Aerobic Microbial Cellulase Systems

Abstract

tons pr oduc ed by plants eac h year (1). M ost terrestrialcellulose isdeg raded by celluloly tic micr oo rganisms, pr imar ily fung iand bact eria,althoug hsome cellulose isrecy cled by Þr e and by phot odeg radation (2). A erobic micr oorganisms arerespo nsible for m u ch ofthe cellulose deg rada tion in soils,bu tther e also are ma ny species ofcelluloly tic anaer obic soil bact eria suc h as C lostridiu m therm ocellu m ,C. cellulovor ans ,andA cetivibrio cellulosolvens(3Ð5). Termit es and some other insects are very impor tant in cellu-lose deg radation, especially in tropical regions, and most cellulose deg rading insects containsymbiotic celluloly tic micr oorganisms, even thoug h man y termit es and other insects pro-duc e cellulases (6, 7). Some very celluloly tic termit es utiliz e aerobic symbiotic celluloly ticfung i to breakdo w n plant mat erial in their nests and then eat the fung i and residual plantmat erial (8). R uminants, suc h as cows, sheep , and deer , also are impor tant cellulose de-grading organisms but all of the cellulose that the y u tilize is d eg raded by sym biotic rumenmicr oorganisms, pr imar ily bact eria.The rumen is an extremely anaer obic en vironment andthis area has been review ed recently (9, 10), so that these organisms w illnot be discussedfu rth er h ere.M icroorganisms catalyz e cellulose deg radat ion by p roducing enzy mes called cellulases,whic h hyd rolyze the ! -1-4 linkages pr esent in cellulose. Almost allcelluly tic micr oorganismssecr ete their cellulases outside their cellwall, as bact eria and fung iare unable to transpor tinsoluble mat erials, like cellulose, inside the cell.T he soluble sugars pr oduc ed by cellulasedigestion of cellulose are transpor ted inside th e cell an d m etabolized . N ative cellu lose isvery resistant to hydr olysis because itis insoluble and contains crystalline regions in w hic hthe adjac ent cellulose molecules ha ve strong interactions, suc h as hydr ogen bonds andhydr ophobic stacking .T h us, the speciÞc activities of indi vidual cellulases are m uc h low ertha n tho se ofmo st enzy mes. H ow ever,in term s ofca talytic enha nc ement, cellulases are veryactive enzy mes, as the half-life of crystalline cellulose in wat er at neut ral p H is estimat edto be about 100 million years. It tak es conc ent rated sulfur ic acid at 125uC to hydr olyz enati ve cellulose at a reasonable rate.W hen cellulases are assa yed on low molecular w eightsoluble subst rates, the y sho w n or mal M ich aelis-M ent en kinetics and some ha ve hig h speciÞcactivities, sho w ing that the y are basically similar to other enzy mes. H ow ever,when cellulasesare assayed on in solu ble su bstrates, th ey h ave very d ifferen t p rop erties, as th e assays are