= 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.<Characterization and sequencing of the active site of 1-aminocyclopropane-1-carboxylate synthase by W. K. Yip and others>[Abstract: PLP dependence and biosynthetic role]{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.<Reversible inhibition of tomato fruit senescence by antisense RNA by P. W. Oeller and others>[Abstract: antisense intervention and external-gas reversal]{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.<Characterization and sequencing of the active site of 1-aminocyclopropane-1-carboxylate synthase by W. K. Yip and others>[Abstract: inactivation by S-adenosylmethionine]{ref}
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