Next to cellulose, the most abundant organic material on earth islignin. This tightly cross-linked polymer serves terrestrial plants intwo ways: as a barrier to microbial attack, and as structuralscaffolding to keep trees standing up.
Lignin's only industrial use, apart from low-grade fuel, is in hardfiber-board.
Cellulose, on the other hand, is as useful as lignin is useless. Inpaper-making, a basic process is to separate the two. Removinglignin from wood pulp requires chlorine and other harsh, polluting,environmentally unfriendly chemicals.
Meanwhile in field and forest, friendly white-rot fungi munch on thelignin of fallen trees and other dead plants, for recycling in nature.How they degrade the undegradable polymer remains a mystery. In1982, biochemists discovered an enzyme that some fungi secrete todo their demolition job, and named it lignin peroxidase.
They're still trying to figure out how to adapt this fungal catalyticenzyme to delignify paper pulp.
One researcher has just discovered that it's not that simple: Not allwhite-rot fungi, he found, make lignin peroxidase. Some may bedeploying an unsuspected, non-enzymatic mechanism instead.
Yesterday's Proceedings of the National Academy of Sciences(PNAS) carries a report titled "Fungal degradation of recalcitrantnonphenolic lignin structures without lignin peroxidase."
The First Cut Is The Hardest
Its senior author, biochemist Kenneth Hammel at the Department ofAgriculture's Forest Products Laboratory in Madison, Wisc.,explained to BioWorld Today: "We want to understand what the firstreactions are when the fungus breaks down lignin in wood. Theinitial cutting of the polymer is the hard part of the reaction. Since allwork in the past has been done with purified enzymes in very simpletest-tube systems, we wondered _ and a lot of people wondered _whether what had been found so far was enough to explain how ithappened."
Specifically, Hammel and his co-workers tackled the question ofwhether some white-rot fungi, which apparently fail to make ligninperoxidase in the laboratory, are nonetheless competent todepolymerize lignin. So they compared two species, one(Phanerochaete chrysosporium), a known enzyme producer, withanother, (Ceriporiopsis subvermispora), a reported non-producer.
They sicced both fungal variants onto two synthetic lignin models ofascending polymeric intractability. The first, P. chrysosporium,rapidly reduced all three to residual carbon dioxide. The second, C.subvermispora polished off only the less recalcitrant one.
"However in wood, its natural environment," Hammel's PNAS paperreported, "C. subvermispora mineralized [i.e., totally degraded] bothmodels as rapidly as P. chrysosporium did."
Further experiments confirmed that the latter fungus "hasmechanisms for degradation of nonphenolic [i.e., the most-difficult-to-oxidize] lignin that are as efficient as those in P. chrysosporium,but which do not depend on lignin peroxidase."
To find this alternative mechanism, Hammel conducted otherexperiments, which he reported in an issue of the weekly FEBSLetters (Vol. 394, 1994), published by the Federation of EuropeanBiochemical Societies.
He explained, "An enzyme is involved called manganese peroxidase.Instead of oxidizing the lignin directly, it is oxidizing othermolecules, very small ones, which themselves then diffuse away andoxidize the lignin by chemical reactions involving free radicals,rather than enzymatic catalysis. And then the lignin just falls apart."
The co-discoverer of lignin peroxidase 12 years ago is Ming Tien,who teaches biochemistry and molecular biology at PennsylvaniaState University in University Park, Pa. "What Ken Hammel hasdone in his PNAS paper," Tien told BioWorld Today, "is to provide,on fairly safe ground, some positive data that there is indeed analternate mechanism."
Tien continued, "In the FEBS Letters, what he did was provide apossible alternate mechanism for a lignin-peroxidase-like activity that's independent. That's generation of freeradicals in lipid-like substrates. Whether this is truly significant willbe borne out by future research."
As to potential applications beyond wood pulp, Tien thinksHammel's work "ties in with other research going on in thebioremediation arena, using the hydroxyl free radical for degradingenvironmental pollutants." He recalled that "Even when the enzymewas discovered, we first went from biopulping to `Well, it looks as ifit isn't going to work so well.' Then we eventually evolved intobioremediation, because these fungi are notorious for degrading allsorts of stuff."
Tien started his own company, Tienzyme Inc., at State College, Pa.,five years ago, to work on fungal strain improvement forbioremediation and enzyme production.
A Transgenic Fungus To Spare Cellulose
Hammel suggests one potential "biotechnological scenario," basedon the fungus's appetite for cellulose. "They only remove the ligninfrom the wood," he said, "because they want to eat the cellulose. Thelignin is only the wrapper on their candy bar."
This fungal gluttony, he added, "is really bad if you want to usethese fungi to make paper, because the cellulose in paper must beundamaged." Some species, such as those in his PNAS paper, pauseafter nibbling away the lignin, before wolfing down the cellulose bymeans of another enzyme, cellulase.
"One manipulation that the genetic engineers are interested in"Hammel went on, "is taking those fungi that degrade lignin veryrapidly and knocking out their cellulase genes, giving them sugar toeat instead, and putting them in wood or wood pulp, where theywon't damage the cellulose." n
-- David N. Leff Science Editor
(c) 1997 American Health Consultants. All rights reserved.