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1-aminocyclopropane-1-carboxylate synthase is a pyridoxal phosphate-dependent enzyme that converts S-adenosylmethionine into 1-aminocyclopropane-1-carboxylate, commonly abbreviated ACC, with methylthioadenosine as the other principal product. ACC is a precursor in the plant pathway producing the gaseous signal ethene, usually called ethylene in plant physiology. The synthase makes the precursor; a subsequent enzyme catalyzes the ethylene-forming step.^[ref]

The precursor/enzyme distinction helps interpret a perturbation. Reducing synthase activity can reduce pathway output even if the downstream machinery remains able to respond. In a 1991 tomato experiment, expression of Antisense RNA against the synthase inhibited fruit ripening. Applying ethylene or propylene reversed the effect, supporting a role for the pathway's signal in triggering ripening rather than merely accompanying it.^[ref]

For an original pathway example, write substrate $\to$ ACC $\to$ signal. Blocking the first arrow and restoring the response by supplying the final signal places the block upstream of that response. It does not by itself prove that every intermediate remains unchanged or that this enzyme controls all developmental responses.

The enzyme's name also does not imply an unlimited constant catalytic rate. Biochemical work showed substrate-dependent inactivation, so reaction progress and the amount of active enzyme can change together during an assay.^[ref]

Methylthioadenosine is a sulfur-containing nucleoside, commonly called 5′-methylthioadenosine. Relative to adenosine, its terminal 5′ hydroxyl group is replaced by $-\mathrm{S}-\mathrm{C}{\mathrm{H}}_3$, a thioether group. Its formula is ${\mathrm{C}}_{11}{\mathrm{H}}_{15}{\mathrm{N}}_5{\mathrm{O}}_3\mathrm{S}$.^[ref]

In plant Ethylene biosynthesis, cleavage of S-adenosylmethionine produces 1-aminocyclopropane-1-carboxylate and methylthioadenosine. The fragments follow different subsequent reactions: the cyclic intermediate supplies Ethylene, whereas the sulfur-containing fragment can be recycled. In 1979, feeding differently labelled methionine to apple tissue connected the sulfur/methyl-labelled portion to a sulfur-containing sugar product, helping distinguish the two routes.^[ref]

S-adenosylmethionine is an activated methionine derivative in which an adenosyl group is bonded to the sulfur atom. The positively charged sulfur bears a transferable methyl group; S-adenosylmethionine therefore couples cellular metabolism to transfer of methyl groups onto other molecules, including DNA and proteins.^[ref]

The same starting molecule can also enter a different reaction. 1-aminocyclopropane-1-carboxylate synthase converts S-adenosylmethionine into 1-aminocyclopropane-1-carboxylate and methylthioadenosine, on the route to plant Ethylene. Here the enzyme directs fragmentation and ring formation rather than simple methyl group transfer. Naming a common substrate does not by itself identify which reaction occurred.^[ref]