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=== CRP structure ===
=== CRP structure ===


The crystal structure of CRP was determined by using SAP as the search model  <ref name="Thompson"/>. The structure of CRP has been determined by X-ray crystallography at 3 Å resolution. Like SAP, the protein consists of five protomers, uncovalently bonded and nonglycosylated that are arranged symmetrically around a central pore <ref name="Volanakis">PMID: 11532280</ref>. Each subunit is a 25 kDa protein consisting of 224 residues  <ref name="uniprot">[http://www.uniprot.org/uniprot/P02741 UniProtKB - P02741 (CRP_HUMAN)]</ref>. The diameter of the CRP pentamer is 102 Å, the inner pore diameter is 30 Å and the diameter of a subunit is 36 Å <ref name="agrawal">PMID: 19799114 </ref><ref name=”Volanakis”/>.
The crystal structure of CRP was determined by using SAP as the search model  <ref name="Thompson"/>. The structure of CRP has been determined by X-ray crystallography at 3 Å resolution. Like SAP, the protein consists of five protomers, uncovalently bonded and nonglycosylated that are arranged symmetrically around a central pore <ref name="Volanakis">PMID: 11532280</ref>. Each subunit is a 25 kDa protein consisting of 224 residues  <ref name="uniprot">[http://www.uniprot.org/uniprot/P02741 UniProtKB - P02741 (CRP_HUMAN)]</ref>. The diameter of the CRP pentamer is 102 Å, the inner pore diameter is 30 Å and the diameter of a subunit is 36 Å <ref name="agrawal">PMID: 19799114 </ref><ref name="Volanakis"/>.
Ser53, His95, Cys97, Asp112, Gly113, Gly136, Gly154, Val165, Leu166, Ile171, and Gly196 are the highly conserved residues in the primary sequence of CRP <ref name="kumar"/>.
Ser53, His95, Cys97, Asp112, Gly113, Gly136, Gly154, Val165, Leu166, Ile171, and Gly196 are the highly conserved residues in the primary sequence of CRP <ref name="kumar"/>.
Each subunit consist of two antiparallel <scene name='71/719862/Sheet/1'>β-sheets</scene> <ref name="uniprot"/> with a flattened jellyroll topology <ref name="Volanakis"/> and a long <scene name='71/719862/Helix/1'>α-helix</scene> (residues 168-176) lies folded against the β-sheets. The predominant structure is β-sheet <ref>PMID: 1382589</ref> but short helical regions can be noticed for the residues 43 and 185 <ref name="kumar"/>.  
Each subunit consist of two antiparallel <scene name='71/719862/Sheet/1'>β-sheets</scene> <ref name="uniprot"/> with a flattened jellyroll topology <ref name="Volanakis"/> and a long <scene name='71/719862/Helix/1'>α-helix</scene> (residues 168-176) lies folded against the β-sheets. The predominant structure is β-sheet <ref>PMID: 1382589</ref> but short helical regions can be noticed for the residues 43 and 185 <ref name="kumar"/>.  
In each subunit, we can distinguish 2 faces: A and B. Two calcium ions are bound 4 Å apart by protein sidechains coming from loops collected at the concave face (B) and this is the site of ligand binding <ref name=”Thompson”/>. The A face is recognizable by the presence of a single α-helix, meaning that this face of the pentamer shows five helices. There is also a marked furrow of 24 Å long, 7.5 Å deep and 12.4 Å wide. The side walls are constructed from Ser5, Arg6, Gln203, Pro206, Trp187, Arg188, Asn160, Gly177, Leu176, Tyr175, His95 and Asp112 <ref name=”Thompson”/>. The outer part of the furrow is positively charged but the inner part terminates halfway through the pentamer pore at residue Asp112, providing a ring of negative charges lining the pore <ref name=”Thompson”/>. This face can interact with C1q and Fc receptors <ref name="duclos">PMID: 15531769 </ref><ref name=”Volanakis”/>. Asp112 seems to be an important residue for recognition of Cq1 by CRP <ref name=”Thompson”/>. Each subunit in CRP is rotated by 22° towards the fivefold axis such that the helices of face A are 5 Å closer to the axis and the calcium sites on face B move away by an equivalent amount.  
In each subunit, we can distinguish 2 faces: A and B. Two calcium ions are bound 4 Å apart by protein sidechains coming from loops collected at the concave face (B) and this is the site of ligand binding <ref name="Thompson"/>. The A face is recognizable by the presence of a single α-helix, meaning that this face of the pentamer shows five helices. There is also a marked furrow of 24 Å long, 7.5 Å deep and 12.4 Å wide. The side walls are constructed from Ser5, Arg6, Gln203, Pro206, Trp187, Arg188, Asn160, Gly177, Leu176, Tyr175, His95 and Asp112 <ref name="Thompson"/>. The outer part of the furrow is positively charged but the inner part terminates halfway through the pentamer pore at residue Asp112, providing a ring of negative charges lining the pore <ref name="Thompson"/>. This face can interact with C1q and Fc receptors <ref name="duclos">PMID: 15531769 </ref><ref name=”Volanakis”/>. Asp112 seems to be an important residue for recognition of Cq1 by CRP <ref name="Thompson"/>. Each subunit in CRP is rotated by 22° towards the fivefold axis such that the helices of face A are 5 Å closer to the axis and the calcium sites on face B move away by an equivalent amount.  


