Remote oxidative modifications induced by oxygen free radicals modify T/R allosteric equilibrium of a hyperthermophilic lactate dehydrogenase

L-Lactate dehydrogenase (LDH) is a model protein allowing to shed light on the fundamental molecular mechanisms that drive the acquisition, evolution and regulation of enzyme properties. In this study, we test the hypothesis of a link between thermal stability of LDHs and their capacity against unfolding induced by reactive oxygen species (ROS) generated by γ-rays irradiation. By using circular dichroism spectroscopy, we analysed that high thermal stability of a thermophilic LDH favours strong resistance against ROS-induced unfolding, in contrast to its psychrophilic and mesophilic counterparts that are less resistant. We suggest that a protein's phenotype linking strong thermal stability and resistance against ROS damages would have been a selective evolutionary advantage. We also find that the enzymatic activity of the thermophilic LDH that is strongly resistant against ROS-unfolding is very sensitive to inactivation by irradiation. To address this counter-intuitive observation, we combined mass spectrometry analyses and enzymatic activity measurements. We demonstrate that the dramatic change on LDH activity was linked to remote chemical modifications away from the active site, that change the equilibrium between low-affinity tense (T-inactive) and high-affinity relaxed (R-active) forms. We found the T-inactive thermophilic enzyme obtained after irradiation can recover its LDH activity by addition of the allosteric effector 1, 6 fructose bis phosphate. We analyse our data within the general framework of allosteric regulation, which requires that an enzyme in solution populates a large diversity of dynamically-interchanging conformations. Our work demonstrates that the radiation-induced inactivation of an enzyme is controlled by its dynamical properties.

Environmental Effects on Reactive Oxygen Species Detection-Learning from the Phagosome

SIGNIFICANCE

Reactive oxygen species (ROS) fulfill numerous roles in biology ranging from signal transduction to the induction of cell death. To advance our understanding of these sometimes contradictory roles, quantitative, specific, and sensitive ROS measurements are required.

RECENT ADVANCES

Several organic or genetically encoded probes were successfully developed for ROS detection.

CRITICAL ISSUES

In some cases, ROS production occurs in a harsh environment such as low pH or high concentration of proteases. However, the ROS sensor may be sensitive to such environmental conditions and therefore becomes inaccurate. While the sensitivity of many ROS sensors to pH is known, many other environmental conditions remain unexplored. This article illustrates the interference between ROS sensors and their environment using the phagosome as an example. In the phagosome, pH changes, high concentration of ROS, and the presence of many proteases generate a hostile and rapidly changing environment.

FUTURE DIRECTIONS

Difficulties due to cell movement and continuous formation of new phagosomes can be reduced by ratio measurements, if appropriate dyes are identified. For detection in live cells and subcellular locations, fluorescent proteins (FPs) offer several advantages and are used to create biosensors for ROS. Some FPs are directly sensitive to certain ROS as shown here. Although this may compromise their use in an environment with high levels of ROS, it can also be exploited for ROS measurement directly with the FPs themselves. For all types of ROS detection, we suggest a set of basic guidelines for testing the environmental sensitivity of an ROS sensor. Antioxid. Redox Signal. 25, 564-576.

An analytical workflow for the molecular dissection of irreversibly modified fluorescent proteins

Owing to their ability to be genetically expressed in live cells, fluorescent proteins have become indispensable markers in cellular and biochemical studies. These proteins can undergo a number of covalent chemical modifications that may affect their photophysical properties. Among other mechanisms, such covalent modifications may be induced by reactive oxygen species (ROS), as generated along a variety of biological pathways or through the action of ionizing radiations. In a previous report [1], we showed that the exposure of cyan fluorescent protein (ECFP) to amounts of (•)OH that mimic the conditions of intracellular oxidative bursts (associated with intense ROS production) leads to observable changes in its photophysical properties in the absence of any direct oxidation of the ECFP chromophore. In the present work, we analyzed the associated structural modifications of the protein in depth. Following the quantified production of (•)OH, we devised a complete analytical workflow based on chromatography and mass spectrometry that allowed us to fully characterize the oxidation events. While methionine, tyrosine, and phenylalanine were the only amino acids that were found to be oxidized, semi-quantitative assessment of their oxidation levels showed that the protein is preferentially oxidized at eight residue positions. To account for the preferred oxidation of a few, poorly accessible methionine residues, we propose a multi-step reaction pathway supported by data from pulsed radiolysis experiments. The described experimental workflow is widely generalizable to other fluorescent proteins, and opens the door to the identification of crucial covalent modifications that affect their photophysics.

