The effect of Trichoderma cellulases on the fine structure of a bleached softwood kraft pulp

The mode of action of several Trichoderma reesei cellulase preparations was investigated in relation to their effect on the crystallinity and degree o f polymerization of a fully bleached softwood kraft pulp. One complete (Cel luclast 1.5L, Novo Industri) and six modified cellulases (Rohm Enzyme Finla nd Oy, former Primalco Ltd Biotec) were used tinder a relatively low enzyme loading of 40 mg protein g(-1) cellulose in a reaction mixture in which an excess of exogenous cellobiase activity was added (Novozym 188). This enzy me lending was used To assess the mode of action of the enzymes under both non-saturating and non-inhibitory conditions. The modified cellulase prepar ations were obtained from T. reesei by deleting the following components of the cellulase system: EG I and/or EG II or CBH I and/or CBH II. Characteri zation of hydrolysates by gel permeation chromatography (GPC) showed that s hort incubation times of 4 h with CBH I-deficient enzymes (CBH I- and CBH I /II-mutants) resulted in the highest shifts in the molecular weight distrib ution of cellulose. Compared to EG-deficient enzymes (EG I; EG II- and EG I /II-mutants), the lack of CBH II resulted in equivalent changes in both pol ydispersities and average degrees of polymerisation of cellulose. This sugg ested that the role of CBH II in hydrolysis was similar to that of both end oglucanases I and II. The effectiveness of all mutant enzymes in hydrolysin g cellulose was much lower than that obtained when a complete cellulase sys tem such as Celluclast 1.5L was used. Our results also indicated that the l ack of CBH I was very detrimental to hydrolysis and depolymerization of cel lulose and that EG I has a greater effect on the substrate than EG II. Howe ver, there were no detectable changes in the degree of crystallinity of par tially hydrolysed pulps, regardless of the enzyme system used. (C) 1999 Els evier Science Inc. All rights reserved.