PDBsum entry 2oay

Immune system,hydrolase inhibitor

PDB id: 2oay

Contents

Protein chain

368 a.a.

Ligands

NAG

GOL ×3

Waters ×70

References listed in PDB file

Key reference

Title

C1 inhibitor serpin domain structure reveals the likely mechanism of heparin potentiation and conformational disease.

Authors

L.Beinrohr, V.Harmat, J.Dobó, Z.Lörincz, P.Gál, P.Závodszky.

Ref.

J Biol Chem, 2007, 282, 21100-21109. [DOI no: 10.1074/jbc.M700841200]

PubMed id

17488724

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.

Figure 6.
FIGURE 6. Showcase of mutations. A, studies failed to detect any effect of the V458M polymorphism, although the conserved hydrophobic core (around strands s2B, s3B, and s4B) is involved (53). The reason is that the side chain of Met^458 (space-filled) sits in the biggest cavity of rC1-inh (orange cloud); hence, spatial differences are tolerated. B, the A436T mutation affects a residue whose side chain becomes buried upon RCL incorporation. Similar mutants of other serpins usually result in cleavable noninhibitory serpins, because loop insertion is hindered. Unexpectedly, the A436T mutant C1-inh is found predominantly in noncleavable loop-inserted forms (51). In the modeled mutant structure, the side chain of Thr^436 forms a novel hydrogen bond network with the side chain of His^421 and the backbone of Gln^422. This makes the RCL-inserted protein more stable.

Figure 7.
FIGURE 7. Different mechanisms of the polyanion potentiation of serpins. Serpins are shown as ellipses, proteinases as circles, and polyanions as lines in this schematic representation. Charged binding regions on proteins are marked with black and charge signs. A, the most prominent type of serpin activation by the "bridging" mechanism is depicted (19, 20). Antithrombin binds tightly to a specific site on the heparin chain. Thrombin binds to the same chain, but with lower affinity. Only thrombin diffuses one-dimensionally toward antithrombin until the encounter. Even then, they bind to different segments on heparin. B, the similar "co-occupation" mechanism proposed for protein C inhibitor is shown (29). Protein C inhibitor and protein C bind to the same polyanion chain, but neither binds with high affinity. Both proteins migrate along the chain until the encounter. In the Michaelis complex, they bind to the same site on the polyanion. C, the "sandwich" mechanism is shown. C1-inh binds a short polyanion with low affinity. Binding neutralizes surface charge at a specific region. Proteinase is now attracted to this surface, which happens to be the contact site in the encounter complex.

The above figures are reprinted by permission from the ASBMB: J Biol Chem (2007, 282, 21100-21109) copyright 2007.