Mechanisms of wood degradation by brown-rot fungi: chelator-mediated cellulose degradation and binding of iron by cellulose

Abstract

Iron, hydrogen peroxide, biochelators and oxalate are believed to play important roles in cellulose degradation by brown-rot fungi. The effect of these compounds in an ‘enhanced’ Fenton system on α-cellulose degradation was investigated specifically in regard to molecular weight distribution and cellulose–iron affinity. This study shows that the degradative ability of an ultrafiltered low molecular weight preparation of chelating compounds isolated from the brown-rot fungus Gloeophyllum trabeum (termed ‘Gt chelator’) increased with increasing Gt chelator concentration when the FeIII to Gt chelator ratio was greater than about 30:1. When this ratio was less than 30:1, increasing Gt chelator concentration did not accelerate cellulose degradation. In excess hydrogen peroxide, cellulose degradation increased and then decreased with increasing iron concentration when FeIII was present in excess of the Gt chelator. The critical ratio of FeIII to Gt chelator varied depending on the concentration of hydrogen peroxide in the system. Increasing iron concentration above a critical iron:chelator ratio inhibited cellulose degradation. The optimum pH for cellulose degradation mediated by Gt chelator was around 4.0. A comparison of the effects of 2,3-DHBA (a chelator that reduces iron similarly to Gt chelator) and Gt chelator with respect to cellulose degradation demonstrated the same pattern of cellulose degradation. Cellulose–iron affinity studies were conducted at three pH levels (3.6, 3.8, 4.1), and the binding constants for cellulose–FeIII, cellulose–FeII and cellulose–FeIII in the presence of Gt chelator were calculated. The binding constants for cellulose–FeIII at all three pH levels were much higher than those for cellulose–FeII, and the binding constants for cellulose–FeIII in the presence of Gt chelator were very close to those for cellulose–FeII. This is probably the result of FeIII reduction to FeII by Gt chelator and suggests that chelators from the fungus may be able to sequester iron from cellulose and reduce it in near proximity to the cellulose and thereby better promote depolymerization. The free radical generating system described has potential for use in a variety of industrial processing and pollution control applications.