Complement

Fragments.

Appended lowercase letters indicate complement fragments. Usually, a lowercase b indicates the larger, active (membrane-binding) fragment and a lowercase a, the smaller, release fragment (released on cleavage of the parent molecule). However, C2 is inconsistent: C2a is the larger active fragment and C2b the smaller release fragment. Other letters represent fragments of b fragments.

Subunits.

The subunits of C1 are as follows:

Various notations combining the C1 subunits convey the stoichiometry (relative quantities of subunits) of the complex; all such styles are acceptable:

(C1r)₂

C1r₂C1s₂

C1qC1r₂C1s₂

C1qr₂s₂

C1s-C1r-C1r-C1s

Isotypes of C4 have capital letters appended:

Protein chains have Greek letters appended:

C8α

C8β

C8γ

C3α is the α chain of C3.

Cleavage of C3α produces C3a and C3b.

An i signifies inactive forms:

iC3 or C3i

iC3b

Complement components that form a complex are written in a series without spaces:

C4b2a3b

C4bC2

Sometimes a hyphen is used to indicate a series:

C5b67 or C5b-7

C5-9

An asterisk shows nascent or metastable state:

C4b*

C3b*

C5b*

C5b-7*

Convertase complexes are linked complement fragments that activate other complement components. For example, the convertase that activates C3 is known as C3 convertase. As in the following examples, the convertases have different compositions, depending on which complement pathway generated them:

(Note: Occasionally, authors have changed the designation of the activated moiety of C2 from C2a to C2b, to be consistent with other complement components.^18,19(p8) A tipoff to the change is the designation of classical pathway C3 convertase as C4b2b.)

A bar over the suffix was proposed in 1968 to designate activated complement, eg: $$\begin{array}{ccc}\text{C}\overline{\text{4}}& \text{C}\stackrel{¯¯¯}{\text{423}}& \text{C}\stackrel{¯¯¯¯}{\text{4b2a}}\end{array}$$ but this convention has fallen from use.

Complement Regulators.

Complement regulators include the following:

(*Not the same as protein S; see 15.7.4, Hemostasis, Inhibition of Coagulation and Fibrinolysis.)

Complement Receptors.

Complement receptors include the following: