Competitive photosensitized oxidation of tyrosine and methionine residues in enkephalins and their model peptides

Does the presence of ground state complex between a PR-10 protein and a sensitizer affect the mechanism of sensitized photo-oxidation?

The mechanisms of one-electron protein oxidation are complicated and still not well-understood. In this work, we investigated the reaction of sensitized photo-oxidation using carboxybenzophenone (CB) as a sensitizer and a PR-10 protein (MtN13) as a quencher, which is intrinsically complicated due to the complex structure of the protein and multiple possibilities of CB attack. To predict and examine the possible reactions precisely, the 3D structure of the MtN13 protein was taken into account. Our crystallographic studies revealed a specific binding of the CB molecule in the protein's hydrophobic cavity, while mass spectrometry identified the amino acid residues (Met, Tyr, Asp and Phe) creating adducts with the sensitizer, thus indicating the sites of ³CB* quenching. In addition, protein aggregation was also observed. The detailed mechanisms of CB quenching by the MtN13 molecule were elucidated by an analysis of transient products by means of time-resolved spectroscopy. The investigation of the transient and stable products formed during the protein photo-oxidation was based on the data obtained from HPLC-MS analysis of model compounds, single amino acids and dipeptides. Our proposed mechanisms of sensitized protein photo-oxidation emphasize the role of a ground state complex between the protein and the sensitizer and indicate several new and specific products arising as a result of one-electron oxidation. Based on the analysis of the transient and stable products, we have demonstrated the influence of neighboring groups, especially in the case of Tyr oxidation, where the tyrosyl radical can be formed via a direct electron transfer from Tyr to CB* or via an intramolecular electron transfer from Tyr to Met radical cation Met > S^●+ or thiyl radical CysS^● from neighboring oxidized groups.

Sensitized photoreduction of selected benzophenones. Mass spectrometry studies of radical cross-coupling reactions

The hydrogen atom transfer reaction (HAT) between selected benzophenones (benzophenone BP, 3-carboxybenzophenone 3CB, and 4-carboxybenzophenone 4CB) and 2-propanol was reinvestigated focusing on stable product analysis. As expected, the primary species of these HAT's are the respective diphenyl and dimethyl ketyl radicals that eventually undergo several radical coupling reactions leading to stable photoproducts. However, the mechanisms of these free radical reactions remain unclear and open to question. In this report, we focus on the detailed analysis of the stable photoproducts of these reactions using liquid chromatography coupled with high-resolution mass spectrometry (LC-ESI-QTOF-MS/MS). Products of photopinacolization (benzpinacol and two diastereoisomers of 4CB and 3CB dimers) and isomeric radical cross-coupling adducts of respective diphenyl and dimethyl ketyl radicals were separated chromatographically, and their structures were determined by high-resolution MS/MS, and the mechanisms of the reactions are discussed.

Photo- and Radiation-Induced One-Electron Oxidation of Methionine in Various Structural Environments Studied by Time-Resolved Techniques

Oxidation of methionine (Met) is an important reaction that plays a key role in protein modifications during oxidative stress and aging. The first steps of Met oxidation involve the creation of very reactive and short-lived transients. Application of complementary time-resolved radiation and photochemical techniques (pulse radiolysis and laser flash photolysis together with time-resolved CIDNP and ESR techniques) allowed comparing in detail the one-electron oxidation mechanisms initiated either by ^●OH radicals and other one-electron oxidants or the excited triplet state of the sensitizers e.g., 4-,3-carboxybenzophenones. The main purpose of this review is to present various factors that influence the character of the forming intermediates. They are divided into two parts: those inextricably related to the structures of molecules containing Met and those related to external factors. The former include (i) the protection of terminal amine and carboxyl groups, (ii) the location of Met in the peptide molecule, (iii) the character of neighboring amino acid other than Met, (iv) the character of the peptide chain (open vs cyclic), (v) the number of Met residues in peptide and protein, and (vi) the optical isomerism of Met residues. External factors include the type of the oxidant, pH, and concentration of Met-containing compounds in the reaction environment. Particular attention is given to the neighboring group participation, which is an essential parameter controlling one-electron oxidation of Met. Mechanistic aspects of oxidation processes by various one-electron oxidants in various structural and pH environments are summarized and discussed. The importance of these studies for understanding oxidation of Met in real biological systems is also addressed.

