Oxidation reactions catalyzed by horseradish peroxidase (HRP) involve the conversion of ferric heme to the ferryl state by hydrogen peroxide (H₂O₂). As a result, HRP is typically considered unreactive without the presence of H₂O₂. Herein, we report the first use of histidine (His) and HRP in the presence of daylight and oxygen to initiate photocatalytic oxidation reactions without the addition of H₂O₂. We demonstrate that the oxidation reactions of norbixin (NBX) molecules proceed 2-times faster using this HRP/His system in comparison with HRP/H₂O₂. Most importantly, we also demonstrate that new species of NBX oxidized products are formed using HRP/His. In this work, we investigate the key steps of the HRP mechanism in the presence of His using electron paramagnetic resonance spectroscopy, liquid chromatography-mass spectrometry, and ultraviolet-visible spectrophotometry. Bearing in mind that H₂O₂ is normally thought indispensable for initiating the oxidation reactions of HRP, this work suggests a potential paradigm shift in our understanding of possible HRP mechanisms in the catalysis of biochemical reactions. The use of His, a natural essential amino acid, to mediate the formation of ferryl heme intermediates suggests the HRP/His system could serve as a natural catalyst for applications in photochemistry and bioelectrocatalysis.