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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 crystallograph at 3 Å resolution.

Primary structure: The C-reactive protein is a homopentamer in which each subunit is a 25 kDa protein consisting of 224 residues ^[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].

Secondary structure: The secondary structure is formed of one and two antiparallel ^[4]. The predominant structure is β-sheet ^[5] but short helical regions can be noticed for the residues 43 and 185 ^[3]. The residues Glu197 and Lys123 of CRP form an intermolecular ion pair ^[1].

Tertiary and quaternary structure: Like SAP, the protein consists of five protomers, uncovalently bonded and nonglycosylated that are arranged symmetrically around a central pore ^{[6] [7]}. 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 ^[8]. Thereafter, the ligand-binding face of the molecule will be called B and the opposite face will be called A. The B face binds two Ca²⁺ ions and a phosphocholine per subunit. The A face, recognizable by the presence of an α-helix and a deep cavity, can interact with C1q and Fc receptors ^[7].

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 ^[9]. 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 ^[8].

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 Ca^{2+ [8]}. The choline function of PC interacts with the two key residues Phe66 and Glu81, therefore PC lies inside the PC-binding site ^{[3] [8]}. 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 Ca^{2+ [8]}. 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 ^[7]. CRP can also bind chromatin, histones, small nuclear ribonucleoproteins nuclear envelop proteins and nucleosomes Ca²⁺-dependently ^[8].

PC and Ca²⁺-binding site ^[10]

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 ^[8]. The CRP is therefore part of the acute phase response which is a rapid concentration variation of plasma proteins ^[11].

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^[11]. 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

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References

1. ↑ ^{1.0 1.1 1.2 1.3 1.4 1.5} 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:http://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:http://dx.doi.org/10.1007/s12010-011-9270-7
4. ↑ ^{4.0 4.1} UniProtKB - P02741 (CRP_HUMAN)
5. ↑ 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
6. ↑ Volanakis JE. Human C-reactive protein: expression, structure, and function. Mol Immunol. 2001 Aug;38(2-3):189-97. PMID:11532280
7. ↑ ^{7.0 7.1 7.2} 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:http://dx.doi.org/10.1385/IR:30:3:261
8. ↑ ^{8.0 8.1 8.2 8.3 8.4 8.5 8.6} Agrawal A, Singh PP, Bottazzi B, Garlanda C, Mantovani A. Pattern recognition by pentraxins. Adv Exp Med Biol. 2009;653:98-116. PMID:19799114
9. ↑ 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
10. ↑ The PyMOL Molecular Graphics System, Version 1.8 Schrödinger, LLC.
11. ↑ ^{11.0 11.1} Szalai AJ. The biological functions of C-reactive protein. Vascul Pharmacol. 2002 Aug;39(3):105-7. PMID:12616974