Circular dichroism studies of human factor H. A regulatory component of the complement system.

Factor H of the human complement system exhibits an unusual circular dichroism spectrum. The CD spectrum of Factor H exhibits a positive extreme at 230 nm and a negative extreme at 190 nm. No apparent alpha-helical or beta-sheet conformations were present in the native protein structure. However, when the disulfide bridges are reduced, followed either by reoxidation or alkylation, the structure of Factor H is modified so that it now exhibits conventional protein secondary structure as determined from its CD spectra in the far ultraviolet region. Factor H also fails to mediate its regulatory function of inhibiting the alternative pathway convertase once the disulfides have been ruptured and conformational rearrangement has occurred. CD studies indicate that minor conformational changes take place when Factor H and C3b associate in free solution.