Description
Secreted protein acidic and rich in cysteine, also known as Osteonectin, is a protein that in humans is encoded by the SPARC gene. The human gene is 26.5 kb long, and contains 10 exons and 9 introns and is located on chromosome 5q31-q33. SPARC is an acidic, cysteine-rich glycoprotein consisting of a single polypeptide chain that can be broken into 4 domains: 1) an Ca++ binding domains near the glutamic acidic-rich region at the amino terminus(domain I), 2) a cysteine- rich(domain II), 3) a hydrophilic region(domain III) and 4) an EF hand motif at the carboxy terminus region(domain IV). SPARC is a glycoprotein in the bone that binds sodium. It is secreted by osteoblasts during bone formation, initiating mineralization and promoting mineral crystal formation. It also shows affinity for collagen in addition to bone mineral calcium. A correlation between SPARC over expression and ampullary cancers and chronic pancreatitis has been found