Electrochemical properties of tyrosine phenoxy and tryptophan indolyl radicals in peptides and amino acid analogs

DeFelippis, Michael R.; Murthy, Cherla P.; Broitman, Federico; Weinraub, D.; Faraggi, M.; Klapper, Michael H.

doi:10.1021/j100161a081

Cited by 171 publications

(235 citation statements)

References 3 publications

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“…6B and Table S1), respectively. The derived reduction potential of 1.24 V for W306 + /W306 is larger than that of tryptophan in aqueous solution, 1.15 V vs. NHE (30), consistent with its surface-exposed position with a less aqueous environment. Clearly, along the tryptophan triad, a strong reduction-potential gradient exists from the higher W382 to lower W306, driving unidirectional electron flow from W306 to W382 and resulting in ultrafast cascade ET dynamics.…”

Section: Characterizing Terminal Electron Donor (W306) and Completingmentioning

confidence: 55%

Determining complete electron flow in the cofactor photoreduction of oxidized photolyase

Liu¹,

Tan

Guo³

et al. 2013

Proc. Natl. Acad. Sci. U.S.A.

170

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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. With various site-directed mutations, we identified all possible electron donors in the enzyme and determined their ET timescales. The excited cofactor behaves as an electron sink to draw electron flow from a series of encircling aromatic molecules in three distinct layers from the active site in the center to the protein surface. The dominant electron flow follows the conserved tryptophan triad in a hopping pathway across the layers with multiple tunneling steps. These ET dynamics occur ultrafast in less than 150 ps and are strongly coupled with local protein and solvent relaxations. The reverse electron flow from the flavin is slow and in the nanosecond range to ensure high reduction efficiency. With 12 experimentally determined elementary ET steps and 6 ET reaction pairs, the enzyme exhibits a distinct reduction-potential gradient along the same aromatic residues with favorable reorganization energies to drive a highly unidirectional electron flow toward the active-site center from the protein surface.protein electron transfer | flavin photoreduction | femtosecond dynamics | electron flow directionality | reduction potential funnel P hotolyase and cryptochrome are evolutionally related and contain a flavin adenine dinucleotide (FAD) as the catalytic cofactor with a unique bent structure in the active sites, but the two perform different functions: photolyase repairs UV-damaged DNA and cryptochrome functions as a photoreceptor for regulation of plant growth or synchronization of circadian rhythm (1-5). The active state of the cofactor in vivo is in the anionic hydroquinone form (FADH -) in photolyase (6), but currently the redox status of flavin in cryptochrome is under debate with some studies suggesting flavin to be in oxidized (FAD), whereas others claiming anionic (FAD -/FADH -) states for the functional form in vivo (7-10). However, in vitro, the cofactor is oxidized to FAD and/or FADH • in photolyase but only appears in the oxidized FAD state in cryptochrome. Thus, it appears that reduction of the oxidized FAD is necessary for its transformation into the active state. Photoinduced electron transfer (ET) is an effective way for such redox state changes and it is conceivable that the photoreduction of FAD could be a primary process for signal initiation in cryptochrome (11).In this study, we used Escherichia coli photolyase (EcPL) as a model system for systematic studies of electron flow into the excited cofactor FAD. As shown in Fig. 1, more than 10 aromatic acid residues (W and Y) encircle the flavin cofactor around the active site (12). These aromatic residues not only are cri...

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Section: Characterizing Terminal Electron Donor (W306) and Completingmentioning

confidence: 55%

Determining complete electron flow in the cofactor photoreduction of oxidized photolyase

Liu¹,

Tan

Guo³

et al. 2013

Proc. Natl. Acad. Sci. U.S.A.

170

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“…Tyrosine is more readily oxidized when its phenolic group is deprotonated. Reported redox potentials are 1.2 V, 0.93 V, and 0.72 V at pH 2, 7, and 13, respectively (DeFelippis et al, 1991;Harriman, 1987). We can assume that, in our experiments (below 9.5), the midpoint tyrosine potential is closed to 1 V. Hence, a good radical mediator for tyrosine oxidation should display a midpoint potential exceeding 1 V. Very few phenols have a midpoint potential high enough to oxidize tyrosine.…”

Section: Discussionmentioning

confidence: 68%

Wheat prolamine crosslinking through dityrosine formation catalyzed by peroxidases: Improvement in the modification of a poorly accessible substrate by ?indirect? catalysis

Michon

Wang

Ferrasson

et al. 1999

Biotechnol. Bioeng.

