C3b Protein

C3b is derived from native C3 upon cleavage and release of C3a. It is prepared by cleavage with the alternative pathway C3 convertase. C3b is central to the function of all three pathways of complement. Initiation of each pathway generates proteolytic enzyme complexes (C3 convertases) which are bound to the target surface. These enzymes cleave a peptide bond in C3 releasing the anaphylatoxin C3a and activating C3b. For a brief time (~60 μs) this nascent C3b is capable of reacting with and covalently coupling to hydroxyl groups on the target surface. Carbohydrates are the favored target, but protein hydroxyls and amino groups also react. This process of tagging the target surface with C3b is called opsonization.