Zinc-induced Self-association of Complement C3b and Factor H: IMPLICATIONS FOR INFLAMMATION AND AGE-RELATED MACULAR DEGENERATION*

The sub-retinal pigment epithelial deposits that are a hallmark of age-related maculardegeneration contain both C3b and millimolar levels of zinc. C3 is the central protein ofcomplement, whereas C3u is formed by the spontaneous hydrolysis of the thioester bridge in C3.During activation, C3 is cleaved to form active C3b, then C3b is inactivated by Factor I and FactorH to form the C3c and C3d fragments. The interaction of zinc with C3 was quantified using analyticalultracentrifugation and x-ray scattering. C3, C3u, and C3b associated strongly in >100μm zinc, whereas C3c and C3d showed weak association. With zinc, C3 forms solubleoligomers, whereas C3u and C3b precipitate. We conclude that the C3, C3u, and C3b association withzinc depended on the relative positions of C3d and C3c in each protein. Computational predictionsshowed that putative weak zinc binding sites with different capacities exist in all five proteins,in agreement with experiments. Factor H forms large oligomers in >10 μm zinc. Incontrast to C3b or Factor H alone, the solubility of the central C3b-Factor H complex was muchreduced at 60 μm zinc and even more so at >100 μm zinc. Theremoval of the C3b-Factor H complex by zinc explains the reduced C3u/C3b inactivation rates by zinc.Zinc-induced precipitation may contribute to the initial development of sub-retinal pigmentepithelial deposits in the retina as well as reducing the progression to advanced age-relatedmacular degeneration in higher risk patients.