The biosynthesis of estrogen is initiated by the synthesis of
Androstenedione, 19-carbon steroid hormone, from
cholesterol. Then this compound is converted to estrogens
Estrone or Estradiol, either
immediately or through Testosterone, which may also be
derived from cholesterol.
Reduction of Androstenedione to
Testosterone requires Hydroxysteroid (17-beta) dehydrogenase
2 (HSD17B2) ,  and
Hydroxysteroid (17-beta) dehydrogenase 3
(HSD17B3) , , . The reverse reaction, oxidation of
Testosterone at 17-position to form
Androstenedione, is catalyzed by monooxygenase Cytochrome
P450, family 2, subfamily C, polypeptide 19
Androstenedione undergoes a three-step A-ring
aromatization to Estrone catalyzed by monooxyganases:
aromatase Cytochrome P450, family 19, subfamily A, polypeptide 1
(CYP19)  and Cytochrome P450, family 11,
subfamily B, polypeptide 1 (CYP11B1) . The
first intermediate reaction is the formation of
19-Hydroxyandrostenedione , which then is
converted to Androst-4-en-3,17,19-trione ,  followed by oxidation to Estrone , .
Another pathway of Estrone biosynthesis is oxidation of
17-alpha-Estradiol by Estradiol 17a-dehydrogenase
Aromatase CYP19 also catalyzes oxidation of
Testosterone to Estradiol
, . The first step is formation of
19-Hydroxytestosterone , which in turn is
oxidized to 19-Oxotestosterone , and then to
Estradiol , , similar to
Androstenedione catabolism to
Another steroid substrate, which undergoes a
CYP19-catalyzed three-step aromatization, is
16alpha-Hydroxyandrostenedione , . The final product of that oxidation is
Interconversion of Estradiol and
Estrone requires Hydroxysteroid (17-beta) dehydrogenases 1
(HSD17B1) , 7
(HSD17B7)  and 8
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