Reduction of the 17,18‐double bond in the D‐ring during chlorophyll biosynthesis is catalyzed by the rare, naturally occurring photoenzyme protochlorophyllide oxidoreductase (POR). A conserved tyrosine residue has been suggested to donate a proton to C18 of the substrate in the past decades. Taylor and colleagues scrutinized the model with a powerful tool that utilized a modified genetic code to introduce fluorinated tyrosine analogues into POR. The presented results show that the suggested catalytically critical tyrosine is unlikely to participate in the reaction chemistry but is required for substrate binding, and instead, a cysteine residue preceding the lid helix is proposed to have the role of proton donor. Reduction of the 17,18‐double bond in the D‐ring during chlorophyll biosynthesis is catalyzed by the rare, naturally occurring photoenzyme protochlorophyllide oxidoreductase. Previously, a conserved tyrosine residue had been suggested to donate a proton to C18 of the substrate. Aoife Taylor and colleagues show that the suggested tyrosine is unlikely to participate in the reaction chemistry but is required for substrate binding, and instead, a cysteine residue preceding the lid helix is proposed to be the proton donor.