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Competitive and non-competitive inhibitors can affect the reaction rates in a metabolic pathway.

Red line (no inhibitor)

The graph levels off because all of the active sites are occupied with the substrate.

Orange line (competitive inhibitor)

There is a gradual increase in reaction rate because competitive inhibitors are occupying only some of the enzyme active sites. As substrate concentration increases, the substrate molecules outnumber the inhibitor so the reaction rate reaches the maximum.

Green line (non-competitive inhibitor)

Most enzyme molecules have become inactive but some are unaffected by the inhibitors so reaction rate remains low. An increase in substrate concentration does not increase reaction rate.