Sandbox Reserved 1121: Difference between revisions

=== Ca²⁺-binding site ===

CRP is a calcium dependent structure. The face B binds two Ca²⁺ ions per subunit. The ligands are bound asymmetrically in both Ca²⁺-binding sites. The residues of the site 1 include Asp60, Asn61, Glu138, Asp140, the carbonyl oxygen of the residue 139 and one oxygen of Asp60. The site 1 provides a total of five ligands for the Ca²⁺ ion. The site 2 residues include Gln138, Asp140, Gln150 and non-coordinating Glu147. The site 2 provides four ligands for the Ca²⁺ ion. The two Ca²⁺-binding sites are of equal affinity for Ca²⁺ and are only differentiated by the carbonyl oxygen ligand. In presence of Ca²⁺, the two Ca²⁺-binding sites are overlapping in a loop. When the Ca²⁺ sites are free, the residues 140 to 150 form a loop away from the molecule<ref name="Thompson"/>. This releases the proteolysis site <ref name="agrawal"/>, which induces the cleavage of CRP between Asn145 and Phe146 by nagarase protease and between Phe146 and Glu147 by pronase <ref name="ramadan">PMID: 12037301</ref>. Therefore Ca²⁺ protects CRP form proteolytic cleavage and from degradation <ref name="agrawal"/>.

=== Phosphocholine-binding site ===

PC stands for phosphocholine. It is a phospholipid in cell membranes and a plasma lipoprotein <ref name="Thompson"/>. The PC-binding site is a hydrophobic pocket constituted by the residues Leu64, Phe66, Thr76 and the two Ca²⁺ <ref name="agrawal"/>.

The choline function of PC interacts with the two key residues Phe66 and Glu81, therefore PC lies inside the PC-binding site <ref name="kumar"/> <ref name="agrawal"/>.

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 <ref name="duclos"/>.

CRP can also bind chromatin, histones, small nuclear ribonucleoproteins nuclear envelop proteins and nucleosomes Ca²⁺-dependently <ref name="agrawal"/>.

== Function ==