Sandbox Reserved 954: Difference between revisions

==General structure==

Serpins are a superfamily of functionally distinct but structurally conserved proteins. <ref>JBC Papers in Press. Published on July 2, 2001 as Manuscript R100016200 THE SERPINS ARE AN EXPANDING SUPERFAMILY OF

SerpinB3 means serin protease inhibitor, clade B (ovalbumin), member 3. The particularity of Serpin B3 is to target proteases wich have a nucleophilic cysteine instead of serine in their catalytic site.  

SCCA1 is a <scene name='60/604473/Trimeric/1'>trimeric protein</scene><ref> PMID : 19166818 </ref>

. Like all for serpins, <scene name='60/604473/One_subunit/1'>one subunit</scene> has three β sheets termed <scene name='60/604473/A_beta_sheet/3'>A (7 stranded)</scene>, <scene name='60/604473/B_beta_sheet/2'>B (5 stranded)</scene> and <scene name='60/604473/C_beta_sheet/2'>C (6 stranded)</scene> and <scene name='60/604473/Alpha_helices/1'>11  α helices (hA to hK)</scene> <ref>Gary A. Silverman1*, Phillip I. Bird2

STRUCTURALLY SIMILAR BUT FUNCTIONALLY DIVERSE PROTEINS, http://www.jbc.org/content/early/2001/07/02/jbc.R100016200.full.pdf DOI : 2001/07/02/jbc.R100016200.full.pdf </ref> <ref>PDB, Crystal structure of human squamous cell carcinoma antigen 1 http://www.rcsb.org/pdb/explore/remediatedSequence.do?structureId=2ZV6&bionumber=1 DOI : pdb/explore/remediatedSequence.do?structureId=2ZV6&bionumber=1</ref> .  

The most important part of Serpins is an exposed region of 20 amino acids near the C terminus named the reactive center loop (<scene name='60/604473/The_rcl_loop_scene/3'>RCL</scene>). The amino-acids of <scene name='60/604473/The_rcl_loop_scene/3'>RCL</scene> are very conservated for Serpin B3 and  allow the specificity interaction of the inhibitor for the target protease<ref> PMID : PMC24842</ref>.

Structural studies on serpins revealed that inhibitory members of the family undergo an unusual conformational change, termed the Stressed to Relaxed (S to R) transition. During this structural transition the <scene name='60/604473/The_rcl_loop_scene/3'>RCL</scene> inserts into <scene name='60/604473/A_beta_sheet/3'>A β-sheet</scene> and forms an extra fourth β strand . The serpin conformational change is key to the mechanism of inhibition of target proteases. <scene name='60/604473/Rcl_insertion_into_beta_sheet/1'>Some amino-acids of RCL</scene> wich belong to a consensus sequence for inhibitory serpins are thought to permit efficient and rapid insertion of the <scene name='60/604473/The_rcl_loop_scene/3'>RCL</scene> into the <scene name='60/604473/A_beta_sheet/3'>A β-sheet</scene>.<ref> James C Whisstocka, 2, Richard Skinnera, 2, Robin W Carrella, Arthur M Leska, Conformational changes in serpins: I. the native and cleaved conformations of α1-antitrypsin1, http://www.sciencedirect.com/science/article/pii/S0022283699935209 DOI:pii/S0022283699935209</ref>

When attacking a substrate, proteases catalyze peptide bond cleavage in a two-step process. Initially, the catalytic cysteine performs a nucleophilic attack on the peptide bond of the substrate. This releases the new N-terminus and forms an new bond between the enzyme and the substrate. This covalent enzyme-substrate complex is called an acyl enzyme intermediate. Subsequent to this, this bond is hydrolysed and the new C-terminus is released.  

The <scene name='60/604473/The_rcl_loop_scene/3'>RCL</scene> of a serpin acts as a substrate for its cognate protease.  

The <scene name='60/604473/The_rcl_loop_scene/3'>RCL</scene> is cleaved at a scissile bond between two residues <scene name='60/604473/P1_scene/3'>Ser354 termed P1 (N-terminal of the cleavage event) and Ser355 termed P1’ (C-terminal of the cleavage event)</scene>. The <scene name='60/604473/P1_scene/3'>P1 and P1' residues</scene> are critical for serpin specificity and mutation of these residues results in the loss or conversion of inhibitory activity. The protease recognize <scene name='60/604473/Amino_acids_for_protease_recog/1'>amino-acids of the RCL</scene> that allow its docking.

Prior to hydrolysis of the acyl-enzyme intermediate, the serpin rapidly undergoes the S-to-R transition. Since the <scene name='60/604473/The_rcl_loop_scene/3'>RCL</scene> is still covalently attached to the protease via the ester bond, the S-to-R transition moves the protease from the top to the bottom of the serpin. http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0104935 At the same time, the protease is distorted into a conformation, where the acyl enzyme intermediate is hydrolysed extremely slowly. The the active site of the enzyme would be expected to break.The protease thus remains covalently attached to the target protease and is thereby inhibited.  

Further, since the serpin has to be cleaved to inhibit the target protases, inhibition consumes the serpin as well. Serpins are therefore irreversible enzyme inhibitors.  

SCCA1 also acts as an inhibitor of UV-induced apoptosis via suppression of the activity of c-Jun NH(2)-terminal kinase (JNK1). It is known that JNK1 is responsible for UV-induced apoptotic cell death and SCCA-1 is up-regulated in UV-irradiated and sun-exposed cells. SCCA1 binds to phosphorylated JNK1 and is transferred into the nucleus after UV irradiation.

Indeed the reactive center loop (<scene name='60/604473/The_rcl_loop_scene/3'>RCL</scene>) of SCCA1 is very flexible and located away from the center of SCCA1. The inhibitory effect of SCCA1 on the kinase activity of JNK1 is lost when the <scene name='60/604473/The_rcl_loop_scene/3'>RCL</scene> was truncated. Furthermore, a mutant protein created by replacing <scene name='60/604473/Phe_352/1'>one amino-acid</scene> in <scene name='60/604473/The_rcl_loop_scene/3'>RCL</scene> maintain the suppressive activity to JNK1, whereas the inhibitory effect to proteinase is obviously decreased.

This indicate that the exposed <scene name='60/604473/The_rcl_loop_scene/3'>RCL</scene> of the native and mutated forms is essential for the JNK1 inhibitory activity, and the JNK1-interaction site is different from the site of proteinase. Actualy JNK1 may bind the the <scene name='60/604473/The_rcl_loop_scene/3'>RCL</scene> region, except for the proteinase-binding site, or the center of <scene name='60/604473/A_beta_sheet/3'>A β-sheet</scene>. http://www.sciencedirect.com/science/article/pii/S0006291X09000953