Substrate Binding Primes Human Tryptophan 2,3-Dioxygenase for Ligand Binding

Conformational Plasticity in Human Heme-Based Dioxygenases

Human indoleamine 2,3-dioxygenase 1 (hIDO1) and human tryptophan dioxygenase (hTDO) are two important heme proteins that degrade the essential amino acid, l-tryptophan (Trp), along the kynurenine pathway. The two enzymes share a similar active site structure and an analogous catalytic mechanism, but they exhibit a variety of distinct functional properties. Here we used carbon monoxide (CO) as a structural probe to interrogate how the functionalities of the two enzymes are encoded in their structures. With X-ray crystallography, we detected an unexpected photochemical intermediate trapped in a crystal of the hIDO1-CO-Trp complex, where CO is photolyzed from the heme iron by X-rays at cryogenic temperatures (100 K). The CO photolysis triggers a large-scale migration of the substrate Trp, as well as the photolyzed CO, from the active site to a temporary binding site, Sa*. It is accompanied by a large conformational change to an active site loop, JK-Loop^C, despite the severely restricted protein motion under the frozen conditions, which highlights the remarkable conformational plasticity of the hIDO1 protein. Comparative studies of a crystal of the hTDO-CO-Trp complex show that CO and Trp remain bound in the active site under comparable X-ray illumination, indicating a much more rigid protein architecture. The data offer important new insights into the structure and function relationships of the heme-based dioxygenases and provide new guidelines for structure-based design of inhibitors targeting them.

Kinetic Study of Ligand Binding and Conformational Changes in Inducible Nitric Oxide Synthase

Nitric oxide synthases (NOSs) are heme enzymes that generate highly reactive nitric oxide from l-arginine (l-Arg) in a complex mechanism that is still only partially understood. We have studied carbon monoxide (CO) binding to the oxygenase domain of murine inducible NOS (iNOS) by using flash photolysis. The P420 and P450 forms of the enzyme, assigned to a protonated and unprotonated proximal cysteine, through which the heme is anchored to the protein, show markedly different CO rebinding properties. The data suggest that P420 has a widely open distal pocket that admits water. CO rebinding to the P450 form strongly depends on the presence of the substrate l-Arg, the intermediate N^ω-hydroxy-l-arginine, and the cofactor tetrahydrobiopterin. After adding these small molecules to the enzyme solution, the CO kinetics change slowly over the hours. This process can be described as a relaxation from a fast rebinding, metastable species to a slowly rebinding, thermodynamically stable species, which we associate with the enzymatically active form. Our results allow us to determine kinetic parameters of l-Arg binding to the ferrous deoxy iNOS protein for the first time and also provide clues regarding the nature of structural differences between the two interconverting species.

Different Mechanisms of Catalytic Complex Formation in Two L-Tryptophan Processing Dioxygenases

The human heme enzymes tryptophan 2,3-dioxygenase (hTDO) and indoleamine 2,3 dioxygenase (hIDO) catalyze the initial step in L-tryptophan (L-Trp) catabolism, the insertion of dioxygen into L-Trp. Overexpression of these enzymes causes depletion of L-Trp and accumulation of metabolic products, and thereby contributes to tumor immune tolerance and immune dysregulation in a variety of disease pathologies. Understanding the assembly of the catalytically active, ternary enzyme-substrate-ligand complexes is not yet fully resolved, but an essential prerequisite for designing efficient and selective de novo inhibitors. Evidence is mounting that the ternary complex forms by sequential binding of ligand and substrate in a specific order. In hTDO, the apolar L-Trp binds first, decreasing active-site solvation and, as a result, reducing non-productive oxidation of the heme iron by the dioxygen ligand, which may leave the substrate bound to a ferric heme iron. In hIDO, by contrast, dioxygen must first coordinate to the heme iron because a bound substrate would occlude ligand access to the heme iron, so the ternary complex can no longer form. Consequently, faster association of L-Trp at high concentrations results in substrate inhibition. Here, we summarize our present knowledge of ternary complex formation in hTDO and hIDO and relate these findings to structural peculiarities of their active sites.

Chain-chain complexation and heme binding in haemoglobin with respect to the hydrophobic core structure

Heme binding by proteins and protein-protein complexation are the processes strongly related to the biological activity of proteins. The mechanism of these processes has not been still recognised. These phenomena are presented using haemoglobin as the example. Half of the mature haemoglobin (one α-chain and one β-chain) treated as a dissociation step in haemoglobin degradation reveals a specific change in heme binding after dissociation. This phenomenon is the object of analysis that interprets the structure of both complexes (tetramer and dimer) with respect to their hydrophobic core structure. The results suggest the higher stability of the complex in the form of one α-chain and one β-chain with respect to the hydrophobic core.