The serum amyloid P component (SAP) is the identical serum form of the amyloid P component (AP), a 25 kDa

pentameric protein A pentameric protein is a quaternary protein structure that consists of five protein subunits. Examples Ligand-gated ion channels Five sub-units come together to form a channel. Each channel consist of two alpha chain, one beta, one gamma an ...

first identified as the pentagonal constituent of in vivo pathological deposits called "

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

". APCS is its human

gene In biology, the word gene has two meanings. The Mendelian gene is a basic unit of heredity. The molecular gene is a sequence of nucleotides in DNA that is transcribed to produce a functional RNA. There are two types of molecular genes: protei ...

In amyloidosis

SAP 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. These conditions are characterised by the ordered aggregation of normal globular proteins and

peptide Peptides are short chains of amino acids linked by peptide bonds. A polypeptide is a longer, continuous, unbranched peptide chain. Polypeptides that have a molecular mass of 10,000 Da or more are called proteins. Chains of fewer than twenty am ...

s into insoluble fibres, which disrupt tissue architecture and are associated with cell death. SAP is thought to decorate and stabilise aggregates by preventing

proteolytic Proteolysis is the breakdown of proteins into smaller polypeptides or amino acids. Protein degradation is a major regulatory mechanism of gene expression and contributes substantially to shaping mammalian proteomes. Uncatalysed, the hydrolysis o ...

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 A SAP scan is a type of nuclear medicine imaging test which uses iodine-123 (123I) and serum amyloid P component (SAP) to diagnose amyloidosis. In patients with amyloidosis, large deposits of SAP coat the affected organs, in addition to the low l ...

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">-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 a distinctive flattened β-jellyroll structure similar to that of the legume lectins. The name "pentraxin" is derived from the Greek words for five (penta) and berries (ragos), relating to the radial symmetry of five monomers forming a ring approximately 95 across and 35 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 ...

(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 are also found in distant invertebrates such as the

horseshoe crab Horseshoe crabs are arthropods of the family Limulidae and the only surviving xiphosurans. Despite their name, they are not true crabs or even crustaceans; they are chelicerates, more closely related to arachnids like spiders, ticks, and scor ...

(Limulus polyphemus).

References

{{Amyloidosis Blood proteins Acute-phase proteins