B. Goodell et al., LOW-MOLECULAR-WEIGHT CHELATORS AND PHENOLIC-COMPOUNDS ISOLATED FROM WOOD DECAY FUNGI AND THEIR ROLE IN THE FUNGAL BIODEGRADATION OF WOOD, Journal of biotechnology, 53(2-3), 1997, pp. 133-162
A review of information, as well as new research results related to lo
w molecular weight metal chelators isolated from wood decay fungi is p
resented. Low molecular weight compounds with metal chelating capabili
ty have been found to be produced by several white and brown rot wood
decay fungi. Phenolate derivative chelators from the brown rot fungus
Gloeophyllum trabeum (Gt-chelator) have been isolated by ultrafiltrati
on (< 1000 MW) and extraction with ethyl acetate. Selected chelators h
ave been purified via HPLC and structural analysis performed by GC-MS
analysis. The chelators isolated have not only a high affinity for the
ferric form of iron but importantly, will mediate the reduction of th
is metal in redox cycling processes at pH values below neutrality. Dep
ending on iron concentration and other environmental parameters, iron
may remain bound to the chelator or can be released in the reduced fer
rous form. The reduced state of iron can react with oxidants such as h
ydrogen peroxide where a Fenton type reaction will occur to generate o
xygen radicals. The presence of the chelator in wood degraded by G. tr
abeum has been demonstrated by ELISA and TEM immunolabelling studies.
Data are presented on the reduction of iron by Gt-chelator, the genera
tion of oxygen radical species, and the degradation of cellulosic and
phenolic compounds mediated by the Gt chelator. Also demonstrated are
the 'redox cycling' of the chelator as well as one-electron oxidation
reactions mediated by the Gt chelator in the presence of Mn or Fe ions
and hydrogen peroxide. A reaction scheme is presented and the role of
oxalic acid as a phase transfer agent for iron in conjunction with lo
w molecular weight, diffusible chelators in early through advanced sta
ges of wood degradation is discussed. We show that oxalate plays a rol
e in increasing the availability of iron via sequestration from iron (
hydr)oxide complexes, allowing the Gt chelators, in a pH dependent rea
ction, to access this metal from the weak oxalate chelator. Chelators
isolated from the brown rot fungus were found to reduce iron or other
transition metals and sequester these metals. The chelator has a suffi
ciently low molecular mass to penetrate into the wood cell wall as sho
wn in previous work. Data are presented showing that reactive oxygen s
pecies generated in the presence of the chelator can mediate the degra
dation of cellulosic and chlorinated phenolic compounds. We hypothesiz
e that the Gt chelator fosters the production of reduced metals within
the buffered environment of the wood cell wall and when in proximity
to reactive oxygen species, such as hydrogen peroxide or other oxidant
s (produced in part via oxidation of the catecholate Gt chelators as w
ell as cell wall lignocellulose components in the presence of the chel
ated metals), will react to form hydroxyl radicals which are capable o
f depolymerizing and oxidizing lignocellulose compounds. This hypothes
is helps to explain the action of brown rot fungi in wood cell wall bi
odegradation and provides a mechanism to explain how brown rot fungi '
control' the production of oxygen radicals, allowing their formation w
ithin the wood cell wall where they are effective against lignocellulo
se components, as opposed to being randomly generated in the extracell
ular environment around the fungal hyphae. (C) 1997 Elsevier Science B
.V.