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This page is reserved for a work from two students in ESBS (A.Butet and E.Rosati) Thanks.
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| Resources: | FirstGlance, OCA, RCSB, PDBsum | ||||||||
| Coordinates: | save as pdb, mmCIF, xml | ||||||||
C-reactive protein, CRP
CRP, C-reactive ProteinCRP, C-reactive Protein
- The C Reactive Protein is a protein of the acute phase, the first described, exclusively synthetized by the liver.
- The CRP was first isolated by Tillett and France in 1930, in patients' serum presenting an acute inflammation. This protein reacted to the polysaccharide C of the pneumocoque, that is where its name comes from. <1>
- The CRP contributes to innate host defense, and plays an important role in inflammatory reactions. It can bind to specific molecules typically exposed during cell death or found on the surfaces of pathogens. It is used as biological marker to reveal an inflammatory reaction and tissue damage. <2>
StructureStructure
Gene structure, familyGene structure, family
- The CRP gene is located on chromosome 1q23. It is composed of two exons and one intron. This gene is regulated by interleukin-6, the principal inducer of the gene during the acute phase. CRP is secreted by hepatocytes.<1>
- The Human CRP belongs to the pentraxin family, proteins having five identical, non-covalently associated subunits that form a symmetrical homopentameric ring. The pentraxin family is highly conserved in evolution.<3>
SizeSize
- Each subunit contains 206 amino acid residues (approximately 23kDa) and is non-glycosylated. The outside diameter of the pentamer is 102 Å, the diameter of the inner core is 30 Å, and the diameter of the protomer is 36 Å. <2>
Detailed strucutreDetailed strucutre
- Each protomer consists of two anti-parallel β sheets (the lectin fold) with an α helix on the effector face of the protein. The ligand biding site is located on the concave face of the protein, and is composed of loops with 2 calcium ions bound 4 Å apart by protein side-chains.
- The recognition face contains the phosphocholine binding site which consists of two coordinated calcium ions next to a hydrophobic pocket in which the phosphocholine stays.
- There are interprotomer interactions between the subunits: three salt bridges are included and the 115-123 loop of one protomer and the 40-42 and 197-202 regions of adjacent protomers are involved. Moreover, the subunits can rotate by 15-20° around an axis parallel to the central alpha-helix.
- Thanks to this rotation, the alpha-helices can lie closer to the axis of the pentamere, therefore bringing the bound Ca2+ further away from it. On each subunit, we can find phosphocholine bound in a shallow surface pocket. With the help of phosphate groups and Glu81 via the choline moiety, the phosphocholine can interact with the two protein-bound ions.
- Moreover, the structure of CRP is different in ill patients. Indeed, in some pathological conditions, the Human CRP is glycosylated. Analysis of the structure showed the systematic absence of two peptide fragments, one at the N-terminus (loop 1-6) in all patients, the other near the C-terminus (loop 189-191) in patients with osteogenic sarcoma and Cushing's syndrome. In a healthy individual, glycosylation sites are inacessible due to the presence of the N-terminal. The loss of these two fragments exposed two potential glycosylation sites on a cleft door. The functional areas of the pentraxin structure remain the same since the Ca2+ and phosphocholine sites are on the opposite site of the pentraxin molecule. <2>
BiomarkerBiomarker
- The CRP concentration is a very useful nonspecific biochemical marker of inflammation and/or tissue damage. It is used since 1977 in the diagnosis and the supervision of the evolution of the infections, because the normalization of its rate is an indication that the infectious phenomenon is mastered. The C Reactive Protein is also a predictive biomarker for cardiovascular disease risk. <4>
- In healthy young adult, the median concentration of CRP is 0.8 mg/l. This value may increase from less than 50 μg/l to more than 500 mg/l (10,000-fold), following an acute-phase stimulus. After a single stimulus, serum concentration of CRP is rising above 5 mg/l in around 6 hours. This increase is proportional to the intensity of the inflammation. The peak is reached during 48 hours. When the stimulus ceases, the circulating CRP concentration falls rapidly. <3>
- There is individual variability in baseline CRP, resulting from non-genetic or genetic factors. Indeed, a polymorphism in the CRP gene intron and promoter has been described, that pertubs expression level.
