Complement factor I

Complement factor I, also known as C3b/C4b inactivator, is a protein that in humans is encoded by the CFI gene. Complement factor I (factor I) is a protein of the complement system, first isolated in 1966 in guinea pig serum, that regulates complement activation by cleaving cell-bound or fluid phase C3b and C4b. It is a soluble glycoprotein that circulates in human blood at an average concentration of 35 μg/mL.

Complement factor I, also known as C3b/C4b inactivator, is a protein that in humans is encoded by the CFI gene. Complement factor I (factor I) is a protein of the complement system, first isolated in 1966 in guinea pig serum, that regulates complement activation by cleaving cell-bound or fluid phase C3b and C4b. It is a soluble glycoprotein that circulates in human blood at an average concentration of 35 μg/mL. Crystal structure the crystal structure of human Factor I has been deposited as PDB: 2XRC. ==Clinical significance== Dysregulated factor I activity has clinical implications. Loss of function mutations in the Complement Factor I gene lead to low levels of factor I which results in increased complement activity. Factor I deficiency in turn leads to low levels of complement component 3 (C3), factor B, factor H and properdin in blood, due to unregulated activation of C3 convertase, and to low levels of IgG, due to loss of iC3b and C3dg production. In addition to the following diseases, low factor I is associated with recurrent bacterial infections in children. ===Age-related macular degeneration=== Research suggests that mutations in the CFI gene contribute to development of age-related macular degeneration. This contribution is thought to be due to the dysregulation of the alternative pathway, leading to increased inflammation in the eye. ===Atypical hemolytic uremic syndrome=== Atypical hemolytic uremic syndrome is caused by complement overactivation. Heterozygous mutations in the serine protease domain of the CFI gene account for 5-10% of cases. OMIM entries on Atypical Hemolytic-Uremic Syndrome The MEROPS online database for peptidases and their inhibitors:

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