to 218) and His57 side chain in the non-canonical conformation. In its inhibited form, the self-inhibitory loop prevents access of Factor B to Factor D. When the self-inhibited conformation of Factor D is approached by the C3bB complex, C3bB displaces the salt bridge in Factor D and results in a new salt bridge between the Arg234 of Factor B and Asp189 of Factor D. The displacement of the Factor D salt bridge results in a realignment of the self-inhibitory loop and a rotation of the active site histidine side chain, creating the canonical form of Factor D. Cleavage of the scissile bond in Factor B then ensues, releasing fragment Ba and forming C3bBb, the alternative pathway C3-convertase. Regulation Factor D is synthesized by the liver and adipocytes with the latter being the major source. The pro-form of Factor D that is secreted is cleaved by MASP-3 to form the active sequence that circulates in the body. Factor D maintains an extremely high substrate specificity, and as a result has no known natural inhibitors in the body. However, most of Factor D remains in the self-inhibited form that limits substrate access to the catalytic site. Factor D has a molecular weight of 23.5 kD and is present at a concentration of 1.8 mg/L of blood in healthy humans. The synthesis rate of Factor is approximately 1.33 mg/kg/day, and most of Factor D is eliminated through the kidney after catabolism in proximal tubules after re-absorption. The net effect is a high fractional metabolic rate of 60% per hour. In patients with normal kidney function, no Factor D was detectable in urine. However, in patients with renal disease, Factor D was found at elevated levels. The alternative pathway is capable of operating even at low levels of Factor D, and deficiencies in levels of Factor D are rare. Role in Diseases A point mutation resulting in the replacement of a serine codon (Ser42 in the unprocessed methionine form of Factor D) with a stop codon (TAG) in the Factor D gene on chromosome 19 has been documented as a cause of Factor D deficiency. Deficiency in Factor D may cause an increased susceptibility to bacterial infections, specifically Neisseria infections. The mode of inheritance of Factor D deficiency is autosomal recessive, and individuals with a mutation on only one allele may not experience the same susceptibility to reoccurring infections. In a patient with reoccurring