1-aminocyclopropane-1-carboxylate synthase

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]