Studies on the role of hematin in the catalytic mechanism of tryptophan pyrrolase

Abstract

1. 1. The metalloporphyrin nature of Pseudomonas tryptophan pyrrolase and its catalytic mechanism have been explored. 2. 2. The Pseudomonas enzyme contains a dissociable metalloporphyrin moiety, the displacement of which results in the loss of catalytic activity; addition of hematin, in the absence or presence of exogenous reductant, restores the catalytic activity. Thus tryptophan pyrrolase catalyzes the oxygenation of tryptophan to formylkynurenine with both hematin and heme capable of serving as active coenzyme forms. 3. 3. Prior incubation of tryptophan with the enzyme-hematin complex prevents inhibition by the competitive inhibitor, protoporphyrin IX. Interaction between the substrate, tryptophan, and the enzyme-hematin complex results in an apparent fixation of hematin to the enzyme. 4. 4. Certain aspects of the catalytic mechanism were explored by studying, spectrophotometrically, variations in the oxidation-reduction state of the hematin coenzyme. Anaerobic incubation of the enzyme with tryptophan results in a substrate-dependent reduction of the enzyme-hematin to heme. Admission of oxygen leads to immediate reoxidation of the heme to hematin. Thus hematin may participate actively in the catalytic process undergoing cyclic reduction and oxidation. 5. 5. Several possible reaction mechanisms are discussed in which electron transfer from one substrate, tryptophah, to the other substrate, oxygen, through the hematin coenzyme, leads to the conversion of tryptophan and oxygen to reactive chemical species, thus catalyzing their interaction.