Aromatase, the 503-amino acid long protein product of the CYP19A1 gene on chromosome 15q21.1, is an enzyme that removes the C-19 methyl group via two oxidation steps and oxidizes the 3- and 17-positions of androgenic steroids to convert them into estrogenic steroids (estrogen). It oonverts androstenedione, testosterone and 16-hydroxytestosterone into estrogen, 17-estradiol and 17-,16--estriol, respectively., ,  Because some tumors are estrogen-sensitive, aromatase inhibitors are sometimes used to treat cancer, mostly in post-menopausal women because aromatase inhibitors do not block estrogen synthesis in the ovaries.
Structure and function
Aromatase is the only enzyme in vertibrates that is capable of synthesizing all of the estrogen steroids from androgens, and has therefore been of great interest as a treatment for estrogen-sensitive tumors. Despite decades of research into aromatase, its structure was only determined in 2009. The structure of human placental aromatase cytochrome P450 bound to ligands androstenedione (cyan) and heme (yellow) was determined by x-ray crystallography.
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