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:
“When a house is tottering to its fall,
The strain lies heaviest on the weakest part,
One tiny crack throughout the structure spreads,
And its own weight soon brings it toppling down.”
—Ovid (Publius Ovidius Naso)
“A structure becomes architectural, and not sculptural, when its elements no longer have their justification in nature.”
—Guillaume Apollinaire (1880–1918)
“The syntactic component of a grammar must specify, for each sentence, a deep structure that determines its semantic interpretation and a surface structure that determines its phonetic interpretation.”
—Noam Chomsky (b. 1928)