They bind to a particular target protein, mainly to an enzyme, thereby directly changing its activity, i.e. increasing or decreasing. An example is salicylic acid, which is a modulator of catalase isozymes activity and jasmonate, which modulates phenylalanine ammonia lyase activity.

Sources: doi:10.1039/C2NP20053H,

doi: 10.1016/j.plaphy.2011.12.005

Thank you so much Diego Batista