Heme is responsible for enhanced singlet oxygen deactivation in cytochrome c

On the production of singlet oxygen by the isoalloxazine ring in free and protein-bound flavin cofactors

Flavin cofactors, flavin adenine dinucleotide (FAD) and flavin mononucleotide (FMN), as a part of flavoenzymes play a critical role in the catalysis of multiple reactions predominantly of a redox nature. Question arises why nature developed two very similar cofactors with an identical functional part - isoalloxazine ring. We believe that an answer is related to the fact that the isoalloxazine ring belongs to endogenous photosensitizers able to produce reactive and potentially harmful singlet oxygen, 1O2, with high efficiency, ΦΔ,FMN ∼ 0.6. In fact, in contrast with one main conformation of FMN in water, the presence of the adenosine mononucleotide in FAD induces a dynamic equilibrium of two main conformations - closed (∼80 %) and open (∼20 %). The presence of predominant closed conformation of FAD in water has a significant impact on the ΦΔ,FAD value, which is nearly 10-fold lower, ΦΔ,FAD ∼ 0.07, than that of FMN. On the other hand, based on our analysis of a non-homologous dataset of FAD containing 105 proteins, ∼75 % enzyme-bound FAD exists predominantly in open conformations but the ΦΔ values are significantly decreased, ΦΔ < 0.03. We addressed these contradictory observations by analysis of: (i) dependence of ΦΔ,FAD value on opening the FAD conformation by urea and (ii) amino acid propensities for isoalloxazine binding site. We demonstrated that urea-induced destabilization, in 7 M vs 0 M urea, of the closed FAD conformation leads to a ∼ 3-fold increase of ΦΔ, proving the causative relation between ΦΔ value and the flavin cofactor conformation. Detailed examination of the flavoproteins dataset clearly indicated positive propensities of three amino acids: glycine, cysteine, and tryptophan for isoalloxazine ring binding site. We hypothesize that both the closed conformation of free FAD and the arrangement of the isoalloxazine binding site is important for prevention of potentially harmful 1O2 production in cells.

pH modulates efficiency of singlet oxygen production by flavin cofactors

Flavin adenine dinucleotide (FAD) and flavin mononucleotide (FMN) are frequently used interchangeably in the catalysis of various reactions as part of flavoenzymes because they have the same functional component, the isoalloxazine ring. However, they differ significantly in their conformational properties. The inclusion of two planar rings in the structure of FAD greatly increases the range of possible conformations compared to FMN. An exemplary instance of this is the remarkable disparity in singlet oxygen efficiency production, Φ Δ, between FMN and FAD. Under neutral pH conditions, FAD has low photosensitizing activity with Φ Δ ∼ 0.07 while FMN demonstrates high photosensitizing activity with Φ Δ ∼ 0.6. Both adenine rings and isoalloxazine in FAD contain pH titratable groups. Through comprehensive analysis of the kinetics of the transient absorbance of the triplet state and the phosphorescence of singlet oxygen from FAD and FMN, we determined the correlation between different conformational states and the pH-dependent generation of singlet oxygen. Based on our findings, we may deduce that within the pH range of pH 2 to pH 13, only two out of the five potential structural states of FAD are capable of efficiently producing singlet oxygen. There are two open conformations: (i) an acidic FAD conformation with a protonated adenine ring, which is around 10 times more populated than the neutral open FAD conformation, and (ii) a neutral pH FAD conformation, which is significantly less populated. The FAD conformer with a protonated adenine ring at acidic pH generates singlet oxygen with approximately 50% efficiency compared to the constantly open FMN at neutral pH. This may have implications for singlet oxygen synthesis in acidic environments.

Singlet oxygen quenching as a probe for cytochrome c molten globule state formation

Singlet oxygen refers to the nonradical metastable excited state of molecular oxygen that readily oxidizes various cellular components. Its behavior in different biological systems has been studied for many years. Recently, we analyzed the effect of singlet oxygen quenching by heme cofactor in cytochrome c (cyt c). Here, we have exploited this effect in the investigation of conformational differences in the molten globule states of cyt c induced by different sodium anions, namely sulfate, chloride and perchlorate. The high efficiency of heme toward quenching singlet oxygen enabled us to use this property for the analysis of the otherwise experimentally difficult-to-determine parameter of heme upon exposure to solvents as highly similar conformational states of cyt c in the molten globule states are induced by different salts at acidic pH. Our results from singlet oxygen quenching experiments correlate well with other spectroscopic methods, such as circular dichroism and fluorescence measurements, and suggest increasing availability of heme in the order: perchlorate < chloride < sulfate. Based on our findings we propose that singlet oxygen phosphorescence measurements are useful in determining the differences in the protein conformation of their heme regions, particularly regarding the relative heme exposure to the solvent.