Structure
Both heavy and light chains bear Asn-linked glycans, on three distinct glycosylation sites each.
The fI heavy chain has four domains: a FIMAC domain, a Scavenger Receptor Cysteine Rich (SRCR) domain and two LDL-receptor Class A domains; the heavy chain plays an inhibitory role in maintaining the enzyme inactive until it meets the complex formed by the substrate (either C3b or C4b) and a cofactor protein (Factor H, CR1, MCP or C4BP). Upon binding of the enzyme to the substrate:cofactor complex, the heavy:light chain interface is disrupted, and the enzyme activated by allostery. The LDL-receptor domains contain one Calcium-binding site each.
The fI light chain is the serine protease domain containing the catalytic triad responsible for specific cleavage of C3b and C4b. Conventional protease inhibitors do not completely inactivate Factor I but they can do so if the enzyme is pre-incubated with its substrate: this supports the proposed rearrangement of the molecule upon binding to the substrate.
Genetic polymorphism in Factor I has been observed and recently explained in terms of variants R201S, R406H, R502L.
Crystal structure the crystal structure of human Factor I has been deposited as PDB 2XRC.
Read more about this topic: Complement Factor I
Famous quotes containing the word structure:
“For the structure that we raise,
Time is with materials filled;
Our to-days and yesterdays
Are the blocks with which we build.”
—Henry Wadsworth Longfellow (1809–1882)
“The structure was designed by an old sea captain who believed that the world would end in a flood. He built a home in the traditional shape of the Ark, inverted, with the roof forming the hull of the proposed vessel. The builder expected that the deluge would cause the house to topple and then reverse itself, floating away on its roof until it should land on some new Ararat.”
—For the State of New Jersey, U.S. public relief program (1935-1943)
“A committee is organic rather than mechanical in its nature: it is not a structure but a plant. It takes root and grows, it flowers, wilts, and dies, scattering the seed from which other committees will bloom in their turn.”
—C. Northcote Parkinson (1909–1993)