Merck, C1740-1MG, Complement component C1q from human serum

C1q, together with C1r and C1s, in the ratio of 1:2:2, form the C1 complex which is the first component of the classical complement pathway. C1q is composed of 18 polypeptide chains (six A, six B, six C) (MW 460 kDa). All contain an 81-aa collagen-like region composed of (Gly-Xaa-Yaa) repeating sequences close to the N-terminus. Three chains (A₁B₁C₁) form a triple helix with the C-terminus forming the globular heads which may be structurally and functionally distinct domains.