Purification and characterization of two exo-cellobiohydrolases from the brown-rot fungus Coniophora puteana (Schum ex Fr) Karst

Two extracellular exo-cellobiohydrolases (EC 3.2.1.91) were purified to homogeneity from the culture filtrate of the brown-rot fungus Coniophora puteana (Schum ex Fr) Karsten, strain EMPA 62. The purification scheme involved three successive chromatographic steps, namely Q Sepharose fast flow, Superose 12, and Fractogel TSK DEAE-650S. The two enzymes, named cellobiohydrolase (CBH) I and CBH II, were purified by a factor of 4.6 and 3.9, respectively, with an activity recovery of 9 and 19% of total, respectively. On sodium dodecyl sulfate-polyacrylamide gel electrophoresis both enzymes migrated as single bands according to a M(r) of 52,000 for CBH I and 50,000 for CBH II; by FPLC gel filtration (TSK G3000 SW) the M(r)'s were higher (65,000 and 60,000). Both enzymes were glycosylated, had similar isoelectric points (pI 3.6 and 3.55) and nearly identical pH optima for activity close to 5. Endoglycosidase H digestion gave two distinct polypeptides where the molecular weight was lowered by 6.5 kDa for CBH I and by 2.5 kDa for CBH II. The specific activities for the hydrolysis of p-nitrophenyllactoside (pNPL) were nearly identical for both enzymes (0.46 versus 0.40 mumol mg-1 min-1 at 40 degrees C) and the Km values (6.8 and 4.3 mM at 30 degrees C) were also very close. Both enzymes were competitively inhibited by cellobiose: with pNPL as substrate, Ki values of 1.2 mM for CBH I and 2.4 mM for CBH II were determined. The two enzymes acted in an identical fashion on cellulose (either amorphous or crystalline) and on cellodextrins, liberating mainly cellobiose, but were inactive on dyed carboxymethylcellulose. Cellobiose was not hydrolyzed whereas cellotriitol was hydrolyzed to equimolar amounts of cellobiose and glucitol: these results support the interpretation that these enzymes are exo-cellobiohydrolases. Their presence in a brown-rot fungus is a new fact.

Isolation and characterization of the cellobiose dehydrogenase from the brown-rot fungus Coniophora puteana (Schum ex Fr.) Karst

The cellobiose dehydrogenase secreted by Coniophora puteana (Schum ex Fr) Karsten during growth on cellulose was isolated by successive anion-exchange chromatography on Q Sepharose fast flow and on TSK DEAE-650S and gel filtration on Superose 12. The enzyme was recovered at a 41% yield with a 43-fold increase in specific activity. The purified sample was homogeneous by polyacrylamide gel electrophoresis (PAGE), sodium lauryl sulfate (SDS)-PAGE, and electrophoretic titration curve analysis and stained positively for glycoprotein (periodic acid/Schiff base reaction) and hemoprotein (peroxidase reaction). By isoelectric focusing over a narrow pH range two distinct bands were observed: a major band (pI 3.9) flanked by a minor band on its acidic side. FPLC gel filtration on TSK G3000 SW revealed a M(r) of 192,000, whereas on SDS-PAGE a single band, corresponding to a M(r) of 111,000, was observed. The enzyme contained 13% sugar as mannose and upon digestion with endoglycosidase H, its molecular weight was lowered by 11 kDa. The enzyme showed a visible spectrum compatible with that of a b-type cytochrome containing a flavin cofactor. It was able to oxidize cellobiose, cellodextrins, and lactose at their C1-reducing group, with dichlorophenol indophenol as oxidant. Oxygen consumption (oxidase reaction in a Clark electrode) was not at a detectable rate. Km and Vmax for cellobiose oxidation were 84 microM and 2.98 mumol mg-1 min-1, respectively, but the enzyme was strongly substrate (cellobiose) inhibited (Kis 5.4 mM).