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The C-reactive protein has been given this name because it precipitates the C polysaccharide in the cell wall.[3]
Structure
CRP structure
Ser53, His95, Cys97, Asp112, Gly113, Gly136, Gly154, Val165, Leu166, Ile171, and Gly196 are the highly conserved residues in the primary sequence of CRP.[3]
The C-reactive protein is a homopentamer of non-covalently bound subunits. Each subunit is a 25 Da protein consisting of 224 residues bound together. The secondary structure is formed of four α-helices and three antiparallel β-sheets (five-stranded, three-stranded and seven-stranded).[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.[6]
The diameterof the CRP pentamer is 102 Å, the inner pore diameter is 30 Å and the diameter of a subunit is 36 Å. [7]
Ca2+ binding-site
CRP is a calcium dependent structure. Effectively, 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 performed through Ca2+ binding too. 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.[8]
The two Ca2+-binding sites are overlapping in a loop. In the absence of Ca2+, the loop changes conformaion and releases the proteolysis site. Therefore Ca2+ protects CRP form proteolytic cleavage.
PC binding site
PC stands for phosphocholine. It is a phospholipid in cell membranes and a plasma lipoproteins.[6] Phe-66 and Glu-81 are the two key residues that enable the binding of PC. [3]
Function
Biomedical interest
Healthy humans have CRP rate which is generally about 1 μg/mL.[3] 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.[6] CRP can be defined as a target for the development of cardioprotection and neuroprotection.[3]
Structural highlights
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