=== Ca<sup>2+</sup>-binding site ===
=== Ca<sup>2+</sup>-binding site ===

Revision as of 14:14, 28 January 2016

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Human C-reactive protein complexed with phosphocholine

Human C-Reactive Protein (CRP) is an acute phase protein belonging to the highly conserved pentraxin protein family, like as its homologue, the Serum Amyloid P component (SAP) [1]. Although it is a normal serum protein, its circulating concentration raises rapidly and extensively in a cytokine-mediated in response to an infection, an inflammation or a tissue injury. In this way, serum CRP rate is empirically dose to detect many human disease [1]. CRP was named this way because it was found that the protein could precipitate the "C" polysaccharide derived from Streptococcus pneumoniae cell wall [2][3].


Structure

CRP structure

The crystal structure of CRP was determined by using SAP as the search model [1]. The structure of CRP has been determined by X-ray crystallography at 3 Å resolution. Like SAP, the protein consists of five protomers, uncovalently bonded and nonglycosylated that are arranged symmetrically around a central pore [4]. Each subunit is a 25 kDa protein consisting of 224 residues [5]. The diameter of the CRP pentamer is 102 Å, the inner pore diameter is 30 Å and the diameter of a subunit is 36 Å [6][4]. Ser53, His95, Cys97, Asp112, Gly113, Gly136, Gly154, Val165, Leu166, Ile171, and Gly196 are the highly conserved residues in the primary sequence of CRP [3]. Each subunit consist of two antiparallel β-sheets [5] with a flattened jellyroll topology [4] and a long α-helix (residues 168-176) lies folded against the β-sheets. The predominant structure is β-sheet [7] but short helical regions can be noticed for the residues 43 and 185 [3]. In each subunit, we can distinguish 2 faces: A and B. Two calcium ions are bound 4 Å apart by protein sidechains coming from loops collected at the concave face (B) and this is the site of ligand binding [1]. The A face is recognizable by the presence of a single α-helix, meaning that this face of the pentamer shows five helices. There is also a marked furrow of 24 Å long, 7.5 Å deep and 12.4 Å wide. The side walls are constructed from Ser5, Arg6, Gln203, Pro206, Trp187, Arg188, Asn160, Gly177, Leu176, Tyr175, His95 and Asp112 [1]. The outer part of the furrow is positively charged but the inner part terminates halfway through the pentamer pore at residue Asp112, providing a ring of negative charges lining the pore [1]. This face can interact with C1q and Fc receptors [8][9]. Asp112 seems to be an important residue for recognition of Cq1 by CRP [1]. Each subunit in CRP is rotated by 22° towards the fivefold axis such that the helices of face A are 5 Å closer to the axis and the calcium sites on face B move away by an equivalent amount.

Ca2+-binding site

CRP is a calcium dependent structure. In fact, Ca2+ is required for PC binding, and more precisely for the formation of the PC-binding site thanks to structural rearrangements. The protection against denaturation and proteolysis is also performed through Ca2+-binding. In the absence of Ca2+, hCRP is cleaved between Asn145 and Phe146 by nagarse protease, and between Phe146 and Glu147 by pronase. Asp60, Asn61, Glu138, Asp140 and the main-chain carbonyl of Gln139 residues allow the first calcium ion binding, and the second is performed through Glu138, Asp140, Glu147 and Gln150 [10]. The two Ca2+-binding sites are overlapping in a loop. In the absence of Ca2+, the loop changes conformation and releases the proteolysis site. Therefore Ca2+ protects CRP form proteolytic cleavage [6].

