Poonam Singh, Monika Batra, Mohamad Aman Jairajpuri
Protein Conformation and Enzymology Lab, Department of Biosciences, Jamia Millia Islamia, New Delhi-110025, India.


Serine protease inhibitors (Serpins) are prone to conformation defects, however little is known about the factors and mechanisms which promote its conformational change and misfolding. Several point mutations occur in helix B which results in pathological conditions due to polymerization. Helix B mutations in α-1antichymotrypsin (Leu55Pro) and α1-antitrypsin (Phe51Leu, Ser53Phe and Val55Pro) can cause lung (emphysema) and liver diseases (cirrhosis). Protein C-inhibitor (Ser52Phe and Ser54Leu) and antithrombin (Pro80Ser/Thr, Thr85Met/Lys, Cys95Arg and Leu99Phe) have mutations which can result in angioedema and thrombosis, similarly in neuroserpin Ser53Pro and Ser56Arg are linked to hereditary disorder called familial encephalopathy with neuroserpin inclusion bodies (FENIB). Helix B analysis for residue burial and cavity was undertaken to understand its role in serpin structure function. A structural overlap and an accessible surface area analysis showed the deformation of strand 6B and exposure of helix B at N-terminal end in cleaved conformation but not in the native and latent conformation of various inhibitory serpins. A cleaved polymer like conformation of antitrypsin also showed deformation of s6B and helix B exposure. Cavity analysis showed that helix B residues were part of the largest cavity in most of the serpins in the native state which increase in size during the transformation to cleaved and latent states. Helix B exposure facilitates smooth insertion of RCL as strand4A and transition of proteinase to the opposite end of the sheet β A, which is mediated by stability changes, hydrogen bond switch and large cavity formation. Conformational changes in helix B variants that allow insertion of RCL of another molecule might trigger the large cavity formation resulting in trapping of inserted RCL in an irreversible complex that leads to polymerization. Our data for the first time shows a credible role of helix B in the mechanism of serpin inhibition and polymer formation across many inhibitory serpins. The results indicate that deformation of the strand 6B at the edge of helix B might confer extra flexibility to the N-terminal end of helix B residue to lift the helix so that strand 4A can smoothly pass further down during the process of protease inhibition. Read More …

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