Sensitized photo-oxidation of plant cytokinin-specific binding protein - Does the environment of the thioether group influence the oxidation reaction? From primary intermediates to stable products

The reactions of protein oxidation play a significant role in many biological processes, especially in diseases development. Therefore, it is important to understand, how the protein molecule behaves in the presence of oxidants. In the present work, photo-oxidation of phytohormone-binding plant protein (VrPhBP) was investigated using light and 3-carboxybenzophenone (3CB) as a sensitizer (one electron oxidant). The protein interacts with the sensitizer in the ground state forming a weak binding complex leading to the presence of bound and free 3CB in solution. The early events and transient species (such as radicals and radical ions) formed during irradiation were characterised by transient spectroscopy showing the formation of the sulphur radical cation Met>S^●+ (stabilized by (S∴N)⁺)and the tyrosyl radical TyrO^● on VrPhBP. Thus the 3CB excited triplet state was quenched by the Met and Tyr residues and mostly by Met (based on the deconvoluted transient absorption spectra).The presence of a Tyr side chain in the vicinity of a Met residue results in intramolecular electron transfer from Tyr to the Met>S^●+ radical cation, leading to regeneration of the thioether side chain and formation of TyrO^●. The presence of other side chains close to Met, such as Arg or Lys can induce the stabilization of Met>S^●+ via the formation of two-centered three-electron bonded species (S∴N)⁺. The transient species were additionally confirmed by stable product analysis. Based on SDS-PAGE, chromatography and mass spectrometry, the formation of methionine sulphoxide and Met-3CB adduct was identified together with di-Tyr cross links. On the basis of the experimental results the overall mechanism of VrPhBP photo-oxidation, from its early events to the formation of stable products, is described. In addition, a good correlation between the mechanisms of photooxidation of model compounds such as Met derivatives and peptides and those for real biological systems is emphasized.

Heavy metals in urban road dusts from Kolkata and Bengaluru, India: implications for human health

Air pollution and dust pollution are major urban environmental issues, with road dust being a potential source and a pathway for human exposure. The developing megacities of India, where the population may spend a significant portion of their working lives close to the roadside, including consuming street food, have obvious source-pathway-receptor linkages. The aim of this study in Kolkata and Bengaluru, India, was to evaluate the risk to human health from inorganic components of road dust. Samples were collected and analysed from a cross section of urban environments for a wide range of anthropogenic and geogenic elements, some such as antimony showing an increase in response to vehicle activity. Calculated enrichment factors relative to crustal abundance demonstrated significant enrichment in common heavy metals and less commonly reported elements, e.g. molybdenum, antimony, that may be used as contaminant markers. Factor analysis gave multielement signatures associated with geography, vehicle traffic and local industry. The bio-accessibility of road dusts in terms of ingestion was determined using the BARGE method with more than 50% of zinc and lead being available in some cases. A formal human health risk assessment using the US EPA framework showed that lead in Kolkata and chromium in Bengaluru were the elements of most concern amongst chromium, nickel, copper, zinc and lead. However, the only risk combination (hazard index) shown to be significant was lead exposure to children in Kolkata. Ingestion dominated the risk pathways, being significantly greater than dermal and inhalation routes.

Unexpected light emission from tyrosyl radicals as a probe for tyrosine oxidation

Tyrosine residues (Tyr) on proteins are a favoured site of one-electron oxidation due to their low one-electron reduction potentials. In this work, light-induced oxidation of Tyr residues was investigated using direct ionisation (via 266 nm light excitation) and sensitized photo-oxidation (by 3-carboxybenzophenone as sensitizer and 355 nm). Light emission (fluorescence) was observed at 410-440 nm as a result of Tyr oxidation. This novel light emission process is shown to be dependent on the solvent and aromatic ring substituents, however it does not depend on pH. It is proposed, that after initial formation of tyrosine phenoxyl radicals (TyrO^●) by one electron-oxidation, the TyrO^● absorbs a second photon to give an excited state species that undergoes subsequent light emission. The intensity of this emission depends on the Tyr concentration, and the detection of this emission can be used to identify and quantify one-electron formation of oxidized Tyr residues on proteins.

Sulfur Radical-Induced Redox Modifications in Proteins: Analysis and Mechanistic Aspects

SIGNIFICANCE

The sulfur-containing amino acids cysteine (Cys) and methionine (Met) are prominent protein targets of redox modification during conditions of oxidative stress. Here, two-electron pathways have received widespread attention, in part due to their role in signaling processes. However, Cys and Met are equally prone to one-electron pathways, generating intermediary radicals and/or radial ions. These radicals/radical ions can generate various reaction products that are not commonly monitored in redox proteomic studies, but they may be relevant for the fate of proteins during oxidative stress. Recent Advances: Time-resolved kinetic studies and product analysis have expanded our mechanistic understanding of radical reaction pathways of sulfur-containing amino acids. These reactions are now studied in some detail for Met and Cys in proteins, and homocysteine (Hcy) chemically linked to proteins, and the role of protein radical reactions in physiological processes is evolving.

CRITICAL ISSUES

Radical-derived products from Cys, Hcy, and Met can react with additional amino acids in proteins, leading to secondary protein modifications, which are potentially remote from initial points of radical attack. These products may contain intra- and intermolecular cross-links, which may lead to protein aggregation. Protein sequence and conformation will have a significant impact on the formation of such products, and a thorough understanding of reaction mechanisms and specifically how protein structure influences reaction pathways will be critical for identification and characterization of novel reaction products.

FUTURE DIRECTIONS

Future studies must evaluate the biological significance of novel reaction products that are derived from radical reactions of sulfur-containing amino acids. Antioxid. Redox Signal. 26, 388-405.