
The thick arrows represent reactions that are catalyzed by enzymes, whereas the thin arrows represent reactions that are present, but with no evidence of the corresponding enzymes. When determining chokepoint reactions, we only consider the catalyzed reaction. (1) A chokepoint, because it produces a unique product; (2) a chokepoint, because it consumes a unique substrate; (3) a chokepoint, because it consumes a unique substrate and produces a unique product.











