Effects of ionizing radiation on tyrosine

Box, Harold C.; Budzinski, Edwin E.; Freund, Harold G.

doi:10.1063/1.1682295

Cited by 43 publications

(66 citation statements)

References 15 publications

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“…With the Mn cluster removed by NH2OH, the rereduction of Yz* is slowed sufficiently to allow some fraction to be trapped by quick freezing (36) DOR spectruni displayed in trace a in Fig. 3 is not obtained at field Biophysics: Gilchrist et aL biological and model tyrosine radicals (30,33,34,37). However, the strong transition at -32 MHz is shifted up in frequency significantly from the strongly coupled f3 methylene proton peak (-29 MHz) of YD-(trace c in Fig.…”

Section: Methodsmentioning

confidence: 98%

Proximity of the manganese cluster of photosystem II to the redox-active tyrosine YZ.

Gilchrist

Ball

Randall

et al. 1995

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

266

309

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Electron spin echo electron-nuclear double resonance (ESE-ENDOR) experiments performed on a broad radical electron paramagnetic resonance (EPR) signal observed in photosystem II particles depleted of Ca2+ indicate that this signal arises from the redox-active tyrosine Yz. The tyrosine EPR signal width is increased relative to that observed in a manganese-depleted preparation due to a magnetic interaction between the photosystem II manganese cluster and the tyrosine radical. The manganese cluster is located asymmetrically with respect to the symmetry-related tyrosines Yz and YD. The distance between the Yz tyrosine and the manganese cluster is estimated to be approximately 4.5 A. Due to this close proximity of the Mn cluster and the redox-active tyrosine Yz, we propose that this tyrosine abstracts protons from substrate water bound to the Mn cluster.The photosystem II (PS II) component of the plant photosynthetic apparatus oxidizes water at an oxygen-evolving complex (OEC) that consists of a tetranuclear cluster of manganese ions along with essential cofactors calcium and chloride (1). The overall architecture of PS II shows homologies to the reaction centers of the purple non-oxygen-evolving bacteria, including the C2 symmetry found in these reaction centers (2-4). In PS II this symmetry appears to persist to the sites of two important tyrosine residues, Yz (tyrosine-161 of the Dl polypeptide, with residues designated for Synechocystis sp. 6803) and YD (tyrosine-160 of the D2 polypeptide) (5-7). The Yz tyrosine serves as an electron transfer intermediate between the chargeseparating chlorophyll moiety P680 and the manganese cluster of the OEC. The symmetry-related tyrosine YD, which is typically present as a dark-stable neutral radical (YD-), is bypassed in the fast electron transfer between P680 and the OEC. Whether the Mn cluster is located on the C2 symmetry axis has been the subject of much debate. Instead, the PS II symmetry may be broken at the OEC level, locating the Mn cluster closer to the active electron transfer intermediate Yz.Calcium depletion of PS II particles by NaCl/EGTA washing (8-10) or low-pH citrate treatment (11-13) eliminates oxygen-evolving activity. In such Ca2+-depleted PS II particles, illumination at a temperature of 273 K leads to the formation of a broad (130-180 G full width at half maximum) g = 2 EPR signal. Other treatments that block oxygen evolution such as acetate or fluoride incubation lead to similar signals upon such illumination, though the signal widths vary appreciably, depending on the details of the treatment and the resulting extrinsic polypeptide composition (14-18). The broad g = 2 signal is thought to arise from a radical center, with the large linewidth caused by a magnetic interaction with the Mn cluster (18,19). UV (19) and IR (20) absorption changes in PS II that are observed concomitantly with the formation of the radical have been interpreted to favor an oxidized histidine as the origin of the broad radical signal. Alternatively, on the basis of EPR s...

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Section: Methodsmentioning

confidence: 98%

Proximity of the manganese cluster of photosystem II to the redox-active tyrosine YZ.

