= 1-aminocyclopropane-1-carboxylic acid
<1-aminocyclopropane-1-carboxylic acid> is a non-proteinogenic <alpha-amino acid> with an amino group and a <carboxyl group> attached to the same carbon of a <cyclopropane> ring. Its molecular formula is $\mathrm{C_4H_7NO_2}$, and it is commonly abbreviated ACC. The named acid and its conjugate base, <1-aminocyclopropane-1-carboxylate>, distinguish protonation states of the same carbon framework.<1-aminocyclopropane-1-carboxylic acid in ChEBI>[CHEBI:18053, Definition, Formula and conjugate-base relationship]{ref}
Its role in plant <ethene> biosynthesis was established through precursor experiments. In 1979, labelled <methionine> supplied to apple tissue produced ethylene in air but accumulated as ACC under nitrogen. Labelled ACC was itself converted into ethylene in air; further precursor and inhibitor comparisons supported its placement between <S-adenosylmethionine> and ethylene.<Ethylene biosynthesis: Identification of 1-aminocyclopropane-1-carboxylic acid as an intermediate in the conversion of methionine to ethylene by D. O. Adams and S. F. Yang>[Abstract, atmosphere-dependent accumulation and precursor tests]{ref}
The informative observation was not merely that ACC was present. Accumulation under one condition, conversion under another and differential effects on upstream and downstream precursors jointly constrained the pathway. A metabolite's concentration alone cannot establish which reaction produces or consumes it, because both processes affect its measured pool.
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