The human complement factor H protein family consists of the complement and immune regulators factor H, the factor H-like protein 1(FHL-1) and five factor H-related proteins (CFHR-1 to -5). Members of the H-related protein family are exclusively composed of individually folded protein domains, termed short consensus repeats (SCRs) or complement control modules. The genes of this family have been located in human chromosome 1q32, which is known as the regulators of complement activation (RCA) gene clusters.
Complement factor H-related protein 1 (CFHR1) is a 43 kDa, secreted member of the factor H family of glycoproteins. It is produced by hepatocytes and circulates as two differentially glycosylated isoforms (37 kDa and 43 kDa). Complement Factor H-related 2 (CFHR2) is synthesized in the liver and secreted into plasma. It may be involved in complement regulation. CFHR2 can also be associated with lipoproteins and may play a role in lipid metabolism.
CFHR-5 has been identified initially as a universal component of complement deposits, and detected in glomerular immune deposits. The pattern of deposits is similar to other complement components, suggesting that CFHR-5 may play a role in complement activation and regulation. It is synthesized in the liver and consists of 9 SCRs. CFHR-5 exhibits similar characteristics as those of factor H in heparin binding, CRP binding, and lipoprotein association. Weak factor I-dependent cofactor activity for C3b cleavage has also been observed.