Homeowners take a lot of trouble, and spend a lot of money, fightingthe insects and molds that munch wood to a powder. To chew upboards and beams, these marauders use, not teeth but enzymes,especially those that break down cellulose.Cellulose is a complex, crystalline carbohydrate that makes up thestructural walls of plants. It is "the most abundant renewable resourceon earth," according to a paper in last Friday's Science, "accountingfor about half of the organic material in the biosphere."That the planet was not totally drowned long ago by dead plant growthis due to the fungi, the termites and other cellulose-degrading forms oflife that make a living by decomposing woody biomass.While exterminators struggle to control the enzyme-deploying verminthat attack wood, the fuel, fruit and textile industries, among others,are cultivating, culturing, fermenting and now cloning, these samecellulases. Their goal is to optimize the enzymic action of cellulase, toimprove the production of alcohol, the "stone-washing" of blue jeansand the clarification of fruit juices, to name only three majorconsumers of cellulose-degrading enzymes.During World War II, a biochemist named Elwyn Reese discoveredwhy army tents pitched in the jungles of Southeast Asia didn't lastlong. A hairy fungus was turning the cellulose in their canvas intoglucose and a disaccharide called cellobiose. The mold that secretedthis potent enzyme acquired the name Trichoderma reesei.T. reesei remains the star of cellulose degradation, and a prime targetof research into how such enzymes work, and how it can be made towork better.That paper in the current Science reports discovery of a flattenedtunnel 40 angstroms in length (one 10-millionth of a millimeter long)that apparently serves as this key enzyme's active cleavage site.Its principal author is structural biologist T. Alwyn Jones of Sweden'sUppsala University. He told BioWorld Today, "this totally enclosedtunnel acts as a means of sucking up the cellulose into the active site,and then it clips it." Jones added, "We imagine that thiscellobiohydrolase enzyme is very important for the breakdown ofcellulose, because of the size of the tunnel. So once it's chains arethreaded in, it's hard for them to get out again. They stick on, disruptneighboring chains, and make it easier for other enzymes to nibbleaway at the cellulose."Fungal geneticist Michael Ward told BioWorld Today that "The mainthing [in Jones's crystallographic tunnel mapping] is that it gives astructural basis for known differences between cellulases, glucanasesand the set of cellobiohydrolase (CBH) enzymes." He explained thatCBH is known to act preferentially from the ends of the cellulosechain. "So the chain has to be fed in from one end into that tunnel,which gives further understanding as to why those enzymes can onlyattack at the ends, and can't grab on to the middle."Ward, who is a senior scientist in the molecular biology group atGenencor International, of South San Francisco, observed, "Withoutthose enzymes, we'd all be swamped in cell walls."Biochemist Michael Himmel leads the technology development taskforce at the Department of Energy's National Renewable Energy Labin Golden, Colo. "These enzymes," he told BioWorld Today, "areimportant to people interested in biomass conversion to fuels andchemicals. With crystal structures now available, enzyme engineeringcan be used to improve and extend the activities of these enzymes."Ward added that Jones's Science paper "has a potentially veryimportant impact for us. It allows us to pursue modifications of theCBH enzyme by site-directed mutagenesis." He aims to use thisgenetic engineering for "relieving the enzyme of end-productinhibition." This occurs when a fungus, for instance, degrades enoughcellulose to satisfy its carbon-source appetite, so shuts off enzymicactivity. Genetically altering the enzyme might deal with this shut-down, he said, "but we can do it effectively only if someone has donethe X-ray crystal structure."When biochemist Sharon Shoemaker worked at Cetus and Genencorin the 1980s, she made major contributions to cloning and expressingthe cellulose-degrading enzymes. Today she continues her studies ofnative and recombinant cellulases as executive director of the Instituteof Food and Agricultural Research at the University ofCalifornia/Davis.Shoemaker told BioWorld Today that, "elucidation of the structuralfeatures of cellobiohydrolase from Trichoderma reesei, includingdiscovery of the 4A tunnel, is a significant contribution to ourunderstanding of the enzymic binding, and subsequent breakdown ofcellulose, our most abundant natural polymer. This information," sheadded, "will provide scientists with the basis for designing enzymes,giving new products and uses for this abundant renewable resource." n
-- David N. Leff Science Editor
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