Complement factor H (fH) is purified from normal human serum. It is an essential regulatory component of the alternative pathway of complement. It is critical for prevention of complement activation on host cells and tissues, especially the kidney. It has two functional activities: 1) it controls the formation and decay of the alternative pathway C3/C5 convertase (decay accelerating activity) and 2) it acts as a cofactor for factor I which proteolytically inactivates C3b when C3b is bound to factor H (cofactor activity).
Factor H is a 155,000 Da protein composed of 20 homologous domains arranged like beads on a semi-flexible string. The N-terminal 5 domains bind to C3b and inhibit binding of factor B thus reducing the formation of C3/C5 convertase. Factor H also binds to preformed C3/C5 convertases (C3b,Bb and C3b,Bb,C3b) and causes rapid release of the catalytic subunit Bb (decay acceleration). These activities are essential for controlling the spontaneous activation of the alternative pathway amplification process in plasma. In addition, factor H controls the formation and decay of these enzymes when C3b is attached to the surface of particles.
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