{"title":"On the production of singlet oxygen by the isoalloxazine ring in free and protein-bound flavin cofactors","authors":"","doi":"10.1016/j.bpc.2024.107333","DOIUrl":null,"url":null,"abstract":"<div><div>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, <sup>1</sup>O<sub>2</sub>, with high efficiency, Φ<sub>Δ,FMN</sub> ∼ 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 Φ<sub>Δ,FAD</sub> value, which is nearly 10-fold lower, Φ<sub>Δ,FAD</sub> ∼ 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 Φ<sub>Δ</sub> values are significantly decreased, Φ<sub>Δ</sub> < 0.03. We addressed these contradictory observations by analysis of: (i) dependence of Φ<sub>Δ,FAD</sub> 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 Φ<sub>Δ</sub>, proving the causative relation between Φ<sub>Δ</sub> 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 <sup>1</sup>O<sub>2</sub> production in cells.</div></div>","PeriodicalId":8979,"journal":{"name":"Biophysical chemistry","volume":null,"pages":null},"PeriodicalIF":3.3000,"publicationDate":"2024-10-10","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":"0","resultStr":null,"platform":"Semanticscholar","paperid":null,"PeriodicalName":"Biophysical chemistry","FirstCategoryId":"99","ListUrlMain":"https://www.sciencedirect.com/science/article/pii/S0301462224001625","RegionNum":3,"RegionCategory":"生物学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":null,"EPubDate":"","PubModel":"","JCR":"Q2","JCRName":"BIOCHEMISTRY & MOLECULAR BIOLOGY","Score":null,"Total":0}
引用次数: 0
Abstract
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.
期刊介绍:
Biophysical Chemistry publishes original work and reviews in the areas of chemistry and physics directly impacting biological phenomena. Quantitative analysis of the properties of biological macromolecules, biologically active molecules, macromolecular assemblies and cell components in terms of kinetics, thermodynamics, spatio-temporal organization, NMR and X-ray structural biology, as well as single-molecule detection represent a major focus of the journal. Theoretical and computational treatments of biomacromolecular systems, macromolecular interactions, regulatory control and systems biology are also of interest to the journal.