Ligand binding to heme proteins: a comparison of cytochrome c variants with globins

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

The human heme enzyme tryptophan 2,3-dioxygenase (hTDO) catalyzes the insertion of dioxygen into its cognate substrate, l-tryptophan (l-Trp). Its active site structure is highly dynamic, and the mechanism of enzyme-substrate-ligand complex formation and the ensuing enzymatic reaction is not yet understood. Here we have studied complex formation in hTDO by using time-resolved optical and infrared spectroscopy with carbon monoxide (CO) as a ligand. We have observed that both substrate-free and substrate-bound hTDO coexist in two discrete conformations with greatly different ligand binding rates. In the fast rebinding hTDO conformation, there is facile ligand access to the heme iron, but it is greatly hindered in the slowly rebinding conformation. Spectroscopic evidence implicates active site solvation as playing a crucial role for the observed kinetic differences. Substrate binding shifts the conformational equilibrium markedly toward the fast species and thus primes the active site for subsequent ligand binding, ensuring that formation of the ternary complex occurs predominantly by first binding l-Trp and then the ligand. Consequently, the efficiency of catalysis is enhanced because O₂ binding prior to substrate binding, resulting in nonproductive oxidation of the heme iron, is greatly suppressed.

Fourier transform infrared spectroscopy study of ligand photodissociation and migration in inducible nitric oxide synthase

Inducible nitric oxide synthase (iNOS) is a homodimeric heme enzyme that catalyzes the formation of nitric oxide (NO) from dioxygen and L-arginine (L-Arg) in a two-step process. The produced NO can either diffuse out of the heme pocket into the surroundings or it can rebind to the heme iron and inhibit enzyme action. Here we have employed Fourier transform infrared (FTIR) photolysis difference spectroscopy at cryogenic temperatures, using the carbon monoxide (CO) and NO stretching bands as local probes of the active site of iNOS. Characteristic changes were observed in the spectra of the heme-bound ligands upon binding of the cofactors. Unlike photolyzed CO, which becomes trapped in well-defined orientations, as indicated by sharp photoproduct bands, photoproduct bands of NO photodissociated from the ferric heme iron were not visible, indicating that NO does not reside in the protein interior in a well-defined location or orientation. This may be favorable for NO release from the enzyme during catalysis because it reduces self-inhibition. Moreover, we used temperature derivative spectroscopy (TDS) with FTIR monitoring to explore the dynamics of NO and carbon monoxide (CO) inside iNOS after photodissociation at cryogenic temperatures. Only a single kinetic photoproduct state was revealed, but no secondary docking sites as in hemoglobins. Interestingly, we observed that intense illumination of six-coordinate ferrous iNOS oxy-NO ruptures the bond between the heme iron and the proximal thiolate to yield five-coordinate ferric iNOS oxy-NO, demonstrating the strong trans effect of the heme-bound NO.

Fourier transform infrared spectroscopy study of ligand photodissociation and migration in inducible nitric oxide synthase

Inducible nitric oxide synthase (iNOS) is a homodimeric heme enzyme that catalyzes the formation of nitric oxide (NO) from dioxygen and L-arginine (L-Arg) in a two-step process. The produced NO can either diffuse out of the heme pocket into the surroundings or it can rebind to the heme iron and inhibit enzyme action. Here we have employed Fourier transform infrared (FTIR) photolysis difference spectroscopy at cryogenic temperatures, using the carbon monoxide (CO) and NO stretching bands as local probes of the active site of iNOS. Characteristic changes were observed in the spectra of the heme-bound ligands upon binding of the cofactors. Unlike photolyzed CO, which becomes trapped in well-defined orientations, as indicated by sharp photoproduct bands, photoproduct bands of NO photodissociated from the ferric heme iron were not visible, indicating that NO does not reside in the protein interior in a well-defined location or orientation. This may be favorable for NO release from the enzyme during catalysis because it reduces self-inhibition. Moreover, we used temperature derivative spectroscopy (TDS) with FTIR monitoring to explore the dynamics of NO and carbon monoxide (CO) inside iNOS after photodissociation at cryogenic temperatures. Only a single kinetic photoproduct state was revealed, but no secondary docking sites as in hemoglobins. Interestingly, we observed that intense illumination of six-coordinate ferrous iNOS oxy-NO ruptures the bond between the heme iron and the proximal thiolate to yield five-coordinate ferric iNOS oxy-NO, demonstrating the strong trans effect of the heme-bound NO.

Microarray analysis of altered gene expression and the role of ATF3 in HK-2 cells treated with hemin

OBJECTIVE

To identify gene expression changes and the role of activating transcription factor 3 (ATF3) in hemin toxicity in renal tubular epithelial cells, then elucidate molecular mechanisms of hemin toxicity on renal tubular epithelial cells.

METHODS

An oligo array comprising 35,035 genes was used to compare differential gene expression in hemin-treated and non-treated HK-2 cells (human renal proximal tubular epithelial cells), and the role of ATF3 in hemin toxicity was assessed using siRNA technique.

RESULTS

A total of 128 mRNAs were at least twofold up-regulated and 101 mRNAs were at least twofold down-regulated after hemin treatment. Expression levels of ATF3, heat shock protein 70, c-fos, and c-jun were remarkably increased. Hemin also suppressed nuclear factor-kappa B inhibitor α, β-2 adrenergic receptor, and interleukin-6 mRNA amounts more than twofold. We further demonstrated the protective role of ATF3 in hemin cytotoxicity.

CONCLUSIONS

The data suggest that hemin caused multiple changes of gene expression in HK-2 cells, and ATF3 protects against hemin cytotoxicity.