= Androstenedione
<Androstenedione> is a 19-carbon <steroid> with <ketone> groups at carbon positions 3 and 17 and a double bond between positions 4 and 5. The structural name androst-4-ene-3,17-dione encodes these features. Its <molecular formula> is $\mathrm{C_{19}H_{26}O_2}$.<Androstenedione in ChEBI>[CHEBI:16422, Definition, Formula and names]{ref}
<Androstenedione> is also a substrate in <steroid hormone> synthesis. <Aromatase> converts it to <estrone>, with <formate> among the products. The enzyme record lists a separate reaction converting <testosterone> to <estradiol>; the two starting steroids and their products should not be conflated.<Aromatase in IUBMB Enzyme Nomenclature>[EC 1.14.14.14, reactions 1 and 2]{ref}
Carbon accounting gives a useful check on the first transformation: the substrate has 19 carbon atoms, whereas <estrone> has 18, and the remaining carbon appears in <formate>. That accounting describes only the carbon balance; the complete reaction also requires oxygen and reducing equivalents. Thus conversion to an estrogen involves a chemical rearrangement and oxidation process, rather than simply attaching a different biological label to the same steroid structure.<Aromatase in IUBMB Enzyme Nomenclature>[reaction 2 and Comments]{ref}
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