Conformational Plasticity in Human Heme-Based Dioxygenases.

Area of Science:

• Biochemistry
• Structural Biology
• Enzymology

Background:

• Human indoleamine 2,3-dioxygenase 1 (hIDO1) and human tryptophan dioxygenase (hTDO) are heme proteins degrading l-tryptophan (Trp) via the kynurenine pathway.
• Both enzymes share similar active sites and catalytic mechanisms but possess distinct functional properties.

Purpose of the Study:

• To investigate how enzyme structures encode distinct functionalities using carbon monoxide (CO) as a structural probe.
• To compare the structural dynamics of hIDO1 and hTDO under X-ray irradiation.

Main Methods:

• X-ray crystallography at cryogenic temperatures (100 K) with CO and Trp bound to hIDO1 and hTDO.
• Analysis of photochemical intermediates and substrate/ligand binding sites.

Main Results:

• An unexpected photochemical intermediate was observed in hIDO1-CO-Trp, where X-ray irradiation photolyzed CO, causing Trp and CO to migrate to a temporary binding site (Sa*).
• This migration in hIDO1 was accompanied by significant conformational changes in the JK-LoopC, demonstrating high protein plasticity.
• In contrast, CO and Trp remained bound in the hTDO active site under similar conditions, indicating a more rigid structure.

Conclusions:

• hIDO1 possesses remarkable conformational plasticity, allowing large-scale ligand and substrate migration upon CO photolysis.
• hTDO exhibits a significantly more rigid protein architecture compared to hIDO1.
• These findings provide crucial insights into structure-function relationships of heme-based dioxygenases and guide inhibitor design.