Molecular modelling of cytochrome CYP1A1: a putative access channel explains differences in induction potency between the isomers benzo( a)pyrene and benzo( e)pyrene, and 2- and 4-acetylaminofluorene

The present studies were undertaken to provide a rationale for the observation that benzo( a)pyrene and 2-acetylaminofluorene induce the hepatic CYP1 A1 protein, whereas their non-carcinogenic isomers benzo( e)pyrene and 4-acetylaminofluorene are, at best, relatively very weak inducers. Using amino acid sequence alignment, a molecular model of the CYP1A1 was constructed by analogy to CYP101, the bacterial protein for which the 3-dimensional structure is known from X-ray crystallographic analysis. The putative structure of the active site of the CYP1A1 protein shows the presence of two phenylalanine residues preferentially aligned in parallel orientation, presumably functioning as a ‘sieve’ for planar molecules, the established substrates of CYP1A1. The molecular dimensions of this putative access channel show a width and depth of 8.321 and 3.261 Å, respectively. The width of 4-acetylaminofluorene, 8.794 Å, and benzo( e)pyrene, 9.153 Å, precludes their passage through this channel access in contrast to benzo( a)pyrene and 2-acetylaminofluorene having a width of 7.150 and 5.283 Å, respectively, explaining their difference in CYP1A1 induction potential.