- The intron of the gene of the CRP is formed of 278 nucleotides, containing a segment of 39 nucleotides rich in GT bases. This segment could be responsible for the formation of the left-handed helix of the Z-form DNA of the CRP. The persons possessing a particular allele combination have a lower concentration of the CRP. It is probably due to structural modifications of the DNA which would affect the transcription of the gene.
- Within the promoter, several polymorphisms were discovered in transcription factor binding E-box sites, what seems to significantly influence the rate of the CRP into the blood.
- This variability should be taken into account when using the CRP as a predictive biomarker. <1>
LigandLigand
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- In the presence of calcium, CRP is specifically bound to residues. We find phosphocholine in the microbial polysaccharides.
- The wide distribution of phosphocholine in polysaccharides of pathogenic and in cellular membranes allows CRP to recognize a range of pathogenic targets as well as the damaged membranes and necrosed cells of the host. The ligand, the phosphocholine, does not appear normally on the surface of cells, but is exposed by cellular damages caused by phospholipases.
- After this connection, CRP activates the classical complement pathway in the absence of antibody, and opsonizes ligands, with the aim of their phagocytosis.
- Indeed, when CRP is bound to the ligand, it is recognized by the factor CIq, which activates powerfully the classical complement pathway, committing the factor C3. Then there is formation of the membrane attack complex C5-C9 on the surface of the ligand, what entails its phagocytosis.
- <5>
- On the recognition face, there are two binding sites of equal affinity to calcium, consisting of residues for the first calcium ion and residues for the second calcium ion. An interaction appears between the two calcium ions and the oxygens of the phosphate group and the choline group, which stays in a hydrophobic pocket formed by residues . The face of Phe66 (in light blue) is exposed, allowing it to have hydrophobic interactions with the methyl groups of the choline. Glu81 (in magenta) interacts with the positively charged nitrogen on choline.
- The binding site for Clq is found of the effector face, or opposite to the phosphocholine binding site. This site is located at the open shallow end of a cleft, where a depression is formed. The pocket is limited by the loops 86-92 and 112-114 on the protomer's C-terminus, and Tyr175 on the other. Residues Asp112 and Tyr75 are the contact residues for the Clq. The substitution of these residues with Ala results in significantly reduced affinity for CRP. Glu88 causes a conformational changed in CRP which is needed before complementation activation can occur, whereas Asn158 and His38 are needed for the proper geometry at the binding site. Substitution of Ala for Lys114 resulted in more Clq-binding and increased complement activation.
- <2>
- The CRP also binds to receptors of IgG, the FcγR, present on the surface of phagocytic cells. It acts as an opsonin. But we still do not know the exact location of the binding sites of these molecules.
- <2>
ReferencesReferences
1-Technical information, R & D Systems <1>
2- C-reactive Protein, Jennifer Kao and Melissa Martin, 2007. <2>
3- C-reactive protein: a critical update, Mark B. Pepys and Gideon M. Hirschfield, June 15, 2003. <3>
4- C-reactive protein, Wikipedia <4>
5- Human C-reactive protein: expression, structure, and function, Volanakis JE., August 2001. <5>
6- Study Refutes Protein's Role in Heart Attacks, Jennifer Couzin-Frankel, June 30, 2009. <6>
7- C-reactive Protein at the Interface Between Innate Immunity, Inflammation: CRP Structure & Expression, Andres Peisajovich,1 Lorraine Marnell,2 Carolyn Mold,3 Terry W Du Clos 4. <7>
Proteopedia Page Contributors and EditorsProteopedia Page Contributors and Editors
Astrid BUTET and Elise ROSATI