Abstract
The ferric form of L-tryptophan 2,3-dioxygenases from both Pseudomonas acidovorans (ATCC 11299b) and rat liver showed magnetic CD spectra ascribable to a high spin protohemoprotein at neutral pH, whereas the ferric indoleamine 2,3-dioxygenase from rabbit intestine exhibited a spectrum due to a mixture of high and low spin states under comparable conditions. Upon addition of L-tryptophan, the spectra of the former enzymes changed to another type of high spin spectra, while the latter showed a marked increase in the low spin component. From these findings and effects of pH on the spectra, it is suggested that the sixth ligand of ferric L-tryptophan 2,3-dioxygenases is water at a neutral pH and that for the ferric indoleamine 2,3-dioxygenase is a strong field ligand such as an imidazole nitrogen. The mixed spin state observed for the latter enzyme was ascribable to a thermal equilibrium between high and low spin states as judged by low temperature spectroscopy. With the ferrous form, the Soret magnetic CD spectra of these enzymes were all similar, giving those of a typical high spin ferrous protohemoprotein, whereas visible spectra were different from one another, suggesting differences in the electronic structure of the heme and its vicinity. The natural CD spectra of both ferric and ferrous forms of each enzyme showed negative Cotton effects in the Soret region. Their intensities were different from one another, presumably due to some differences in the interaction of the heme with nearby aromatic amino acid residue(s).
Highlights
The ferric form of L-tryptophan 2,3-dioxygenases L-Tryptophan 2,3-dioxygenase and indoleamine 2,3-dioxyfrom both Pseudomonas acidovorans (ATCC11299b) genase are protohemoproteinswhich catalyzethe oxygenative and rat liver showed magnetic CD spectra ascribable ring cleavage of either L-tryptophan or itsanalogues to form to a high spin protohemoprotein at neutral pH, whereas the ferric indoleamine 2,3-dioxygenase from rabbit intestine exhibited a spectrum due to a mixture of high and low spin states under comparable conditions
From these findings and effects of pH on the spectra, it is suggested that the sixth ligand of ferric L-tryptophan 2,3-dioxygenases is waterat a neutral pH and that for the ferric indoleamine 2,3-dioxygenase is a strong field ligand such as an imidazole nitrogen
In order to understand the reaction mechanisms of the heme-containing dioxygenases, we have undertaken magnetic and naturalcircular dichroism studies on thelectronic strucof these enzymes were all similar, giving those of a ture of the heme and its vicinity in these dioxygenases.‘ In typical high spin ferrous protohemoprotein, whereas contrast to otherphysicochemical methods such as EPR and visible spectra were different from one another, sug- NMR, themagneto-optical and optical methods areapplicable gesting differences in the electronic structure of the heme and its vicinity
Summary
Upon addition of L-tryptophan, thespectra of the former enzymes changed to another typeof high spin spectra, while the latter showed a marked increase in the low spin component. From these findings and effects of pH on the spectra, it is suggested that the sixth ligand of ferric L-tryptophan 2,3-dioxygenases is waterat a neutral pH and that for the ferric indoleamine 2,3-dioxygenase is a strong field ligand such as an imidazole nitrogen. Theresults describedin this paper serve as thefist description of MCD2 and CD spectra of theheme-containing dioxygenases andpermittedusto compare theirligand-binding and spinequilibrium properties with those of other types of hemoproteins.MCDandCD spectra of ferric and ferrous low spin forms of these enzymes including an oxygenated form of indoleamine 2,3-dioxygenase.
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