Gilchrist

Ball

Randall

et al. 1995

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

266

309

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“…The ENDOR spectrum of a histidyl radical is known, and both the ring (a) and methylene (/) protons exhibit hfs couplings far too large to correspond with those observed for ES (34). ENDOR results for tyrosyl show that the a-proton couplings are within the range of values for ES but that the f-couplings would likely be larger (20).…”

Section: Methodsmentioning

confidence: 99%

“…Phenoxy radicals normally have g shifts much smaller than that of ES, with bgI < 0.01 for all i = x,y,z (19); the tyrosyl radical, either neutral or ionized, is not reported to have an unusual anisotropy (20). Nitrogen heterocycle radicals likewise have the typically small wx-radical g shifts, bgiI < 0.01 (21).…”

Section: Methodsmentioning

confidence: 99%

Electron-nuclear double resonance of the hydrogen peroxide compound of cytochrome c peroxidase: identification of the free radical site with a methionyl cluster.

Hoffman¹,

Roberts²,

Brown³

et al. 1979

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

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The results of electron-nuclear double resonance and electron paramagnetic resonance (EPR) studies on the hydrogen peroxide compound of yeast cytochrome c peroxidase are inconsistent with previous proposals for the source of the EPR signal in this compound, in particular with its identification with an aromatic amino acid radical such as would arise by oxidation of a tryptophanyl side chain. The present observations lead us to propose that the EPR signal is associated with a cluster containing at least one methionine and in which proximate side chains share the charge created by loss of one electron.

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“…The spin density distribution of the enzyme radical is similar to that of oxidized tyrosine (26)(27)(28). Tyrosine is an easily oxidizable amino acid and the oxidized tyrosyl radicals have been observed in several studies using 1-electron oxidation (26,29,30).…”

Section: The Tyrosyl Radical Spectroscopic Properties and Structurementioning

confidence: 62%

The Tyrosyl Free Radical in Ribonucleotide Reductase

Gräslund¹,

Sahlin²,

Sjöberg³

1985

Environmental Health Perspectives

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The enzyme, ribonucleotide reductase, catalyses the formation of deoxyribonucleotides from ribonucleotides, a reaction essential for DNA synthesis in all living cells. The Escherichia coli ribonucleotide reductase, which is the prototype of all known eukaryotic and virus-coded enzymes, consists of two nonidentical subunits, proteins Bi and B2. The B2 subunit contains an antiferromagnetically coupled pair of ferric ions and a stable tyrosyl free radical. EPR studies show that the tyrosyl radical, formed by loss of an electron, has its unpaired spin density delocalized in the aromatic ring of tyrosine. Effects of ironradical interaction indicate a relatively close proximity between the iron center and the radical. The EPR signal of the radical can be studied directly in frozen packed cells of E. coli or mammalian origin, if the cells are made to overproduce ribonucleotide reductase.The hypothetic role of the tyrosyl free radical in the enzymatic reaction is not yet elucidated, except in the reaction with the inhibiting substrate analogue 2'-azido-CDP. In this case, the normal tyrosyl radical is destroyed with concomitant appearance of a 2'-azido-CDP-localized radical intermediate. Attempts at spin trapping of radical reaction intermediates have turned out negative.In E. coli the activity of ribonucleotide reductase may be regulated by enzymatic activities that interconvert a nonradical containing form and the fully active protein B2. In synchronized mammalian cells, however, the cell cycle variation of ribonucleotide reductase, studied by EPR, was shown to be due to de novo protein synthesis.Inhibitors of ribonucleotide reductase are of medical interest because of their ability to control DNA synthesis. One example is hydroxyurea, used in cancer therapy, which selectively destroys the tyrosyl free radical.

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Effects of ionizing radiation on tyrosine

Cited by 43 publications

References 15 publications

Proximity of the manganese cluster of photosystem II to the redox-active tyrosine YZ.

Proximity of the manganese cluster of photosystem II to the redox-active tyrosine YZ.

Electron-nuclear double resonance of the hydrogen peroxide compound of cytochrome c peroxidase: identification of the free radical site with a methionyl cluster.

The Tyrosyl Free Radical in Ribonucleotide Reductase

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