Análisis de la Gestión o actividad

Knowledge of the low molecular weight iron binding compounds and their mechanism of action in the basidiomycetes is limited but they have been successfully extracted from several species (Table 16). Low molecular iron binding compounds and phenolic lignin breakdown products play a crucial role in the Fenton reaction. Various non-enzymatic low molecular weight iron-reducing compounds have been reported in previous studies, which targeted macromolecular cell wall to attack. The

functional role of these compounds in the degradative system varies from laccase mediator to Fe reducing compound as shown in Table 16.

Table 16. Various compounds released by fungi, which are involved in the break down of lignocellulose and their mechanism of action.

Type of compounds Functional

degradative system

Reference

3-Hydroxy-anthranilic acid Laccase mediator

Arantes et al., (2009)

Veratryl alcohol AVA/LiP

mediator Arantes et al., (2009) 2Cl-1,4DMB (2-cloro-1,4- dimethoxy benzene 2Cl- 1.4DMB/LiP mediator/oxalate Mn3+ Arantes et al., (2009)

Linoleic acid Fatty acid

peroxidation

Arantes et al., (2009) Carboxylic acid Oxalate direct

attack Arantes et al., (2009) 2,5-dimethoxyhydroquinone (2,5-DMHQ) Fe reduction agent Kerem at al., (1999); Jensen et al., (2001) 2,5-dimethoxy-1,4- benzoquinone (2,5-DMBQ) Fe reduction agent Kerem at al., (1999); Shimonokawa et al., (2004); Jensen et al., (2001) 4,5-dimethoxy-1,2- benzoquinoene (4,5-DMBQ) Fe reduction agent Kerem et al., (1999); Shimokawa et al., (2004) 4,5-dimethoxy-1,2-benzenediol (DMC) Fe reduction agent Paszczynski et al., (1999) 2,5-dimethoxyl-1,4-benzenediol (DMH) Fe reduction agent Paszczynski et al., (1999) 2,3 dihydroxynenzoic acid (DHBA) Fe reduction agent Goodell et al., (1997) 3,4-dihydrophenylacetic acid (DOPAC) Fe reduction agent Arantes and Milagres (2006)

Basiodiomycetes can produce more than one iron reducing agent, for example, Gloeophyllum trabeum not only released 2,5-DMHQ (2,5- dimethoxyhydroquinone) but also other quinones such as 4,5-dimethoxy-1,2- benzoquinone (4,5-DMBQ) and 4,5-dimethoxycatechol (DMC) (Jensen et al., 2001)

and it has been referred to as the Gt chelator (Goodell et al., 1997; Shimokawa et al., 2004). In general, these compounds have an aromatic C ring with different arrangement of aryl or alkyl bonds (Figure 34).

Figure 34. The chemical structure of low molecular quinone compounds from Gloeophyllum trabeum (Jensen et al., 2001)

The quinones produced by brown rot fungi have a strong affinity for Fe3+ and could mediate the redox cycling of iron under the low pH conditions associated with fungal cultures. The reduction of Fe3+ to Fe2+, is considered important for initiation of the Fenton reaction. In brown rot fungi, Fe2+ will react with H2O2 to produce active oxygen species, which is involved in the breakdown of lignocellulose (Paszczynski et al., 1999).

According to Jensen et al. (2001), when extracellular quinone or hydroquinone was involved, iron reduction in the brown rot G. trabeum can be explained by the following reactions:

(1) Fe2+ + H2O2 + H+ → Fe3+ + H2O + ·OH (2) H2Q + Fe3+ ↔ HQ· + H+ + Fe2+

(3) HQ· + O2 ↔ Q + ·OOH (4) HQ· + Fe3+ ↔ Q + H+ + Fe2+ (5) Q + 2e- + 2H+ → H2Q

From the reaction, the extracellular quinone or hydroquinones reduce Fe3+, followed by the production of quinone or hydroquinone radicals (HQ·) which then act with O2 to produce the perhydroxy radical (·OOH). Remaining quinone or hydroquinone radicals (HQ·) may also react to reduce the available Fe3+, and finally the quinone undergoes cyclic oxidation-reduction reaction. Numerous studies have tried to quantify quinone or hydroquinone production in both in white and brown rot fungi. The total production of 2,5 dimethoxyhydroquinone and 4,5- dimethoxycatechol in Gloeophyllum striatum reached 43 µM (7.4 mg l-1) (Kramer et al.,2004), while 2,5 dimethoxyhydroquinone and 4,5-dimethoxycatechol levels in G. trabeum, ranged between 51µM to 17µM (Jensen et al., 2002).

2,3 dihydroxybenzoic acid (2,3-DHBA) has been found in Gloeophyllum trabeum. It has been shown that this acts as a phenolate chelator compound with the capacity to reduce the iron complex (Arantes et al., 2009). The mechanism of iron reduction by 2,3-DHBA has been described by Xu and Jordan, (1988), and involves two steps. First, Fe3+ is bound to DHBA, which is followed by the creation of a DHBA-Fe3+ complex, which is subsequently oxidized. The second step involves the simultaneous reduction of Fe3+ by an intermediate quinone form.

Cultures of brown rot fungi often contain significant amounts of oxalic acid, while the white rot fungi have little or no oxalic acid produce during the growth (Kaneko et al., 2005). Oxalic acid is an organic acid commonly occurring in plants, animals and fungi, which play different roles in different organisms. The role of oxalate as a chelating agent in basidiomycetes is unclear (Varela and Tien, 2003). A difference in oxalic acid production between white and brown rot fungi and its potential role in modifying the acidity of the environment has been previously shown (Goodell et al., 2002). In wood decay by white rots the oxalic acid may play multiple roles such as an inhibitor of lignin peroxidase, electron donor for production of NADH, a source of radicals to reduce dioxygen or ferric iron to yield the ferrous iron, and as a chelating agent for lignin degradation. While in the brown rot fungi, oxalic acid may serve as a proton source for enzymatic and non enzymatic hydrolysis of carbohydrates and as a metal chelator (Shimada et al., 1997). Furthermore, Varela and Tien (2003) proposed that the role of oxalic acid was to assist in hydroxyl radical formation (Figure 35) but that higher concentrations had the opposite role.

Figure 35. A schematic showing how a superoxide can be generated through the degradation of oxalic acid (Varela and Tien, 2003).

The oxalic acid binds with the hydroxyl radical (·OH) to form an oxalic acid radical, which can release CO2. The C=O bond radical then reacts with O2 to produce superoxide (Figure 35).

Although similar chelating mechanisms are observed in other types of white or other brown rot fungi, different fungal types may produce different amounts and types of quinone or hydroquinone as Fe reducing agents. In this current work the quinones produced by S. lacrymans culture were examined, in conjunction with the quantity of oxalic acid production during culture. This study showed that S.lacrymans was similar to the brown rot G. trabeum in its production of hydroquinones. This suggests that they both are capable of degrading wheat straw biomass using non-enzymatic conditions or in combination with fungal extracts. These may consist of various enzymes, oxalic acid and quinones or hydroquinone.