Phosphocholine-binding site

PC stands for phosphocholine. It is a phospholipid in cell membranes and a plasma lipoprotein [1]. The PC-binding site is a hydrophobic pocket constituted by the residues Leu64, Phe66, Thr76 and the two Ca2+ [6]. The choline function of PC interacts with the two key residues Phe66 and Glu81, therefore PC lies inside the PC-binding site [3] [6]. The PC-binding site is next to the Ca2+-binding sites on the same face of the CRP protein. The phosphate groupe of PC interacts by coordination with the two Ca2+ [6]. The affinity of CRP for PC increases with the concentration of PC. A surface containing a high density of PC, such as C-polycaccharide, is therefore propitious to the CRP-binding [8]. CRP can also bind chromatin, histones, small nuclear ribonucleoproteins nuclear envelop proteins and nucleosomes Ca2+-dependently [6].

PC and Ca2+-binding site [11]

Function

CRP binds to PC located on the surface of bacteria that infected the organism. The resulting immune response is the phygocytosis of PC-expressing bacteria [6]. The CRP is therefore part of the acute phase response which is a rapid concentration variation of plasma proteins [12].

Biomedical interest

Healthy humans have a CRP rate which is generally about 1 μg/mL[3]. CRP is secreted by the liver into the blood circulation[12]. CRP level is 1000 times higher in a cytokine-mediated response due to tissue injury, infection and inflammation. Therefore the CRP rate in serum is common use to detect the activity of a disease[1]. CRP can be defined as a target for the development of cardioprotection and neuroprotection[3].

Structural highlights

This is a sample scene created with SAT to color by Group, and another to make a transparent representation of the protein. You can make your own scenes on SAT starting from scratch or loading and editing one of these sample scenes.

</StructureSection>

References

  1. ↑ 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 Thompson D, Pepys MB, Wood SP. The physiological structure of human C-reactive protein and its complex with phosphocholine. Structure. 1999 Feb 15;7(2):169-77. PMID:10368284
  2. ↑ Black S, Kushner I, Samols D. C-reactive Protein. J Biol Chem. 2004 Nov 19;279(47):48487-90. Epub 2004 Aug 26. PMID:15337754 doi:https://dx.doi.org/10.1074/jbc.R400025200
  3. ↑ 3.0 3.1 3.2 3.3 3.4 3.5 Kumar SV, Ravunny RK, Chakraborty C. Conserved domains, conserved residues, and surface cavities of C-reactive protein (CRP). Appl Biochem Biotechnol. 2011 Sep;165(2):497-505. doi: 10.1007/s12010-011-9270-7., Epub 2011 May 4. PMID:21541851 doi:https://dx.doi.org/10.1007/s12010-011-9270-7
  4. ↑ 4.0 4.1 4.2 Volanakis JE. Human C-reactive protein: expression, structure, and function. Mol Immunol. 2001 Aug;38(2-3):189-97. PMID:11532280
  5. ↑ 5.0 5.1 UniProtKB - P02741 (CRP_HUMAN)
  6. ↑ 6.0 6.1 6.2 6.3 6.4 6.5 6.6 Agrawal A, Singh PP, Bottazzi B, Garlanda C, Mantovani A. Pattern recognition by pentraxins. Adv Exp Med Biol. 2009;653:98-116. PMID:19799114
  7. ↑ Dong A, Caughey B, Caughey WS, Bhat KS, Coe JE. Secondary structure of the pentraxin female protein in water determined by infrared spectroscopy: effects of calcium and phosphorylcholine. Biochemistry. 1992 Oct 6;31(39):9364-70. PMID:1382589
  8. ↑ 8.0 8.1 Du Clos TW, Mold C. C-reactive protein: an activator of innate immunity and a modulator of adaptive immunity. Immunol Res. 2004;30(3):261-77. PMID:15531769 doi:https://dx.doi.org/10.1385/IR:30:3:261
  9. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named ”Volanakis”
  10. ↑ Ramadan MA, Shrive AK, Holden D, Myles DA, Volanakis JE, DeLucas LJ, Greenhough TJ. The three-dimensional structure of calcium-depleted human C-reactive protein from perfectly twinned crystals. Acta Crystallogr D Biol Crystallogr. 2002 Jun;58(Pt 6 Pt 2):992-1001. Epub, 2002 May 29. PMID:12037301
  11. ↑ The PyMOL Molecular Graphics System, Version 1.8 Schrödinger, LLC.
  12. ↑ 12.0 12.1 Szalai AJ. The biological functions of C-reactive protein. Vascul Pharmacol. 2002 Aug;39(3):105-7. PMID:12616974
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