonds with Asp189 at its bottom, and additionally with Glu192 at its entrance. The carboxy-terminal domain, whose distorted reactive-site loop cannot adopt the canonical conformation, docks to the fibrinogen recognition exosite via extensive electrostatic interactions. The rather acidic polypeptide linking the two domains is displaced from the thrombin surface, with none of its residues involved in direct salt bridges with thrombin. The tight (Ki = 2 x 10(-13) M) binding of rhodniin to thrombin is the result of the sum of steric and charge complementarity of the amino-terminal domain towards the active-site cleft, and of the electrostatic interactions between the carboxy-terminal domain and the exosite.

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 SequenceAnimalsBinding SitesCattleCrystallizationHirudinsInsect HormonesInsect ProteinsMolecular Sequence DataProtein ConformationRhodniusSequence Alignment

Summary

Rhodniin is a highly specific inhibitor of thrombin isolated from the assassin bug Rhodnius prolixus.

Why This Matters for Hirudotherapy

This article reports the 2.6 Å crystal structure of rhodniin, a highly specific thrombin inhibitor from the assassin bug Rhodnius prolixus, in complex with bovine alpha-thrombin. The abstract describes a two-Kazal-domain inhibitor whose N-terminal domain binds the active site in substrate-like fashion (P1 His in the S1 pocket) while the C-terminal domain docks the fibrinogen recognition exosite, yielding tight binding (Ki ~2 x 10^-13 M). For ASH, this is relevant as a comparative example of convergent dual-site thrombin inhibition in a blood-feeding arthropod, conceptually paralleling hirudin's mechanism. The caveat is that no leeches or leech-derived molecules are involved; relevance is indirect and comparative only.

Citation

Two heads are better than one: crystal structure of the insect derived double domain Kazal inhibitor rhodniin in complex with thrombin

van de Locht A et al. · The EMBO journal, 1995

DOI

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