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Appl. Environ. Microbiol., Oct 1996, 3679-3686, Vol 62, No. 10
KA Jensen Jr, W Bao, S Kawai, E Srebotnik and KE Hammel
Many ligninolytic fungi appear to lack lignin peroxidase (LiP), the enzyme
generally thought to cleave the major, recalcitrant, nonphenolic structures
in lignin. At least one such fungus, Ceriporiopsis subvermispora, is
nevertheless able to degrade these nonphenolic structures. Experiments
showed that wood block cultures and defined liquid medium cultures of C.
subvermispora rapidly depolymerized and mineralized a (sup14)C-labeled,
polyethylene glycol-linked, high-molecular-weight (beta)-O-4 lignin model
compound (model I) that represents the major nonphenolic structure of
lignin. The fungus cleaved model I between C(inf(alpha)) and C(inf(beta))
to release benzylic fragments, which were shown in isotope trapping
experiments to be major products of model I metabolism. The
C(inf(alpha))-C(inf(beta)) cleavage of (beta)-O-4 lignin structures to
release benzylic fragments is characteristic of LiP catalysis, but assays
of C. subvermispora liquid cultures that were metabolizing model I
confirmed that the fungus produced no detectable LiP activity. Three
results pointed, instead, to the participation of a different enzyme,
manganese peroxidase (MnP), in the degradation of nonphenolic lignin
structures by C. subvermispora. (i) The degradation of model I and of
exhaustively methylated (nonphenolic), (sup14)C-labeled, synthetic lignin
by the fungus in liquid cultures was almost completely inhibited when the
Mn concentration of the medium was decreased from 35 (mu)M to approximately
5 (mu)M. (ii) The fungus degraded model I and methylated lignin
significantly faster in the presence of Tween 80, a source of unsaturated
fatty acids, than it did in the presence of Tween 20, which contains only
saturated fatty acids. Previous work has shown that nonphenolic lignin
structures are degraded during the MnP-mediated peroxidation of unsaturated
lipids. (iii) In experiments with MnP, Mn(II), and unsaturated lipid in
vitro, this system mimicked intact C. subvermispora cultures in that it
cleaved nonphenolic (beta)-O-4 lignin model compounds between C(inf(alpha))
and C(inf(beta)) to release a benzylic fragment.
Copyright © 1996, American Society for Microbiology
Manganese-Dependent Cleavage of Nonphenolic Lignin Structures by Ceriporiopsis subvermispora in the Absence of Lignin Peroxidase
Institute for Microbial and Biochemical Technology, USDA Forest Products Laboratory, Madison, Wisconsin 53705
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