C3 convertase

C3 Convertase

Structure of C3 convertase

C3 convertase is a key enzyme involved in the complement system, which is an important part of the immune response in vertebrates. It plays a crucial role in the activation of the complement cascade, leading to the elimination of pathogens and the clearance of immune complexes.

Structure

C3 convertase is a complex enzyme that consists of multiple subunits. It can be formed through two different pathways: the classical pathway and the alternative pathway. In the classical pathway, C3 convertase is formed by the cleavage of C4 and C2 proteins, while in the alternative pathway, it is formed by the cleavage of C3 protein.

The structure of C3 convertase varies depending on the pathway involved. In the classical pathway, C3 convertase is composed of C4b2a, where C4b is derived from the cleavage of C4 and C2a is derived from the cleavage of C2. In the alternative pathway, C3 convertase is composed of C3bBb, where C3b is derived from the cleavage of C3 and Bb is derived from the cleavage of factor B.

Function

C3 convertase plays a crucial role in the complement cascade by catalyzing the cleavage of C3 protein into C3a and C3b fragments. C3b is an opsonin that coats the surface of pathogens, marking them for phagocytosis by immune cells. It also participates in the formation of the membrane attack complex (MAC), which can directly lyse pathogens.

C3a, on the other hand, acts as a potent inflammatory mediator by inducing the release of histamine and other pro-inflammatory molecules. It also attracts immune cells to the site of infection or inflammation, enhancing the immune response.

Regulation

The activity of C3 convertase is tightly regulated to prevent excessive complement activation and potential damage to host tissues. Several regulatory proteins, such as factor H and factor I, are involved in the degradation and inactivation of C3 convertase. They ensure that the complement cascade is properly controlled and targeted towards pathogens.

Clinical Significance

Dysregulation of the complement system, including C3 convertase, has been implicated in various diseases. Deficiencies or mutations in regulatory proteins can lead to excessive complement activation, resulting in autoimmune disorders, such as systemic lupus erythematosus (SLE) and atypical hemolytic uremic syndrome (aHUS). On the other hand, deficiencies in C3 convertase components can lead to increased susceptibility to infections.

Understanding the structure and function of C3 convertase is crucial for developing targeted therapies for complement-related diseases. Researchers are actively studying the regulation and manipulation of C3 convertase to identify potential therapeutic targets.

See Also

• Complement system
• Immune response
• Opsonization
• Membrane attack complex