Complement Flashcards
what is complement?
A very complex system of about 50 different proteins found either in serum
or as receptors on white cells
what does complement mediate?
wide range of functions
what does complement involve?
enzymes cascade
Describe enzyme cascades.
- all components circulate as inactive precursors
- comes into contact with appropriate stimulus
- one product of one reaction acts as an enzyme for next reaction and so on
what can the compliment system be triggered by?
3 pathways:
- Classical pathway
- Lectin pathway
- alternative pathway
What do the 3 pathways have in common?
result in proteolytic cleavage
of C3 to C3b (large fragment) and C3a (small)
what is compliment effective at doing?
Getting rid of bacterial infection
what is the central protein of the complement system?
C3
what does deficient in C3 lead to?
recurrent bacterial infections
where is C3b deposited to in all 3 pathways?
surface of micro-organism and marks it out for destruction (lysis/ opsonisation)
How does C3b attach to the microorganisms surface?
- C3 comes into close proximity to the membrane of the pathogen
-thioester bond between Cys and Glu on alpha chain is cleaved
-Thioester bond becomes exposed and unstable
Nucleophilic attack by electrons on OH and -NH2 groups
-covalent bond is formed
what is the first component of the classical pathway?
C1q
Describe the structure of C1q.
-hexavalent
-“Bunch of tulips” arrangement of 18 polypeptides
-6 collagenous base and arm(stalk) regions
connected to 6 globular heads
what is the classical pathway activated by?
IgG and IgM
How does IgG activate the classical pathway?
- 2 IgG molecules are adjacent and in close proximity (control measure to prevent innpaorpriate activation)
- C1q binds 2 of the 6 globular head regions to the IgG molecules
what is different about IgM?
only one molecule but C1q only binds when it goes through a conformational change
what binds to C1q (CP)?
C1r and C1s (2 each)
what do C1r and C1s become (CP)?
proteolytic enzymes