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Structure
According to X-ray analysis, the SAP pentamer consists of five subunits of 204 amino-acid residues, each with a closely similar three-dimensional structure constructed from antiparallel β-strands arranged in two sheets and a long α-helix between strands L and M [2]. Appropriately, the pentraxin family is characterized by distinctive β-jellyroll structures. The five subunits of SAP are arranged in a ring with a hole 20Å in diameter and 35Å deep at the center [2]. While SAP is most commonly characterized as a pentamer, SAP decamers have also been observed. Unlike the pentamer, though, the decamer is less stable and can be readily dissociated by reducing the pH to 5.5 [2].
Several studies have confirmed that SAP has a calcium-binding site, and some have suggested that calcium ligation is likely to be an important local structural determinant. Two large spheres of density which are too heavy to be oxygen atoms and are in positions that imply the presence of calcium ions were identified between 4.0 and 4.3 Å apart in the five subunits [2]. Thus, calcium ions have been recognized as co-factors for SAP. The presence of calcium increases SAP resistance to proteolytic degradation, and all ligand-binding has been proven calcium-dependent at physiological pH and ionic strength.
Potential Functions
Work related to the roles and functions of SAP traces back to the early 1970s. Due to the lack of a natural state of SAP deficiency, it is thought to be an important protein involved in a variety of physiological roles. While reports confirm that SAP is a major calcium-dependent DNA-binding protein in normal serum, there is a lack of evidence explaining the exact functions and reaction mechanisms of SAP. More recent speculations regarding the physiological relevance of SAP have been related to different diseases.
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