ROS production in phagocytes: why, when, and where?

In the phagocytosis field, ROS production by the phagocyte NOX has been associated with pathogen killing for the last 50 years. Since the discovery of nonphagocyte NOX, numerous other roles for ROS production have been identified. Oxidative stress and ROS-mediated signaling have received much attention in recent years. Much lower concentrations of ROS may be required for signaling compared with microbial killing. Based on the discoveries in nonphagocytic cells, it became logical to look for ROS functions distinct from pathogen killing, even in phagocytes. ROS are now linked to various forms of cell death, to chemotaxis, and to numerous modifications of cellular processes, including the NOX itself. ROS functions are clearly concentration-dependent over a wide range of concentrations. How much is required for which function? Which species are required for how much time? Is ROS signaling only a side effect of bactericidal ROS production? One major obstacle to answer these questions is the difficulty of reliable quantitative ROS detection. Signal transduction often takes place on a subcellular scale over periods of seconds or minutes, so the detection methods need to provide appropriate time and space resolution. We present examples of local ROS production, decreased degradation, signaling events, and potentially ROS-sensitive functions. We attempt to illustrate the current limitations for quantitative spatiotemporal ROS detection and point out directions for ongoing development. Probes for localized ROS detection and for combined detection of ROS, together with protein localization or other cellular parameters, are constantly improved.

The single T65S mutation generates brighter cyan fluorescent proteins with increased photostability and pH insensitivity

Cyan fluorescent proteins (CFP) derived from Aequorea victoria GFP, carrying a tryptophan-based chromophore, are widely used as FRET donors in live cell fluorescence imaging experiments. Recently, several CFP variants with near-ultimate photophysical performances were obtained through a mix of site-directed and large scale random mutagenesis. To understand the structural bases of these improvements, we have studied more specifically the consequences of the single-site T65S mutation. We find that all CFP variants carrying the T65S mutation not only display an increased fluorescence quantum yield and a simpler fluorescence emission decay, but also show an improved pH stability and strongly reduced reversible photoswitching reactions. Most prominently, the Cerulean-T65S variant reaches performances nearly equivalent to those of mTurquoise, with QY  = 0.84, an almost pure single exponential fluorescence decay and an outstanding stability in the acid pH range (pK_1/2 = 3.6). From the detailed examination of crystallographic structures of different CFPs and GFPs, we conclude that these improvements stem from a shift in the thermodynamic balance between two well defined configurations of the residue 65 hydroxyl. These two configurations differ in their relative stabilization of a rigid chromophore, as well as in relaying the effects of Glu222 protonation at acid pHs. Our results suggest a simple method to greatly improve numerous FRET reporters used in cell imaging, and bring novel insights into the general structure-photophysics relationships of fluorescent proteins.

Oxidative stress induces mainly human centrin 2 polymerisation

PURPOSE

To determine the human centrin 2 (Hscen 2) protein response to oxidising radicals in vitro and to evaluate the consequences on its biological functions.

MATERIALS AND METHODS

Hscen 2 was submitted to hydroxyl and azide radicals produced by radiolysis in the absence of oxygen. The resulting products were characterised by biochemical, spectroscopic and mass spectrometry techniques. Their thermodynamics parameters of complexation with C-terminal fragment of Xeroderma pigmentosum C protein (C-XPC), one of the Hscen 2 cellular partners, were quantified by isothermal titration calorimetry (ITC).

RESULTS

Both hydroxyl and azide radicals induce centrin 2 polymerisation as we characterised several intermolecular cross-links generating dimers, trimers, tetramers and higher molecular mass species. These cross-links result from the formation of a covalent bond between the only tyrosine residue (Tyr 172) located in the C-terminal region of each monomer. Remarkably, dimerisation occurs for doses as low as a few grays. Moreover, this Hscen2 dimer has a lower affinity and stoechiometry binding to C-XPC.

CONCLUSIONS

These results show that as oxidative radicals induce high proportions of irreversible damages (polymerisation) centrin 2 is highly sensitive to ionising radiation. This could have important consequences on its biological functions.