Tryptophan in horseradish peroxidase.

Fluorescent derivatives of horseradish peroxidase C were prepared by replacing protoheme by protoporphyrin or mesoporphyrin. Calculations according to Förster on energy transfer allowed the determination of the distances of greater than 2.2 nm between tryptophan and porphyrin (heme) and greater than 2 nm between tryptophan and substrate-binding site. The modification of the single tryptophan with 2-hydroxy-5-nitrobenzyl bromide (Koshland's reagent) did not affect the enzyme's activity towards hydrogen peroxide or ascorbate. Modified and unmodified peroxidase showed the same affinity for aromatic substrates.