Dynamics and mechanism of dimer dissociation of photoreceptor UVR8.

Photoreceptors are a class of light-sensing proteins with critical biological functions. UVR8 is the only identified UV photoreceptor in plants and its dimer dissociation upon UV sensing activates UV-protective processes. However, the dissociation mechanism is still poorly understood.

Dynamics and mechanism of light harvesting in UV photoreceptor UVR8.

Photosynthetic pigments form light-harvesting networks to enable nearly perfect quantum efficiency in photosynthesis excitation energy transfer. However, similar light-harvesting mechanisms have not been reported in light sensing processes in other classes of photoreceptors during light-mediated signaling. Here, based on our earlier report, we mapped out a striking energy-transfer network composed of 26 structural tryptophan residues in the plant UV-B photoreceptor UVR8.

A leap in quantum efficiency through light harvesting in photoreceptor UVR8.

Plants utilize a UV-B (280 to 315 nm) photoreceptor UVR8 (UV RESISTANCE LOCUS 8) to sense environmental UV levels and regulate gene expression to avoid harmful UV effects. Uniquely, UVR8 uses intrinsic tryptophan for UV-B perception with a homodimer structure containing 26 structural tryptophan residues. However, besides 8 tryptophans at the dimer interface to form two critical pyramid perception centers, the other 18 tryptophans' functional role is unknown.

The molecular origin of high DNA-repair efficiency by photolyase.

The primary dynamics in photomachinery such as charge separation in photosynthesis and bond isomerization in sensory photoreceptor are typically ultrafast to accelerate functional dynamics and avoid energy dissipation. The same is also true for the DNA repair enzyme, photolyase. However, it is not known how the photoinduced step is optimized in photolyase to attain maximum efficiency.

Dynamics and mechanisms of DNA repair by photolyase.

Photolyases, a class of flavoproteins, use blue light to repair two types of ultraviolet-induced DNA damage, a cyclobutane pyrimidine dimer (CPD) and a pyrimidine-pyrimidone (6-4) photoproduct (6-4PP). In this perspective, we review the recent progress in the repair dynamics and mechanisms of both types of DNA restoration by photolyases. We first report the spectroscopic characterization of flavin in various redox states and the active-site solvation dynamics in photolyases.

Dynamics and mechanism of UV-damaged DNA repair in indole-thymine dimer adduct: molecular origin of low repair quantum efficiency.

Many biomimetic chemical systems for repair of UV-damaged DNA showed very low repair efficiency, and the molecular origin is still unknown. Here, we report our systematic characterization of the repair dynamics of a model compound of indole-thymine dimer adduct in three solvents with different polarity. By resolving all elementary steps including three electron-transfer processes and two bond-breaking and bond-formation dynamics with femtosecond resolution, we observed the slow electron injection in 580 ps in water, 4 ns in acetonitrile, and 1.

Dynamic Determination of Active-Site Reactivity in Semiquinone Photolyase by the Cofactor Photoreduction.

Photolyase contains a flavin cofactor in a fully reduced form as its functional state to repair ultraviolet-damaged DNA upon blue light absorption. However, after purification, the cofactor exists in its oxidized or neutral semiquinone state. Such oxidization eliminates the repair function, but it can be reverted by photoreduction, a photoinduced process with a series of electron-transfer (ET) reactions.

Quenching Dynamics of Ultraviolet-Light Perception by UVR8 Photoreceptor.

UVR8 is a recently discovered UV-B photoreceptor with a homodimer as the active state. UV-B perception of an interfacial tryptophan (W285) causes dissociation of the dimer into two functional monomers. Here, we investigate the molecular mechanism behind UV perception by W285 in UVR8.

Determining complete electron flow in the cofactor photoreduction of oxidized photolyase.

The flavin cofactor in photoenzyme photolyase and photoreceptor cryptochrome may exist in an oxidized state and should be converted into reduced state(s) for biological functions. Such redox changes can be efficiently achieved by photoinduced electron transfer (ET) through a series of aromatic residues in the enzyme. Here, we report our complete characterization of photoreduction dynamics of photolyase with femtosecond resolution.

Dynamic determination of the functional state in photolyase and the implication for cryptochrome.

The flavin adenine dinucleotide cofactor has an unusual bent configuration in photolyase and cryptochrome, and such a folded structure may have a functional role in initial photochemistry. Using femtosecond spectroscopy, we report here our systematic characterization of cyclic intramolecular electron transfer (ET) dynamics between the flavin and adenine moieties of flavin adenine dinucleotide in four redox forms of the oxidized, neutral, and anionic semiquinone, and anionic hydroquinone states. By comparing wild-type and mutant enzymes, we have determined that the excited neutral oxidized and semiquinone states absorb an electron from the adenine moiety in 19 and 135 ps, whereas the excited anionic semiquinone and hydroquinone states donate an electron to the adenine moiety in 12 ps and 2 ns, respectively.

Ultrafast dynamics of nonequilibrium electron transfer in photoinduced redox cycle: solvent mediation and conformation flexibility.

We report here our systematic characterization of a photoinduced electron-transfer (ET) redox cycle in a covalently linked donor-spacer-acceptor flexible system, consisting of N-acetyl-tryptophan methylester as an electron donor and thymine as an electron acceptor in three distinct solvents of water, acetonitrile, and dioxane. With femtosecond resolution, we determined all the ET time scales, forward and backward, by following the complete reaction evolution from reactants to intermediates and finally to products. Surprisingly, we observed two distinct ET dynamics in water, corresponding to a stacked configuration with ultrafast ET in 0.

Electron tunneling pathways and role of adenine in repair of cyclobutane pyrimidine dimer by DNA photolyase.

Electron tunneling pathways in enzymes are critical to their catalytic efficiency. Through electron tunneling, photolyase, a photoenzyme, splits UV-induced cyclobutane pyrimidine dimer into two normal bases. Here, we report our systematic characterization and analyses of photoinitiated three electron transfer processes and cyclobutane ring splitting by following the entire dynamical evolution during enzymatic repair with femtosecond resolution.

Dynamics and mechanism of cyclobutane pyrimidine dimer repair by DNA photolyase.

Photolyase uses blue light to restore the major ultraviolet (UV)-induced DNA damage, the cyclobutane pyrimidine dimer (CPD), to two normal bases by splitting the cyclobutane ring. Our earlier studies showed that the overall repair is completed in 700 ps through a cyclic electron-transfer radical mechanism. However, the two fundamental processes, electron-tunneling pathways and cyclobutane ring splitting, were not resolved.

Dynamics and mechanism of repair of ultraviolet-induced (6-4) photoproduct by photolyase.

One of the detrimental effects of ultraviolet radiation on DNA is the formation of the (6-4) photoproduct, 6-4PP, between two adjacent pyrimidine rings. This lesion interferes with replication and transcription, and may result in mutation and cell death. In many organisms, a flavoenzyme called photolyase uses blue light energy to repair the 6-4PP (ref.

Ultrafast solvation dynamics at binding and active sites of photolyases.

Dynamic solvation at binding and active sites is critical to protein recognition and enzyme catalysis. We report here the complete characterization of ultrafast solvation dynamics at the recognition site of photoantenna molecule and at the active site of cofactor/substrate in enzyme photolyase by examining femtosecond-resolved fluorescence dynamics and the entire emission spectra. With direct use of intrinsic antenna and cofactor chromophores, we observed the local environment relaxation on the time scales from a few picoseconds to nearly a nanosecond.