Hydrocarbon hormone ethylene

S-Adenosylmethionine plays an additional role in plant metabolism, as the precursor to the hydrocarbon hormone ethylene.

Ethylene promotes plant growth and development and induces the ripening of fruit.

The main carbon skeleton of methionine, rather than the methyl group, is split off in this process, to yield 1-aminocyclopropane-1-carboxylic acid.

This unusual reaction is based on the same chemistry that makes AdoMet such a good methyl donor: the positively charged sulfonium makes all three adjacent carbon
centers strongly electrophilic and easily transferred, including the methythioadenosine moiety, as in this case.

The cyclopropane, representing carbons 1-4 of methionine, is fragmented as shown, to yield ethylene, CO2, and cyanide.