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Joined 2026-09-21 Message
Incoming links: Complement receptor 1
Complement factor I 2026-11-05
Complement factor I is a regulatory serine protease of the complement system. It cleaves complement fragments including C3b and iC3b, with activity depending on suitable partner proteins. Its official enzyme classification is EC 3.4.21.45.[ref]
Complement factor I in IUBMB Enzyme Nomenclature. accepted name, Reaction and Comments
For cleavage of C3b, the substrate binds a partner such as complement factor H or complement receptor 1. Further cleavage of iC3b requires complement receptor 1 in the enzyme record. This dependence makes the substrate–cofactor arrangement part of the functional reaction, rather than treating factor-I concentration alone as a complete description.[ref]
Complement factor I in IUBMB Enzyme Nomenclature. Comments, substrate-specific cofactor requirements
A 2009 structural and functional study examined the first four domains of complement factor H. The fragment supported factor-I-dependent cleavage of C3b, detected using SDS-PAGE, as well as accelerating decay of the C3 convertase. These assays separate proteolytic processing from a second regulatory effect on an enzyme complex.[ref]
A useful control comparison therefore holds complement factor I and substrate constant while adding or omitting the required partner. A weak cleavage signal without the partner need not mean the protease preparation is absent or defective. The experiment tests the assembled reaction system.
D. T. Fearon 2026-11-05
D. T. Fearon authored a 1980 study identifying a membrane glycoprotein as the receptor for the complement fragment C3b on human red blood cells, B cells, monocytes and polymorphonuclear leukocytes. The molecule, called gp205 in the paper, is associated with the receptor now called complement receptor 1. The work connected a purified protein's biochemical activity with a receptor found on several classes of blood cells.[ref]
Identification of the membrane glycoprotein that is the C3b receptor of the human erythrocyte, polymorphonuclear leukocyte, B lymphocyte, and monocyte by D. T. Fearon. Abstract, Journal of Experimental Medicine 152 (1980), pp. 20–30; DOI 10.1084/jem.152.1.20.
The route to identification is instructive. The glycoprotein was investigated for its ability to inhibit a C3 convertase and to assist enzymatic inactivation of C3b. Those activities suggested a C3b-binding function, which the receptor experiments tested. Thus a cell membrane receptor need not merely initiate a response: recognition of a complement component can also help regulate the amplification system to which that component belongs.[ref]
Identification of the membrane glycoprotein that is the C3b receptor of the human erythrocyte, polymorphonuclear leukocyte, B lymphocyte, and monocyte by D. T. Fearon. Abstract: convertase destabilization, C3b-inactivator cofactor activity and receptor identification.