RESUMO
Water-Soluble Chlorophyll Proteins (WSCPs) from Brassicaceae are non-photosynthetic proteins which tetramerize upon binding four chlorophyll (Chl) molecules. The bound Chls are highly photostable, despite the lack of bound carotenoids known, in Chl-containing photosynthetic proteins, to act as singlet oxygen and Chl triplet (3Chl) quenchers. Although the physiological function of WSCPs is still unclear, it is likely to be related to their biochemical stability and their resistance to photodegradation. To get insight into the origin of this photostability, the properties of the 3Chl generated in WSCPs upon illumination were investigated. We found that, unlike the excited singlet states, which are excitonic states, the triplet state is localized on a single Chl molecule. Moreover, the lifetime of the 3Chl generated in WSCPs is comparable to that observed in other Chl-containing systems and is reduced in presence of oxygen. In contrast to previous observations, we found that WSCP actually photosensitizes singlet oxygen with an efficiency comparable to that of Chl in organic solvent. We demonstrated that the observed resistance to photooxidation depends on the conformation of the phytyl moieties, which in WSCP are interposed between the rings of Chl dimers, hindering the access of singlet oxygen to the oxidizable sites of the pigments.
Assuntos
Hidrolases de Éster Carboxílico/química , Clorofila A/química , Proteínas de Ligação à Clorofila/química , Oxirredutases atuantes sobre Doadores de Grupo CH-CH/química , Água/química , Sítios de Ligação , Hidrolases de Éster Carboxílico/genética , Hidrolases de Éster Carboxílico/metabolismo , Clorofila A/genética , Clorofila A/metabolismo , Proteínas de Ligação à Clorofila/genética , Proteínas de Ligação à Clorofila/metabolismo , Expressão Gênica , Modelos Moleculares , Oxirredução , Oxirredutases atuantes sobre Doadores de Grupo CH-CH/genética , Oxirredutases atuantes sobre Doadores de Grupo CH-CH/metabolismo , Oxigênio/química , Oxigênio/metabolismo , Pisum sativum/química , Pisum sativum/metabolismo , Fotólise , Fotossíntese/fisiologia , Ligação Proteica , Conformação Proteica em alfa-Hélice , Conformação Proteica em Folha beta , Domínios e Motivos de Interação entre Proteínas , Multimerização Proteica , Estabilidade Proteica , Oxigênio Singlete/química , Oxigênio Singlete/metabolismo , Solubilidade , Triticum/química , Triticum/metabolismo , Água/metabolismoRESUMO
In this paper we address the question of how a protein environment can modulate the absorption spectrum of a chromophore during a molecular dynamics simulation. The effect of the protein is modeled as an external field acting on the unperturbed eigenstates of the chromophore. Using a first-principles method recently developed in our group, we calculated the perturbed electronic energies for each frame and the corresponding wavelength absorption during the simulation. We apply this method to a nanosencond timescale molecular dynamics simulation of the light-harvesting peridinin-chlorophyll-protein complex from Amphidinium carterae, where chlorophyll was selected among the chromophores of the complex for the calculation. The combination of this quantum-classical calculation with the analysis of the large amplitude motions of the protein makes it possible to point out the relationship between the conformational flexibility of the environment and the excitation wavelength of the chromophore. Results support the idea of the existence of a correlation between protein conformational flexibility and chlorophyll electronic transitions induced by light.
