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1-aminocyclopropane-1-carboxylate is the carboxyl-deprotonated form of 1-aminocyclopropane-1-carboxylic acid: a three-carbon ring bears an amino group and a carboxylate group on the same ring carbon. Its specified anionic form has formula ${\mathrm{C}}_4{\mathrm{H}}_6\mathrm{N}{\mathrm{O}}_2^{-}$. The familiar biochemical abbreviation ACC is also often used for the acid without specifying protonation.^[ref]

It is an intermediate between S-adenosylmethionine and Ethylene in plants. The 1979 apple experiments showed that labelled ACC became Ethylene in air and bypassed an inhibitor that blocked conversion from methionine. A supplied downstream intermediate restoring product formation helps locate the blocked reaction upstream.^[ref]

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 ${\mathrm{C}}_4{\mathrm{H}}_7\mathrm{N}{\mathrm{O}}_2$, 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.^[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.^[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.