Tyrosine radical reactions

Figure 1.19 Tyrosine and phenylalanine residues can undergo oxidation to modify their phenyl side-chain groups. Tyrosine can form covalent dimers that link two side chains together via a radical reaction. Both tyrosine and phenylalanine can be modified by oxidation to add oxygen-containing groups directly to their aromatic ring.

Other very convincing evidences for free radical-mediated mechanism of decomposition and reactions of peroxynitrite and nitrosoperoxocarboxylate were demonstrated by Lehnig [140] with the use of CIDNP technique. This technique is based on the effects observed exclusively for the products of free radical reactions their NMR spectra exhibit emission characterizing a radical pathway of their formation. Lehnig has found the enhanced emission in the 15N NMR spectra of N03- formed during the decomposition of both peroxynitrite and nitrosoperoxocarboxylate. This fact indicates that N03- was formed from radical pairs [ N02, H0 ] and [ N02, C03 ]. Emission was also observed in the reaction of both nitrogen compounds with tyrosine supposedly due to the formation of radical pair [ N02, tyrosyl ]. 

In eukaryotes, ribonucleotide reductase is a tetramer consisting of two R1 and two R2 subunits. In addition to the disulfide bond mentioned, a tyrosine radical in the enzyme also participates in the reaction (2). It initially produces a substrate radical (3). This cleaves a water molecule and thereby becomes radical cation. Finally, the deoxyribose residue is produced by reduction, and the tyrosine radical is regenerated. 

The Tyrosine Radicals. - In PS II two tyrosine radicals Y,D and Y z are known. The position of YD and Yz within PS II has been determined by X-ray crystallography 7,19,341 see Fig. 1. Yz is located in between P680 and the tetranuclear Mn-cluster (Mm) in the D1 protein. It is involved in ET between these 2 components and is only observed as a transient radical or in the form of a split signal (see below). YD is found in a symmetry equivalent position to Yz in a hydrophobic pocket of the D2 protein. The Y D is stable in the dark for hours. Y D undergoes slow redox reactions with the lower S states of the OEC (see 4.7) in the dark but is not involved in the main ET pathway that leads to water oxidation. Its detailed function is not fully understood. 

The iron protein has been found in animals, viruses and some prokaryotic organisms. The best studied example is from Escherichia coli.S19 The enzyme consists of two subunits, Bi and B2. Subunit B, has a molecular weight of 160 000 and is made up of two polypeptide chains. It contains redox-active SH groups, which play a role in the reaction, and has two binding sites for substrates and sites for effector molecules. It can bind any of the four ribonucleotide substrates. Subunit B2 has a molecular weight of 78 000 and also has two polypeptide chains. It contains a dimeric oxo-bridged iron site associated with a remarkably stable tyrosine radical. The formation of the active enzyme from B, and B2 requires the presence of magnesium ions. 

A case in point is the combination of a Thy -OH-adduct with a tyrosine-derived phenoxyl radical [reaction (186) Simic and Dizdaroglu 1985],... 

Electrons to reaction centre via tyrosine radical cation H to the aqueous phase... 

Galactose oxidase is an extracellular enzyme secreted by the fungus Dactylium den-droides. It is monomeric (M = 68000), contains a single copper site and catalyses the oxidation of a wide range of primary alcohols to the corresponding aldehydes. The two-electron transfer reaction RCH2OH - RCHO + 2H+ + 2e does not utilise a Cu(III)/Cu(I) couple, but a second redox site, involving a tyrosine radical which mediates the transfer of the second electron. 

Freeze-quenching technique in combination with ESR and Mossbauer spectroscopy was used for monitoring intermediates in the reaction of substrate free 57Fe-P450C8Itl with peroxy acetic acid (Schunemann et al., 2000). In such a condition, the oxidant oxidized the enzyme active site iron (III) to iron (VI) and Tyr 96 into tyrosine radical, 90% and 10% from the starting material, respectively. Thus the tyrosine residue may be involved in the catalytic process. 

Figure 2. Paths of electron transfer in PSII P680, reaction-center chlorophyll that functions as the primary electron donor P680, first excited singlet state ofP680 Pheo, pheophytin QA, primary quinone electron acceptor QB, secondary quinone electron acceptor cyt b559, cytochrome b559 Chlz, redox-active chlorophyll that mediates electron transfer between cytochrome b559 and P680 YD, redox-active tyrosine that gives rise to the dark-stable tyrosine radical Yz, redox-active tyrosine that mediates electron transfer from the Mn complex to P680.

Hydroxyl radical may hydroxylate tyrosine to 3,4-dihydroxyphenylalanine (DOPA). DOPAs are the main residues corresponding to protein-bound reducing moieties able to reduce cytochrome c, metal ions, nitro tetrazolium, blue and other substrates (S32). Reduction of metal ions and metalloproteins by protein-bound DOPA may propagate radical reactions by redox cycling of iron and copper ions which may participate in the Fenton reaction (G9). Abstraction of electron (by OH or peroxyl or alkoxyl radicals) leads to the formation of the tyrosyl radical, which is relatively stable due to the resonance effect (interconversion among several equivalent resonant structures). Reaction between two protein-bound tyrosyl radicals may lead to formation of a bityrosine residue which can cross-link proteins. The tyrosyl radical may also react with superoxide, forming tyrosine peroxide (W13) (see sect. 2.6). 

The original reaction is between a MnOH species and a tyrosine radical forming a MnO moiety. The process is known as a proton coupled electron transfer (PCET) and this reaction step is modelled by the process depicted in Figure 3.131. 

AP0-B2 takes up Fe from the medium, and the tyrosine radical is generated spontaneously if dioxygen is present. The dioxygen may be reduced to give the 0x0 bridging group, with concomitant oxidation of two Fe and tyrosine. These redox reactions are represented in equation (26). 

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