Evidence for a ferryl intermediate in a heme-based dioxygenase

Area of Science:

• Biochemistry
• Enzymology
• Structural Biology

Background:

• Cytochrome P450 monooxygenases are a broad class of enzymes, but humans possess only two heme-based dioxygenases: tryptophan dioxygenase (hTDO) and indoleamine 2,3-dioxygenase (hIDO).
• Both hTDO and hIDO catalyze the initial, rate-limiting step of the kynurenine pathway by oxidizing L-tryptophan to N-formyl kynurenine.
• The precise reaction mechanisms of these heme-based dioxygenases remain incompletely understood despite significant research interest.

Purpose of the Study:

• To elucidate the reaction mechanism of human indoleamine 2,3-dioxygenase (hIDO) and tryptophan dioxygenase (hTDO).
• To investigate the potential involvement of reactive intermediates during the dioxygenase reaction catalyzed by hIDO and hTDO.
• To compare the mechanistic and structural differences between hIDO and hTDO.

Main Methods:

• Experimental investigation of the reaction mechanism of human indoleamine 2,3-dioxygenase (hIDO).
• Characterization of reaction intermediates using spectroscopic or kinetic techniques (details not specified in abstract).
• Comparative analysis of reaction intermediates between hIDO and hTDO.

Main Results:

• Experimental evidence for a key ferryl intermediate during the hIDO-catalyzed dioxygenase reaction was obtained.
• This ferryl intermediate supports a sequential, two-step mechanism involving the insertion of both dioxygen atoms into the substrate.
• The ferryl intermediate was not observed during the hTDO reaction, indicating distinct mechanisms and highlighting structural differences between hIDO and hTDO.

Conclusions:

• The findings propose a paradigm shift in understanding heme-based dioxygenase chemistry, moving from a presumed simultaneous dioxygen incorporation to a consecutive two-step mechanism for hIDO.
• The absence of the ferryl intermediate in hTDO suggests significant stereoelectronic differences between the two human dioxygenases.
• These differences are crucial for modulating the enzymatic reactions and underscore the complexity of heme-based oxygen activation.