Complement Proteins

Front 1

C3

Back 1

Complement Component 3
Made, enters the liver in an inactive form with the thioester bond on the inside of the molecule; cleavage into C3a and C3b allows the thioester bond to become susceptible to chemical attack

Front 2

C3a

Back 2

Soluble portion of the C3 cleavage; acts as a chemoattractant to promote the localization of other immune cells

Front 3

C3b

Back 3

Reactive portion of the C3 cleavage; attaches to the pathogen surface, promoting phagocytosis by immune cells

Front 4

iC3

Back 4

C3 protein that has been hydrolyzed in the alternative pathway; occurs at low rates in blood plasma, but the microenvironment of certain pathogens enhances the process

Front 5

Factor B

Back 5

Inactive complement factor that binds to iC3 and pathogen-bound C3b; contains a binding domain for Factor D to cleave it into Ba and Bb

Front 6

Factor D

Back 6

Protease that cleaves Factor B into Ba and Bb by binding to the iC3B complex

Front 7

Ba

Back 7

Small fragment produced from the cleavage of Factor B; released into the environment

Front 8

Bb

Back 8

Large fragment produced from the cleavage of Factor B; remains attached to iC3

Front 9

iC3Bb

Back 9

C3 convertase that attaches to and cleaves C3 molecules into C3a and C3b; Generally leads to the deposition of C3b molecules on pathogen surface

Front 10

C3bBb

Back 10

Complex of a C3b molecule on a pathogen's surface and the Bb fragment; acts as an alternative C3 convertase by binding to C3 and cleaving it to C3b and C3a

Front 11

Complement Control Proteins

Back 11

Proteins that regulate the alternative C3 pathway by either stabilizing or degrading the C3b at cell surfaces

Front 12

Properdin (Factor P)

Back 12

Binds to the C3bBb on microbe surfaces and prevents its degradation by proteases

Front 13

Factor H

Back 13

Binds to C3b and facilitates its degradation

Front 14

Factor I

Back 14

Serine protease that binds to the Factor H protein and cleaves the C3b molecule to iC3b, which makes it inactive

Front 15

Decay-Accelerating Factor (DAF)

Back 15

Binds to the C3b component of the alternative C3 convertase and causes its dissociation and inactivation

Front 16

Membrane Co-factor Protein (MCP)

Back 16

Binds to C3b and makes it susceptible to cleavage and inactivation by Factor I

Front 17

Complement Receptor 1 (CR1)

Back 17

Receptor on macrophages that bind to a cluster of C3b molecules on a pathogen's surface, facilitating the engulfment and destruction of the pathogen

Front 18

Complement Receptor 3 (CR3)

Back 18

Binds to the iC3b fragment on microbial surfaces, facilitating the engulfment and destruction of the pathogen

Front 19

Complement Receptor 4 (CR4)

Back 19

Binds to the iC3b fragment on microbial surfaces, facilitating the engulfment and destruction of the pathogen

Front 20

C5

Back 20

Complement component 5
Cleaved by the alternative C5 convertase into C5a and C5b

Front 21

C5b

Back 21

Large unit created by the cleavage of the C5 protein; initiates the formation of a membrane-attack complex that makes holes in the membranes of pathogens and some cells

Front 22

C6

Back 22

Binds to and stabilizes C5b; forms a binding site for C7

Front 23

C7

Back 23

Binds to C5b6 and exposes a hydrophobic region that permits attachment to the cell membrane

Front 24

C8

Back 24

Binds to C5b67 and exposes a hydrophobic regions that inserts directly into the cell membrane

Front 25

C9

Back 25

Polymerization on the C5b678 complex to form a membrane-spanning channel that disrupts the cell's integrity, which can lead to cell death

Front 26

CD59

Back 26

Prevents recruitment of C9 by the C5b678 complex

Front 27

C5a

Back 27

Small fragment produced from the cleavage of the C5 protein; promotes inflammation by targeting receptors on mast cells, macrophages, and endothelial cells

Front 28

Protease Inhibitors

Back 28

Proteins that work to trap and inhibit proteases from functioning properly

Front 29

MBL-Associated Serine Protease (MASP) 1 and 2

Back 29

Serine protease zymogens that bind to the main stalk of mannose-binding lectin (2 each); Upon binding to a pathogen surface that contains mannose, one molecule of MASP-2 cleaves itself and MASP-1, which then goes on to cleave C2 and C4

Front 30

C2

Back 30

Serine protease zymogen that is similar in structure/function to Factor B; is cleaved into C2a (large, active fragment) and C2b (small, soluble fragment)

Front 31

C4

Back 31

Similar in structure/function to C3; is cleaved by MASP-1 into C4a (small, soluble fragment) and C4b (large, pathogen-binding fragment); Contains a high-energy thioester bond

Front 32

C4a

Back 32

Similar to C3a; small, soluble fragment produced from the cleavage of C4; acts as a chemoattractant and anaphylotoxin that recruits leukocytes to the site of C4b fixation

Front 33

C4b

Back 33

Similar to C3b; large, active fragment produced from the cleavage of C3 that binds to the surface of a pathogen

Front 34

C4bC2a

Back 34

Classical C3 convertase; binds to and cleaves C3 into C3a and C3b fragments

Front 35

C1

Back 35

Comprised of C1q (6 subunits that acts as binding sites for the Fc regions of IgM or IgG), C1r (protease), and C1s (protease)

Front 36

C1q

Back 36

Binding sites for the Fc regions of IgM or IgG; binding site for C-Reactive Protein

Front 37

C1s

Back 37

Protease in the C1 complex; two molecules are attached to the molecules of C1q and C1r

Front 38

C1r

Back 38

Protease in the C1 complex; two molecules are attached to the molecules of C1q and C1s; C1r cleaves itself, the other C1r, and the two C1s molecules upon activation; cleaves C4 and C2