C1 Inhibitor Serpin Domain Structure Reveals the Likely Mechanism of Heparin Potentiation and Conformational Disease

László Beinrohr,Veronika Harmat,József Dobó,Zsolt Loörincz,Péter Gál,Péter Závodszky

doi:10.1074/jbc.m700841200

László Beinrohr, Veronika Harmat + Show 4 more

Open Access

https://doi.org/10.1074/jbc.m700841200

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Abstract

C1 inhibitor, a member of the serpin family, is a major down-regulator of inflammatory processes in blood. Genetic deficiency of C1 inhibitor results in hereditary angioedema, a dominantly inheritable, potentially lethal disease. Here we report the first crystal structure of the serpin domain of human C1 inhibitor, representing a previously unreported latent form, which explains functional consequences of several naturally occurring mutations, two of which are discussed in detail. The presented structure displays a novel conformation with a seven-stranded beta-sheet A. The unique conformation of the C-terminal six residues suggests its potential role as a barrier in the active-latent transition. On the basis of surface charge pattern, heparin affinity measurements, and docking of a heparin disaccharide, a heparin binding site is proposed in the contact area of the serpin-proteinase encounter complex. We show how polyanions change the activity of the C1 inhibitor by a novel "sandwich" mechanism, explaining earlier reaction kinetic and mutagenesis studies. These results may help to improve therapeutic C1 inhibitor preparations used in the treatment of hereditary angioedema, organ transplant rejection, and heart attack.

Highlights

Active site of proteinase and traps it in an inactive, covalently linked serpin-enzyme complex [2, 3]
Most of the sugars are present in the N-terminal domain and do not affect proteinase inhibition [11, 12], but affinity to endotoxins and selectins depends on the N-glycans
The importance of C1 inhibitor (C1-inh) is underlined by its deficiency, resulting in hereditary angioedema (HAE) [13]

Summary

The abbreviations used are

C1-inh, C1 inhibitor; rC1-inh, recombinant C1 inhibitor; HAE, hereditary angioedema; GAG, glycosaminoglycan; RCL, reactive center loop; MOPS, 3-(N-morpholino)propanesulfonic acid; fXIIa and fXIa, factor XIIa and XIa, respectively. Crystal Structure of C1 Inhibitor polysaccharide (glycosaminoglycan (GAG)). The widely used anticoagulant therapy is based on the prototype of GAG-protein interactions, where a heparin chain binds both the serpin antithrombin and the proteinase thrombin. Allostery plays an important role in the heparin activation of antithrombin [21] and heparin cofactor II [22]. These mechanisms fail to explain the effect of GAGs on C1-inh. We determined the structure of N-terminally truncated human C1-inh to understand how heparin and related polyanions alter its activity

EXPERIMENTAL PROCEDURES

RESULTS

DISCUSSION