Structure of an activity suppressing Fab fragment to cytochrome P450 aromatase: insights into the antibody-antigen interactions

The crystal structure of a Fab fragment (Fab3-2C2) of a monoclonal antibody raised against aromatase cytochrome P450 (P450(arom)) has been determined at 3.0 Angstrom resolution. P450(arom) is a membrane bound enzyme responsib le for the catalysis of androgens to estrogens, the process of aromatizatio n, and hence has been implicated in hormone-dependent breast cancer. The Fa b fragment of MAb3-2C2 IgG suppresses P450(arom) activity in a dose depende nt manner. The Fab3-2C2 molecule crystallizes in the space group P2(1)2(1)2 (1) With a unit cell of a = 154.89 Angstrom, b = 73.51 Angstrom, and c = 36 .90 Angstrom. The crystal structure consists of a light and a heavy chain i n the asymmetric unit, each characterized by the greek-key antiparallel bet a barrel folding seen in all Fab structures. The average elbow angle betwee n the two domains is 143 degrees. Modeling of the interactions between the variable domains of the antibody and a known model of P450(arom) maps the e pitope to a region of the enzyme that is consistent with the available bioc hemical data and the activity-suppressing function of the antibody. The epi tope mapping result is further supported by the inability of MAb3-2C2 IgG t o suppress the activity of, or to interact with placental porcine P450(arom ), which is 81% identical (86% similar) to human P450(arom) but has a few k ey substitutions in the putative epitope region. (C) 1999 Elsevier Science Ltd. All rights reserved.