Evidence for a functionally important histidine residue in human tyrosine hydroxylase

Mutual influence of non-covalent interactions formed by imidazole: A systematic quantum-chemical study

Imidazole is a five-membered heterocycle that is part of a number of biologically important molecules such as the amino acid histidine and the hormone histamine. Imidazole has a unique ability to participate in a variety of non-covalent interactions involving the NH group, the pyridine-like nitrogen atom or the π-system. For many biologically active compounds containing the imidazole moiety, its participation in formation of hydrogen bond NH⋯O/N and following proton transfer is the key step of mechanism of their action. In this work a systematic study of the mutual influence of various paired combinations of non-covalent interactions (e.g., hydrogen bonds and π-interactions) involving the imidazole moiety was performed by means of quantum chemistry (PW6B95-GD3/def2-QZVPD) for a series of model systems constructed based on analysis of available x-ray data. It is shown that for considered complexes formation of additional non-covalent interactions can only enhance the proton-donating ability of imidazole. At the same time, its proton-accepting ability can be both enhanced and weakened, depending on what additional interactions are added to a given system. The mutual influence of non-covalent interactions involving imidazole can be classified as weak geometric and strong energetic cooperativity-a small change in the length of non-covalent interaction formed by imidazole can strongly influence its strength. The latter can be used to develop methods for controlling the rate and selectivity of chemical reactions involving the imidazole fragment in larger systems. It is shown that the strong mutual influence of non-covalent interactions involving imidazole is due to the unique ability of the imidazole ring to effectively redistribute electron density in non-covalently bound systems with its participation.

Identification of Binding Sites in Copper(II)-Peptide Complexes Using Infrared Spectroscopy

Complex formation of the copper(II) ion (CuII) with histidine (H) and H-containing peptides plays a crucial role in various metallo-enzymatic reactions. To elucidate the nature of coordinate bonding in CuII complexes, Fourier-transform infrared spectroscopy and 2D IR spectroscopy were employed to investigate the coordination geometries of CuII with diglycine, l-histidylglycine (HG), glycyl-l-histidine (GH), and glycylglycyl-l-histidine. The coordination of CuII to different peptide groups, including the peptide N- and C-termini, the amide group, and the imidazole of the H side chain, exhibits distinct spectral features. The derived molecular structure of the CuII-HG complex based on these spectral features significantly differs from that of CuII-GH, suggesting a preference of the N-terminus and the steric hindrance of the H side chain in CuII chelation.

Effect of histidine protonation state on ligand binding at the ATP-binding site of human protein kinase CK2

Histidine residues contribute to numerous molecular interactions, owing to their structure with the ionizable aromatic side chain with pKa close to the physiological pH. Herein, we studied how the two histidine residues, His115 and His160 of the catalytic subunit of human protein kinase CK2, affect the binding of the halogenated heterocyclic ligands at the ATP-binding site. Thermodynamic studies on the interaction between five variants of hCK2α (WT protein and four histidine mutants) and three ionizable bromo-benzotriazoles and their conditionally non-ionizable benzimidazole counterparts were performed with nanoDSF, MST, and ITC. The results allowed us to identify the contribution of interactions involving the particular histidine residues to ligand binding. We showed that despite the well-documented hydrogen bonding/salt bridge formation dragging the anionic ligands towards Lys68, the protonated His160 also contributes to the binding of such ligands by long-range electrostatic interactions. Simultaneously, His 115 indirectly affects ligand binding, placing the hinge region in open/closed conformations.

Structural basis of DNA binding by the NAC transcription factor ORE1, a master regulator of plant senescence

Plants use sophisticated mechanisms of gene expression to control senescence in response to environmental stress or aging. ORE1 (Arabidopsis thaliana NAC092) is a master regulator of senescence that belongs to the plant-specific NAC transcription factor protein family. ORE1 has been reported to bind to multiple DNA targets to orchestrate leaf senescence, yet the mechanistic basis for recognition of the cognate gene sequence remains unclear. Here, we report the crystal structure of the ORE1-NAC domain alone and its DNA-binding form. The structure of DNA-bound ORE1-NAC revealed the molecular basis for nucleobase recognition and phosphate backbone interactions. We show that local versatility in the DNA-binding site, in combination with domain flexibility of the ORE-NAC homodimer, is crucial for the maintenance of binding to intrinsically flexible DNA. Our results provide a platform for understanding other plant-specific NAC protein-DNA interactions as well as insight into the structural basis of NAC regulators in plants of agronomic and scientific importance.

Enhancement of the Water Affinity of Histidine by Zinc and Copper Ions

Histidine (His) is widely involved in the structure and function of biomolecules. Transition-metal ions, such as Zn²⁺ and Cu²⁺, widely exist in biological environments, and they are crucial to many life-sustaining physiological processes. Herein, by employing density function calculations, we theoretically show that the water affinity of His can be enhanced by the strong cation–π interaction between His and Zn²⁺ and Cu²⁺. Further, the solubility of His is experimentally demonstrated to be greatly enhanced in ZnCl₂ and CuCl₂ solutions. The existence of cation–π interaction is demonstrated by fluorescence, ultraviolet (UV) spectroscopy and nuclear magnetic resonance (NMR) experiments. These findings are of great importance for the bioavailability of aromatic drugs and provide new insight for understanding the physiological functions of transition metal ions.

