The serum amyloid P component (SAP) is the identical serum form of amyloid P component (AP), a 25kDa pentameric protein first identified as the pentagonal constituent of in vivo pathological deposits called "

amyloid Amyloids are aggregates of proteins characterised by a fibrillar morphology of 7–13 nm in diameter, a beta sheet (β-sheet) secondary structure (known as cross-β) and ability to be stained by particular dyes, such as Congo red. In the hum ...

". APCS is its human gene.

In amyloidosis

AP makes up 14% of the dry mass of amyloid deposits and is thought to be an important contributor to the pathogenesis of a related group of diseases called the

Amyloidoses Amyloidosis is a group of diseases in which abnormal proteins, known as amyloid fibrils, build up in tissue. There are several non-specific and vague signs and symptoms associated with amyloidosis. These include fatigue, peripheral edema, weigh ...

. These conditions are characterised by the ordered aggregation of normal globular proteins and peptides into insoluble fibres which disrupt tissue architecture and are associated with cell death. AP is thought to decorate and stabilise aggregates by preventing proteolytic cleavage and hence inhibiting fibril removal via the normal protein scavenging mechanisms. This association is utilised in the routine clinical diagnostic technique of SAP scintigraphy whereby radio-labelled protein is injected into patients to locate areas of amyloid deposition. The SAP-amyloid association has also been identified as a possible drug target for anti-amyloid therapy, with the recent development and first stage clinical trials of a compound called CPHPC (R-1- -[R-2-carboxy-pyrrolidin-1-yl6-oxohexanoyl.html" ;"title="-2-carboxy-pyrrolidin-1-yl.html" ;"title="-[R-2-carboxy-pyrrolidin-1-yl">-[R-2-carboxy-pyrrolidin-1-yl6-oxohexanoyl">-2-carboxy-pyrrolidin-1-yl.html" ;"title="-[R-2-carboxy-pyrrolidin-1-yl">-[R-2-carboxy-pyrrolidin-1-yl6-oxohexanoylpyrrolidine-2-carboxylic acid), a small molecule able to strip AP from deposits by reducing levels of circulating SAP.

Structure

SAP is a member of the pentraxins family, characterised by calcium dependent ligand binding and distinctive flattened β-jellyroll structure similar to that of the legume lectins. The name "pentraxin" is derived from the Greek word for five (penta) and berries (ragos) relating to the radial symmetry of five monomers forming a ring approximately 95 Å across and 35 Å deep. Human SAP has 51% sequence homology with

C-reactive protein C-reactive protein (CRP) is an annular (ring-shaped) pentameric protein found in blood plasma, whose circulating concentrations rise in response to inflammation. It is an acute-phase protein of hepatic origin that increases following interleukin-6 ...

(CRP), a classical acute phase response plasma protein, and is a more distant relative to the "long" pentraxins such as PTX3 (a cytokine modulated molecule) and several neuronal pentraxins. Both SAP and CRP are evolutionary conserved in all vertebrates and also found in distant invertebrates such as the horseshoe crab (Limulus polyphemus).

References

{{Amyloidosis Blood proteins Acute-phase proteins