The Conformation of Factor H Determines its Von Willebrand Factor Reductase Activity

Full-length factor H (FH), consisting of 20 small consensus repeat domains (150 kDa), is a complement control protein that regulates the activity of the alternative complement pathway. Recombinant factor H (rFH) has exposed free thiols that can also reduce the disulfide bonds linking smaller soluble von Willebrand factor (VWF) multimers into large VWF multimeric forms. In contrast, commercial, highly purified FH (pFH) is prepared under conditions that result in fewer exposed free thiols and little or no VWF reductase activity. pFH chemically altered by either EDTA or urea regains its VWF reductase activity. We used single molecule force studies with atomic force microscopy (AFM) to compare the characteristics of rFH and pFH in the presence or absence of EDTA and urea. Our data indicate that the VWF reductase activity of full-length FH (rFH and pFH) is associated with different conformations and the amount of free thiols.