Role of manganese and veratryl alcohol in the ligninolytic system of Bjerkandera sp. strain BOS55

Abstract

IntroductionLignin is a three dimensional hydrophobic plant polymer derived from the random coupling of phenylpropanoid precursors. The chemical and physical characteristics of lignin require a nonspecific, extracellular oxidative process for biodegradation. White rot basidiomycetes are the only group of organisms having an efficient extracellular ligninolytic system. These fungi produce peroxidases and laccases that are involved in the initial attack of lignin. The peroxidases work with H 2 O 2 which is also enzymatically produced by the fungi by different H 2 O 2 -producing oxidases. For their proper operation, peroxidases require appropriate cofactors which are the best substrates of the enzymes. Lignin peroxidase (LiP) uses the de novo produced secondary metabolite, veratryl alcohol (3,4-dimethoxybenzyl alcohol) as cofactor; whereas, the cofactor of manganese peroxidase (MnP) is the metal Mn(II) which occurs naturally in wood. The in vitro oxidation of lignin preparations by LiP or MnP is only feasible in the presence of veratryl alcohol or manganese, respectively. Additionally, simple aliphatic organic acid metabolites such as oxalate are involved in lignin degradation. On the one hand, oxalate is a good chelator of Mn(III) generated by MnP. This complex is relatively stable forming a low molecular weight diffusible oxidant capable of oxidizing phenolic lignin. Oxalate is also oxidized by ligninolytic enzymes in the presence of their cofactors generating reactive oxygen species that possibly participate in the oxidation of lignin.The low molecular weight cofactors also influence the physiological regulation of white rot fungi. Veratryl alcohol is known to increase LiP activities in white rot fungal cultures, although, veratryl alcohol itself does not induce lip gene transcription. Manganese is essential for the induction of mnp gene expression and MnP activity in various white rot fungi. In contrast, manganese lowers LiP production so that the highest LiP activities are measured under manganese deficiency. Nonetheless manganese has no direct repressive effect on lip gene transcription.Despite the great research efforts conducted so far on various white rot fungi, the true mechanism of lignin degradation is still not fully understood. However, it is becoming clear that low molecular weight cofactors are important catalytic and physiological regulating components of the ligninolytic system.This thesis was dedicated to study the interrelationship between low molecular weight cofactors and ligninolytic enzymes. Bjerkandera sp. strain BOS55 was used as the fungus of study, since it was found to be an outstanding producer of ligninolytic enzymes and a great variety of secondary aryl metabolites. Moreover, this strain was observed to be a good degrader of polycyclic aromatic hydrocarbons (PAH) and biobleacher of kraft pulp in various screenings. The main objective of the thesis was to gain insight into the role of veratryl alcohol and manganese in regulating the physiology and participating in the ligninolytic system of Bjerkandera sp. strain BOS55.Chapter 2In Chapter 2, it was demonstrated for the first time that manganese inhibits the biosynthesis of veratryl alcohol in white rot fungi. This explains at least in part the general observation that LiP production is lowered in the presence of manganese although manganese itself does not repress the transcription of lip genes. The ten-fold increase in veratryl alcohol biosynthesis caused by manganese deficiency stimulated the LiP titres by protecting the enzyme from inactivation by physiological levels of H 2 O 2 . Adding veratryl alcohol to manganese containing cultures of the fungus sustained high LiP titres similar to that found under manganese deficient conditions. Moreover, a good correlation was observed between the LiP titres and veratryl alcohol concentrations irrespective of whether veratryl alcohol was produced by the fungus or added exogenously.Chapter 3In Chapter 3, the mechanism resulting in manganese inhibition of veratryl alcohol biosynthesis was studied. Potential biosynthetic precursors of veratryl alcohol were added to manganese deficient and sufficient cultures of Bjerkandera sp. strain BOS55 in order to bypass the inhibited step. The addition of fully methylated precursors (veratrate, veratraldehyde) equally increased the production of veratryl alcohol irrespective of the manganese concentration. This observation indicated that the reduction of the benzylic acid to benzyl alcohol group is not inhibited by manganese. All the other known precursors such as phenylalanine, cinnamate, benzoate/benzaldehyde as well as the partially hydroxylated benzylic compounds (e.g. 3-hydroxybenzoate, 4-hydroxybenzoate, protocatechuate, vanillate, isovanillate) did increase the veratryl alcohol production in the presence of manganese but never as much as that under manganese deficiency. From these observations it was concluded that no single step along the biosynthetic pathway was inhibited by the presence of manganese. Instead, the availability of phenolic precursors is limited when manganese was added. From this study, we also learned that there are several alternative precursors resulting in increased veratryl alcohol production which can potentially originate from lignin degradation. In addition, it was demonstrated that many of the veratryl alcohol precursors (phenylalanine, cinnamate, benzoate, 4-hydroxybenzoate) enhanced the production of anisyl and chloroanisyl metabolites indicating the existence of common precursors in these biosynthetic pathways. Deuterium labelled benzoate and 4-hydroxybenzoate were converted to a broad spectrum of labelled aryl metabolites.Chapter 4In Chapter 4, the interrelationship between cofactors and peroxidases in cultures grown on natural substrates (beech wood and hemp stem