Heme is responsible for enhanced singlet oxygen deactivation in cytochrome c

The deactivation of singlet oxygen, the lowest electronic excited state of molecular oxygen, by proteins is usually described through the interaction of singlet oxygen with certain amino acids. Changes in accessibility of these amino acids influence the quenching rate and the phosphorescence kinetics of singlet oxygen. In the cellular environment, however, numerous proteins with covalently bound or encapsulated cofactors are present. These cofactors could also influence the deactivation of singlet oxygen, and these have received little attention. To confront this issue, we used cytochrome c (cyt c) and apocytochrome c (apocyt c) to illustrate how the heme prosthetic group influences the rate constant of singlet oxygen deactivation upon acidic pH-induced conformational change of cyt c. Photo-excited flavin mononucleotide (FMN) was used to produce singlet oxygen. Our data show that the heme group has a significant and measurable effect on singlet oxygen quenching when the heme is exposed to solvents and is therefore more accessible to singlet oxygen. The effect of amino acids and heme accessibility on the FMN triplet state deactivation was also investigated.

Investigation of mutations (L41F, F17M, N57E, Y99F_Y134W) effects on the TolAIII-UnaG fluorescence protein's unconjugated bilirubin (UC-BR) binding ability and thermal stability properties

The UnaG protein is a ligand (unconjugated bilirubin) dependent fluorescence protein isolated from Unagi freshwater eel larvae and expressed as fusion in heterologous expression systems. Bilirubin is a tetrapyrrole molecule mainly produced from heme catabolism by the destruction of erythrocytes in the body. Bilirubin can cause kernicterus, a serious condition associated with permanent neurological damage in neonates with the passage of brain tissue. Different methods have been developed for plasma bilirubin analysis and quantification. The use of UnaG fluorescence protein triggered by bilirubin has become a new approach in bilirubin studies. In this study, we aimed to investigate the biophysical characterization of ligand interactions with the proteins obtained as a result of mutations (UnaG^Y99F_Y134W, UnaG^N57E, UnaG^L41F, and UnaG^F17M) on the amino acid sequence of TolAIII-UnaG protein. After the purity levels of the expressed proteins have been analyzed by SDS-PAGE, secondary structures and thermal melting temperatures of the proteins have been examined by circular dichroism spectroscopy. Then determination of excitation and emission points by fluorescence spectroscopy, titration studies have been performed with bilirubin, and dissociation constant was calculated. According to the biophysical characterization studies, UnaG^L41F has the highest affinity and stability among the mutants.

Photodynamic Therapy

Photodynamic therapy is a clinical technique for the treatment of cancers, microbial infections and other medical conditions by means of light-induced generation of reactive oxygen species using photosensitising drugs. The intrinsic fluorescence of many such drugs make them potential theranostic agents for simultaneous diagnosis and therapy. This chapter reviews the basic chemical and biological aspects of photodynamic therapy with an emphasis on its applications in theranostics. The roles of nanotechnology is highlighted, as well as emerging trends such as photoimmunotherapy, image-guided surgery and light- and singlet-oxygen dosimetry.

Two Distinct Fluorescence States of the Ligand-Induced Green Fluorescent Protein UnaG

UnaG is a recently discovered ligand-induced fluorescent protein that utilizes bound bilirubin (BR) as its fluorophore. The fluorescence of the UnaG-BR complex (holoUnaG) compares in quantum efficiency to that of enhanced green fluorescent protein, but it is superior in that the fluorophore formation is instantaneous and not dependent on oxygen; hence, much attention has been paid to UnaG as a new fluorescent probe. However, many important molecular properties of fluorescent probes remain unknown, such as the association/dissociation rates of BR, which determine the stability thereof, and the dispersibility of UnaG in aqueous solutions, which influence the functions of labeled proteins. In this study, we found, in the process of investigating the association rate, that the holoUnaG takes two distinct fluorescence states, which we named holoUnaG₁ and holoUnaG₂. The holoUnaG₁ initially forms after binding BR and then changes to the brighter holoUnaG₂ by a reversible intra-molecular reaction, thereby finally reaching an equilibrium between the two states. Spectroscopic analysis indicated that the intra-molecular reaction was associated not with a chemical change of BR but with a change in the environmental conditions surrounding BR. We also revealed that the molecular brightness ratio and equilibrium population ratio of the two states (holoUnaG₁/holoUnaG₂) were 1:3.9 and 6:4, respectively, using photon number counting analysis. From these results, we have suggested a novel schema, to our knowledge, for the formation of the UnaG and BR complex system and have determined the various rate constants associated therein. Additionally, using analytical ultracentrifugation, we established that UnaG in the apo-state (apoUnaG) and the holoUnaG are monomeric in aqueous solution. These findings provide not only key information for the practical use of UnaG as a fluorescent probe, but also the possibility for development of a brighter UnaG mutant by genetic engineering to constitutive holoUnaG₂.

Reactive oxygen species: Reactions and detection from photosynthetic tissues

Reactive oxygen species (ROS) have long been recognized as compounds with dual roles. They cause cellular damage by reacting with biomolecules but they also function as agents of cellular signaling. Several different oxygen-containing compounds are classified as ROS because they react, at least with certain partners, more rapidly than ground-state molecular oxygen or because they are known to have biological effects. The present review describes the typical reactions of the most important ROS. The reactions are the basis for both the detection methods and for prediction of reactions between ROS and biomolecules. Chemical and physical methods used for detection, visualization and quantification of ROS from plants, algae and cyanobacteria will be reviewed. The main focus will be on photosynthetic tissues, and limitations of the methods will be discussed.