Light-Induced Histidine Adducts to an IgG1 Molecule Via Oxidized Histidine Residue and the Potential Impact of Polysorbate-20 Concentration

PURPOSE

Histidine (His) undergoes light-induced reactions such as oxidation, crosslinking and addition. These reactions are initiated by singlet oxygen (¹O₂) to generate His photo-oxidation products, which are subject to nucleophilic attack by a non-oxidized His residue from another protein or by nucleophilic buffer components such as Tris and His. This report aims to identify light-induced His-adducts to a monoclonal antibody (mAb-1) due to the reaction of His molecules in the buffer with the photooxidized His residues under ICH light conditions. Since polysorbate-20 (PS-20) is a commonly used excipient in biotherapeutics formulation, it is also important to study the impact of PS-20 concentration on protein photostability.

RESULTS

We identified and characterized light-induced His-adducts of mAb-1 by LC-MS/MS. We showed that the levels of light-induced His-adducts generally correlate with the solvent accessibility of His residues in the protein. In addition, the presence of PS-20 at concentrations commonly used in protein drug formulations can significantly increase the levels of light-induced His-adducts.

CONCLUSIONS

Since His residues are present in a conserved region in the Fc domain, and may be present in the complementarity-determining region (CDR), the impact on the biological functions of the His-adducts observed here should be further studied to evaluate the risk of their presence.

The multiple roles of histidine in protein interactions

Background

Among the 20 natural amino acids histidine is the most active and versatile member that plays the multiple roles in protein interactions, often the key residue in enzyme catalytic reactions. A theoretical and comprehensive study on the structural features and interaction properties of histidine is certainly helpful.

Results

Four interaction types of histidine are quantitatively calculated, including: (1) Cation-π interactions, in which the histidine acts as the aromatic π-motif in neutral form (His), or plays the cation role in protonated form (His⁺); (2) π-π stacking interactions between histidine and other aromatic amino acids; (3) Hydrogen-π interactions between histidine and other aromatic amino acids; (4) Coordinate interactions between histidine and metallic cations. The energies of π-π stacking interactions and hydrogen-π interactions are calculated using CCSD/6-31+G(d,p). The energies of cation-π interactions and coordinate interactions are calculated using B3LYP/6-31+G(d,p) method and adjusted by empirical method for dispersion energy.

Conclusions

The coordinate interactions between histidine and metallic cations are the strongest one acting in broad range, followed by the cation-π, hydrogen-π, and π-π stacking interactions. When the histidine is in neutral form, the cation-π interactions are attractive; when it is protonated (His⁺), the interactions turn to repulsive. The two protonation forms (and pK_a values) of histidine are reversibly switched by the attractive and repulsive cation-π interactions. In proteins the π-π stacking interaction between neutral histidine and aromatic amino acids (Phe, Tyr, Trp) are in the range from -3.0 to -4.0 kcal/mol, significantly larger than the van der Waals energies.

Mössbauer, electron-paramagnetic-resonance and X-ray-absorption fine-structure studies of the iron environment in recombinant human tyrosine hydroxylase

Isoforms (1-4) of human tyrosine hydroxylase (TH) have been expressed in Escherichia coli and purified as apoenzymes (metal-free). Apo-human TH binds 1.0 atom Fe(II)/enzyme subunit, and iron binding is associated with an immediate and dramatic (40-fold) increase in specific activity. For X-ray absorption fine structure (XAFS) and electron paramagnetic resonance (EPR) measurements the apoenzyme was reconstituted with 56Fe and for Mössbauer measurements with 57Fe. XAFS measurements at the Fe-K edge of human TH were performed on the native form [Fe(II)-human TH], as well as after addition of stoichiometric amounts of the substrate tetrahydropterin, the inhibitor dopamine and of H2O2. The addition of dopamine or H2O2 oxidizes the ferrous iron of the native human TH to the ferric state. In both redox states the iron is octahedrally coordinated by low-Z backscatterers, thus sulfur coordination can be excluded. From the multiple scattering analysis of the EXAFS region is was surmised that part of the iron coordination is due to (3 +/- 1) imidazols. Addition of tetrahydropterin does not significantly change the iron coordination of the Fe(II) enzyme. The Mössbauer results confirm the valence states and the octahedral coordination of iron as well as the exclusion of sulfur ligation. Both the EPR spectra and the Mössbauer magnetic hyperfine pattern of dopamine- and H2O2-treated native human TH, were analyzed with the spin-Hamiltonian formalism. This analysis provides significantly different features for the two forms of human TH: the ferric iron (S = 5/2) of the H2O2-treated form exhibits a rhombic environment while that of the dopamine-treated form exhibits near-axial symmetry. The specific spectroscopic signature of dopamine-treated human TH, including that of an earlier resonance-Raman study [Michaud-Soret, I., Andersson, K. K., Que, L. Jr & Haavik, J. (1995) Biochemistry 34, 5504-5510] is most likely due to the bidentate binding of dopamine to iron.