F I G U R E 3 5 -2 1
(Also see color figure.) Structures for the immunosuppressant, FK506 (tacrolimus). Tacrolimus is a macrolide isolated
S trep to m yces tsu ku b a en sis.
Shown from left to right in approximately the same orientation are the simple line
structure, a stick- structure, and a space-filling structure. The stick and space-filling structures are based on the
3-dimensional structure of the molecule bound to the immunophilin FKBP. Protein data bank designation 1FKF.
T A B L E 3 5 - 3
Proteins o f the Complement System: Alternative, Classical, Lectin, and Membrane Attack Pathways and Complement
Initiated by attack of nucleophilic cell surface molecules on thioester in C3
and/or spontaneous hydrolysis by H20 of thioester in C3
Description and Role of the
Active Forms in These Pathways
Cleaves B to Ba and Bb
Contains serine protease active site of C3bBb
Active site-containing chain of C3bBb, the C5
Stabilizer of C3bBb
C3 Convertase (C3b,Bb)
C3 (see classical pathway also)
Alternative pathway C3 convertase
Precursor to C3a and C3b
Anaphylatoxin, increases vascular permeability and
causes mast cell degranulation
C3b contains a thioester bond that can attach C3b
covalently to target cells. Forms complex with
factor B, allowing D to cleave B. C3bBb forms
complex with C5, allowing C3bBb to cleave C5.
C3bBb stabilized by binding P to form C3BbP.
Fragments that interact with complement receptor, CR2
Involved in leukocytosis