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“…13 and refs. therein) and for Trp͞Trp ⅐ϩ , it is Ϸϩ1.15 V vs. NHE (14). Knowing the 0-0 transition (S 1 4S 0 ) energy of 2.3 eV, we obtained a net ⌬G°of Ϫ0.75 eV.…”

Section: Resultsmentioning

confidence: 85%

Femtosecond dynamics of a drug–protein complex: Daunomycin with Apo riboflavin-binding protein

Zhong

Pal

Wan

et al. 2001

Proc. Natl. Acad. Sci. U.S.A.

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In this contribution, we report studies of the primary dynamics of the drug-protein complexes of daunomycin with apo riboflavinbinding protein. With femtosecond resolution, we observed the ultrafast charge separation between daunomycin and aromatic amino acid residues of the protein, tryptophan(s). Electron transfer occurs from tryptophan(s) to daunomycin with two reaction times, 1 ps and 6 ps, depending on the local complex structure. The formation of anionic daunomycin radical is crucial for triggering a series of chemical reactions in redox cycling. One of the subsequent reactions is the reduction of dioxygen to form active superoxide by the reduced daunomycin. This catalytic process was found to occur within 10 ps. In the absence of dioxygen, charge recombination takes a much longer time, more than 100 ps. These results, along with similar findings in DNA and nucleotides, elucidate that the ultrafast generation of reduced daunomycin radicals by photoactivation is a primary step for the observed photoenhancement of drug cytotoxicity by several orders of magnitude. We also studied the dependence of the dynamics on protein conformations at different ionic strengths and denaturant concentrations. We observe a sharp transition from the tertiary structure to the unfolding state at 2 M of denaturant concentration. D aunomycin (DM, Fig. 1), an anthracycline antibiotic, is one of the effective drugs used for cancer chemotherapy. It has been shown that the cytotoxicity of DM is enhanced by several orders of magnitude with irradiation of light, but the mechanism by photoactivation is still unknown (1-3). In recent studies from this group (X. Qu, C.W., H.-C. Becker, D.Z., and A.H.Z., unpublished work), we reported the striking observation of the primary dynamics of DM when it intercalates into GC base pairs of DNA or binds to nucleotides. Ultrafast electron-transfer (ET) reactions between DM and the base G were found to occur on the femtosecond to picosecond time scales (4). The reduced DM radicals can catalyze superoxide formation from dioxygen (O 2 ), which then triggers redox cycling. Thus, molecular oxygen becomes an integral part of the drug function.Here, we report our studies of the dynamics of DM with a protein. Many proteins have hydrophobic domains and can interact with DM molecules. Aromatic or sulfur-containing amino acid residues in proteins, such as Trp, Tyr, or cysteine, can often function as reductants and donate electrons. One of the flavoproteins, apo riboflavin-binding protein (apoRfBP) and a riboflavin carrier in plasma, has several Trp residues in its hydrophobic cleft (20 Å wide and 15 Å deep). This protein, a single polypeptide chain of 219 amino acids, was chosen as a prototype (Fig. 1) and the complex of DM-apoRfBP as a model system.As mentioned in the preceding paper (5), the x-ray crystal structure reveals a compact stack of riboflavin (vitamin B 2 ) sandwiched between Trp-156 and Tyr-75 at an equal interplanar distance of 3.7 Å in the hydrophobic cleft of apoRfBP (6). Ultrafast ET between rib...

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Electrochemical properties of tyrosine phenoxy and tryptophan indolyl radicals in peptides and amino acid analogs

Cited by 171 publications

References 3 publications

Determining complete electron flow in the cofactor photoreduction of oxidized photolyase

Determining complete electron flow in the cofactor photoreduction of oxidized photolyase

Wheat prolamine crosslinking through dityrosine formation catalyzed by peroxidases: Improvement in the modification of a poorly accessible substrate by ?indirect? catalysis

Femtosecond dynamics of a drug–protein complex: Daunomycin with Apo riboflavin-binding protein

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