Lecture 5: Complement Flashcards
What is complement?
A series of over 20 proteins circulating in the blood and tissue fluids
What causes complement proteins to become activated?
Become sequentially activated in an enzyme cascade in response to the recognition of molecular components
What is the function of complement?
- opsonise or cause direct killing of the target
- signalling to promote inflammation or chemotaxis
What are the 3 different complement pathways?
Classical, lectin, and alternative
What is a zymogen?
An inactive substance which is converted into an enzyme when activated by another enzyme
What is the specific initiating steps in the lectin pathway?
Mannose binding lectin (MBL) and ficolins recognise and bind carbohydrates on the pathogen surface
What is the specific initiating steps in the classical pathway?
C1q interacts with the pathogen surface or with antibodies bound to the surface
What is the specific initiating steps in the alternative pathway?
C3 undergoes spontaneous hydrolysis to C3(H2O) to initiate eventual deposition of C3 convertase on microbial surfaces
What is similar about all pathways?
All pathways produce a C3 convertase, which cleaves C3. C3b is left bound to microbial surface, while C3a is released
What are the 3 outcomes from complement pathways?
Inflammation, phagocytosis, and membrane disruption
How does complement induce inflammation?
C3a and C5a recruit phagocytic cells to the infection site and promote inflammation
How does complement induce phagocytosis?
Phagocytes with receptors for C3b engulf and destroy the pathogen
How does complement induce membrane disruption?
All pathogens generate a C5 convertase that leads to formation of the membrane attack complex (MAC), which disrupts the cell membrane
What is TED?
A thioester domain found in the alpha chain of C3
How is TED involved and affected by C3 cleavage?
- before cleavage, the thioester bond within TED is protected from reacting
- cleavage releases C3a, and a conformational change of C3b allows the thioester bond to react with a chemical group on the pathogen surface
- can be inactivated by hydrolysis
What are the different C3 convertases in each pathway?
Lectin & classical : C4b2a
Alternative : C3bBb
What are the different C5 convertases in each pathway?
Lectin & classical : C4b2a3b
Alternative : C3b2Bb
What is the function of ficolins and MBL?
Recognise differences in oligosaccharides between different species
Eg. Yeast: terminal mannose residues, vertebrates: terminal sialic acid residues
What does MBL do?
MBL monomers form trimeric clusters of carbohydrate recognition domains, which bind with high avidity to mannose and ficolin residues
What does ficolin do?
Similar structures to MBL but have different carbohydrate binding domains (bind to oligosaccharides containing acetylation sugars)
What is the process of C3 convertase?
- activated MASP-2 (MBL or ficolin associated) cleaves C4, where C4b binds to the microbial surface
- C4b bind C2, which is cleaved by MASP-2
- C2a and C4b bind to form the C4b2a complex
- C4b2a is an active C3 convertase which cleaves C3, where C3b binds to either the microbial surface or the convertase itself
How are IgG involved in the classical pathway?
- Multiple IgG are needed for activation, as they bind to C1q
- Movement of Fc domain opens up C1q binding sites
- Number and position of IgG determines opening and strength of the pathway
Why are structural changes of C1q needed for the classical pathway?
- Structural changes from IgG are needed for activation of C1r and C1a
- C1q will rotate inwards which leads to opening of the CLR stalk
- C1rC1s will pass from an inactive 8 to an active S shape confirmation
What does the enzyme C1s do?
Cleaves C4 to C4a and C4b, as well as C2
What is the dominant complement pathway in plasma during normal haematopoiesis?
Alternative pathway
What is the concept of tick over?
There is constant low level activation of the alternative pathway, through spontaneous hydrolysis of the TED domain which exposes binding site of TED domain with help of cofactor B
What control measures are involved with tick over of the alternative pathway?
- half life of C3b is 60 seconds
- must bind to a surface in 60nm
- rapidly inactivated on host cells
How does factor B help make for C3 convertase?
- C3b is deposited by classical or lectin pathway C3 convertase
- C3b binds factor B, while MASP-3 cleaves pro-factor D
- bound factor B is cleaved by plasma protease factor D, resulting in Ba and Bb
- C3bBb complex is a C3 convertase, cleaving many C3 molecules to C3a and C3b
How does C3 hydrolyse in the plasma to give C3(H20)?
- C3 undergoes spontaneous hydrolysis to C3(H20), which binds to factor B allowing it to be cleaved into factor D
- The C3(H20)Bb complex is a C3 convertase, cleaving more C3 into C3a and C3b
C3b is rapidly inactivated unless it binds to a cell surface - Factor B binds noncovelantly to C3b on a cell surface and is cleaved to Bb by factor D
What is properdin?
A glycoprotein in the plasma that is mainly produced by leukocytes and can positively regulate AP activity by stabilising C3
How does properdin stabilise the alternative pathway?
- pathogens lack complementary regulatory proteins
- binding of properdin stabilises C3bBb complex
- C3bBb complex cleaves C3 and deposits many C3b molecules on the pathogen surface
- results in opsonisation
