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Structural basis for androgen specificity and oestrogen synthesis in human aromatase

Abstract

Aromatase cytochrome P450 is the only enzyme in vertebrates known to catalyse the biosynthesis of all oestrogens from androgens1,2,3. Aromatase inhibitors therefore constitute a frontline therapy for oestrogen-dependent breast cancer3,4. In a three-step process, each step requiring 1 mol of O2, 1 mol of NADPH, and coupling with its redox partner cytochrome P450 reductase, aromatase converts androstenedione, testosterone and 16α-hydroxytestosterone to oestrone, 17β-oestradiol and 17β,16α-oestriol, respectively1,2,3. The first two steps are C19-methyl hydroxylation steps, and the third involves the aromatization of the steroid A-ring, unique to aromatase. Whereas most P450s are not highly substrate selective, it is the hallmark androgenic specificity that sets aromatase apart. The structure of this enzyme of the endoplasmic reticulum membrane has remained unknown for decades, hindering elucidation of the biochemical mechanism. Here we present the crystal structure of human placental aromatase, the only natural mammalian, full-length P450 and P450 in hormone biosynthetic pathways to be crystallized so far. Unlike the active sites of many microsomal P450s that metabolize drugs and xenobiotics, aromatase has an androgen-specific cleft that binds the androstenedione molecule snugly. Hydrophobic and polar residues exquisitely complement the steroid backbone. The locations of catalytically important residues shed light on the reaction mechanism. The relative juxtaposition of the hydrophobic amino-terminal region and the opening to the catalytic cleft shows why membrane anchoring is necessary for the lipophilic substrates to gain access to the active site. The molecular basis for the enzyme’s androgenic specificity and unique catalytic mechanism can be used for developing next-generation aromatase inhibitors.

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Figure 1: The structure of aromatase.
Figure 2: Views of the active site of aromatase.
Figure 3: Steroid–protein interactions and mechanistic implications.
Figure 4: A putative active-site access channel from within the lipid bilayer.

Accession codes

Primary accessions

Protein Data Bank

Data deposits

Atomic coordinates and structure factor files have been deposited with the Protein Data Bank under the accession code 3EQM.

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Acknowledgements

We thank Y. Osawa, who pioneered aromatase research and its purification from human placenta at the institute, for many discussions and encouragement; past graduate students, postdoctoral scientists and research associates for contributions to the initial purification and crystallization efforts; H. Davies for discussions; D. Gewirth, V. Cody, G. DeTitta and J. Griffin for critically reading the manuscript; staff at the Women’s and Children’s Hospital of Buffalo for providing the placenta used in this work; and staffs of the Cornell High Energy Synchrotron Source and the Advanced Photon Source, Argonne National Laboratory, for helping with the synchrotron X-ray data collection. The research is supported in part by grants GM62794 and GM59450 (to D.G.) from the National Institutes of Health.

Author Contributions J.G. and M.E. performed the purification and crystallization of aromatase. W.P. and J.G. contributed to diffraction data collection. D.G. was involved in diffraction data collection and processing. D.G. solved the structure, wrote the manuscript and was responsible for overall planning and supervision of the project.

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Correspondence to Debashis Ghosh.

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Ghosh, D., Griswold, J., Erman, M. et al. Structural basis for androgen specificity and oestrogen synthesis in human aromatase. Nature 457, 219–223 (2009). https://doi.org/10.1038/nature07614

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