wood sawdust) was studied. Beech wood and hemp stem wood substrates, which contain 6 and 25 mg kg -1dry wood of soluble manganese, respectively were favourable for MnP production. Many studies in the past have failed to demonstrate the presence of LiP on natural substrates even in fungi having lip genes. Surprisingly, Bjerkandera sp. strain BOS55 produced very high LiP titres on the wood substrates. The high LiP activity observed suggested that this enzyme may have an important role during wood decay. The significant LiP production in spite of the presence of manganese can be explained by the very high production of veratryl alcohol throughout the incubation. The fact that veratryl alcohol was produced in high amounts although soluble manganese was always present apparently contradicts the findings presented in Chapter 2 . However, the high veratryl alcohol production can be explained by the presence of lignin degradation products such as 4-hydroxybenzoate, protocatechuate, vanillate, isovanillate entering the biosynthetic pathway of veratryl alcohol as was suggested in Chapter 3.Chapter 5Bjerkandera sp. strain BOS55 is a good MnP producer. In Chapter 5, the conditions for optimal MnP production were examined. The highest production was observed in nitrogen rich medium with 0.2 to 1 mM manganese at a pH value of 5.2 and at a temperature of 30 oC. Two interesting phenomena were discovered while studying the physiology of MnP production. Firstly, significant MnP production was also observed in the absence of manganese which previously only has been shown to be the case for Pleurotus spp. However, unlike Pleurotus spp., MnP production in cultures of Bjerkandera sp. strain BOS55 was enhanced in response to increasing Mn levels. Secondly, it was demonstrated that the addition of various organic acid metabolites significantly increased the MnP titres under manganese sufficient conditions. The best results were obtained with glycolate.Chapter 6In Chapter 6, the study on the MnP production was continued in order to better understand why MnP is produced in the absence of manganese and to elucidate the induction mechanism under manganese deficient conditions. In the absence of manganese, oxalate and related organic acids, such as glycolate or glyoxylate were found to induce MnP production. The stimulatory effect of organic acids on MnP production was demonstrated to be due to the increased production of MnP proteins. Additionally, it was shown that the acids induced mnp gene transcription (unpublished data).The major MnP isozyme produced in the absence of Mn and in the presence of glycolate was purified and characterized. Like other MnP isozymes, this enzyme was able to efficiently oxidize Mn. However, unlike other MnP isozymes, it was also able to directly oxidize veratryl alcohol and 1,4-dimethoxybenzene with a very high affinity in the absence of manganese. These nonphenolic substrates are typical substrates of LiP. Methoxyphenols and aromatic amines could also be oxidized in the absence of manganese. The optimal pH for the manganese independent oxidation of all the substrates tested was 3.0 similar to that observed for LiP isozymes. On the other hand, the oxidation of Mn(II) and consequently the manganese dependent oxidation of phenolic substrate reached the highest rate at pH 4.5 as described for many MnP isozymes. The kinetic values in terms of turnover number and affinity for Mn(II) and veratryl alcohol oxidation were similar to those found for other MnP and LiP isozymes. Therefore, the Bjerkandera MnP could be best described as a hybrid enzyme between MnP and LiP, having a binding site for Mn(II) as well as for methoxy aromatics/phenols. This conclusion is supported by the finding that Mn(II) at concentrations greater than 0.1 mM severely inhibited veratryl alcohol oxidation by the enzyme; whereas Mn(II) has no effect on LiP. The fact that this enzyme can oxidize Mn(II) as well as directly oxidize veratryl alcohol and other aromatic amines and phenols clarifies the physiological relevance of the occurrence of this MnP under Mn deficient and sufficient conditions.ConclusionsIn conclusion, this thesis has resulted in new insights into the key regulatory role of manganese in lignin degradation such as the repressive effect of manganese on the production of aryl metabolites. The addition of manganese at concentrations as low as 33μM completely changed the pattern of peroxidases and aryl metabolites production compared to that under manganese deficiency. Additionally, it was observed that MnP is purposefully produced and regulated by organic acids in the total absence of manganese. Under these conditions, a novel type of MnP-LiP hybrid isozyme was isolated which was functional under any manganese regime using either veratryl alcohol or Mn(II) as a cofactor. Under natural conditions, such an enzyme would have considerable physiological significance since soluble manganese is known to be leached out or become oxidized to insoluble MnO 2 during fungal attack, possibly resulting in manganese deficient areas in wood. In manganese deficient areas, veratryl alcohol biosynthesis is stimulated and the enzyme can use this secondary metabolite as an alternative to manganese.Future research should elucidate the newly discovered role of organic acids in regulating MnP in white rot fungi. The physiological significance of this regulation may be due to the role of organic acids as an important source of reduced oxygen radicals. The oxidative stress resulting from radicals may be the signal for MnP gene expression as has been shown to be the case with H 2 O 2 . The radicals are also required for extensive degradation of lignin. Additionally, the presence of organic acids may serve as an early warning for the upcoming presence of bioavailable manganese. The fungal organic acid metabolites are well known for their ability to solubilize insoluble MnO 2 deposits in fungal attacked wood.</p