Complement component 1q

Human protein involved in the complement system

Structure of Complement Component 1q

Complement component 1q (C1q) is a protein complex involved in the complement system, which is part of the immune system. C1q is the first subcomponent of the C1 complex, which is the first component of the classical complement pathway. This pathway is one of the three pathways that activate the complement system, leading to a cascade of immune responses that help clear pathogens from an organism.

Structure

C1q is a large protein complex composed of 18 polypeptide chains, which form six heterotrimeric subunits. Each subunit consists of three different polypeptide chains: A, B, and C. These chains are arranged in a collagen-like triple helix structure. The C-terminal globular heads of C1q are responsible for binding to the Fc region of immunoglobulins, such as IgG and IgM, when they are bound to antigens.

Function

C1q plays a crucial role in the activation of the classical complement pathway. Upon binding to the Fc region of antibodies that are attached to antigens, C1q undergoes a conformational change that activates the C1r and C1s serine proteases. This activation leads to the cleavage of complement component 4 (C4) and complement component 2 (C2), forming the C4b2a complex, also known as the C3 convertase. The C3 convertase then cleaves complement component 3 (C3), leading to the opsonization of pathogens, recruitment of inflammatory cells, and eventual formation of the membrane attack complex.

Role in Disease

Deficiencies in C1q can lead to increased susceptibility to infections and are associated with autoimmune diseases such as systemic lupus erythematosus (SLE). C1q deficiency is a rare genetic disorder that results in impaired clearance of immune complexes and apoptotic cells, contributing to the development of autoimmunity.

Complement Pathway

Complement Pathway Diagram

The complement system consists of three pathways: the classical pathway, the lectin pathway, and the alternative pathway. C1q is specifically involved in the classical pathway, which is primarily activated by antigen-antibody complexes. The complement system plays a vital role in innate immunity, inflammation, and the clearance of immune complexes and apoptotic cells.

Related pages

• Complement system
• Classical complement pathway
• Immunoglobulin
• Systemic lupus erythematosus