Veratryl radical

LiP catalyzes the degradation of lignin. The enzyme is commonly assayed by its ability to oxidize veratryl alcohol to a diffusible veratryl radical cation that can either oxidize other substrates or is further converted by a second electron removal to the aldehyde (Fig. 5.5). This reaction is thought to mimic the in vivo operation of... 

These findings led to the proposition that the veratryl alcohol is degraded via the quinone intermediates (Figure 5) to CO2 through a series of transformations involving lignin peroxidase, perhydroxy radicals and the NADP-dependent aryl alcohol oxidoreductase. Veratraldehyde, the major product of lignin peroxidase catalyzed veratryl alcohol oxidation, is rapidly reduced back to veratryl alcohol it is the further metabolism of the side products of the oxidative process, viz. the quinones and lactones, that drives the overall transformation towards completion (34). 

LiP catalyzes the oxidation of a low-molecular-weight redox mediator, veratryl alcohol, which in mrn mediates one-electron oxidation of lignin to generate aryl cation radicals [100]. The radicals facilitate a wide variety of reactions such as carbon-carbon cleavage, hydroxylation, demethylation, and so on. Dezotti et al. [101] reported enzymatic removal of color from extraction stage effluents using lignin and horseradish peroxidases immobilized on an activated silica gel support. 

A possible reductive role for veratryl alcohol oxidase is proposed in Figure 5. Laccases from C. versicolor can produce both polymerization and depolymerization of lignin (29). In phenolic lignin model dimers, laccase can perform the same electron abstraction and subsequent bond cleavage as found for lignin peroxidase (30). The phenolic radical is however likely to polymerize unless the quinoid-type intermediates can be removed, for example by reduction back to the phenol. Veratryl alcohol oxidase, in... 

Khindaria A, Yamazaki I, Aust SD (1996) Stabilization of the veratryl alcohol cation radical by lignin peroxidase. Biochemistry 35 6418-6424... 

Khindaria A, Nie G, Aust SD (1997) Detection and characterization of the lignin peroxidase compound II- veratryl alcohol cation radical complex. Biochemistry 36 14181-14185... 

In some enzymes, the protein radical appears to participate in substrate oxidation. Evidence exists for the involvement of a surface tryptophan in the oxidation of veratryl alcohol by the ligninase from Phanerochaete chrysosporium [33]. Similarly, tryptophan radicals on the surface of the versatile peroxidases from Pleurotus eryngii and Bjerkandera adjusta [34—36], and a tyrosine in the LiP from Trametes cervina [33], are thought to be involved in substrate oxidation. 

Fig. 5.5 Oxidation of veratryl alcohol to a radical cation that can oxidize other substrates (RX) or can be further oxidized to give veratryl aldehyde...

As already mentioned, in some enzymes radicals generated on surface tryptophan and tyrosine radicals by electron transfer to the ferryl species are involved in abstraction of electrons from substrates [33-36, 40]. Mutation of Trpl71 on the surface of P. chrysosporium LiP to a phenylalanine or serine completely suppresses the veratryl alcohol oxidizing activity of the enzyme [40]. A similar depression in the oxidation of veratryl alcohol occurs on mutation of Trpl64 in the versatile peroxidase from P. eryngii [34, 35]. 

Analogous behavior is also observed with the corresponding 3,4-dimethoxy derivatives [102] with these derivatives, however, because of the increased stability of the radical cations, in acidic solution a decrease in reactivity of three orders of magnitude for the deprotonation reaction is observed on going from MBA to the 3,4-dimethoxybenzyl alcohol radical cation (veratryl alcohol, VA). In the presence of OH the deprotonation site shifts from carbon to oxygen, as previously described for the 4-methoxy derivatives, and a dramatic increase in reactivity is observed, VA + reacting with OH with A = 1.3 x 10 M s (Table 5), a value which is lower than the diffusion limit and which, compared with that measured for MBA +, rather than the decreased O-H acidity of VA , presumably reflects the increased electron density on the aromatic system which disfavors the intramolecular (side-chain to nucleus) electron transfer eventually leading to the benzyloxyl radical. 

Elder, T. 1997. Oxidation of a lignin model compound by the veratryl alcohol cation radical. Results from molecular orbital calculations. Holrforschung 51(l) 47-56. 

The addition of 0.5 equivalent of a two-electron reducing agent like veratryl alcohol generates LiP-Cpd II (with a Soret band at 420 nm), which corresponds to the iron(IV) 0x0 without oxidation of the porphyrin ligand. Site-directed mutations have demonstrated a key role for Trp-171 in veratryl alcohol oxidation, this amino acid being probably the location of an intermediate radical cation in the oxidation cascade. 

Laccases typically oxidize the phenohc units of lignin. Lignin peroxidases (LiPs) oxidize both nonphenohc hgnin structures and veratryl alcohol (VA), a metabolite synthesized by fungi that helps liP to avoid inactivation by H2O2 and whose radical... 

In addition to catalyzing the oxidation of many compounds, LiP is also able to catalyze reductive reactions in the presence of electron donors such as EDTAor oxalate (Fig. 7) (6, 77). Veratryl alcohol is a free radical mediator in these reactions. The electron donors appear to be oxidized by a LiP generated veratryl alcohol cation radical. The resulting anion radical can catalyze the reduction of good electron acceptors such as cytochrome c, nitroblue tetrazolium, and oxygen. Evolution of CO2 from EDTA or oxalate effectively drives the reductive reactions. Similar reactions have also been observed with the manganese dependent peroxidases in the presence of quinones (20). Early work performed in our laboratory showed that these reductive mechanisms are not involved in TNT reduction. However, these reactions may be involved in other steps in TNT metabolism. 

Figure 10. The catalytic cycle of the lignin peroxidase (Wariishi and Gold (54), modified). The thick arrows denote the catalytic cycle in the presence of H2O2 and a reducing substrate like veratryl alcohol. VA = veratryl alcohol cation radical = superoxide radical.

Veratryl alcohol plays an important role as a radical cation mediator in lignin biodegradation (18). The oxidative decarboxylation of oxalate (1) or EDTA (42) by the lignin peroxidase is mediated by the veratryl alcohol cation radical (VA ) leading to an apparent inhibition of the veratryl alcohol oxidase activity of the lignin peroxidase. 

The effect of the initial concentration of veratryl alcohol on the duration of the lag period during oxidation of hydroxylaminodinitrotoluene was investigated (Fig. 11). The length of the lag period was directly related to the veratryl alcohol concentration, up to 2.8 mM, thus indicating its role as a radical cation mediator in the oxidation of hydroxylaminodinitrotoluene. Surprisingly, the activity of the lignin peroxidase that reappeared was also affected by the concentration of veratryl alcohol. The rates were lower when veratryl alcohol was below 1.1 mM (Fig. 11). A value of 167 pM was determined for veratryl alcohol and lignin peroxidase H2 at pH 3.5 (50). 

Harvey, P. J., H. E. Schoemaker, and J. M. Palmer. 1986. Veratryl alcohol as a mediator and the role of radical cations in lignin biodegradation by Phanerochaete chrysosporium. FEBS Lett. 195 242-246. 

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