Fungal cellulase systems. Comparison of the specificities of the cellobiohydrolases isolated from Penicillium pinophilum and Trichoderma reesei.
Reaction patterns for the hydrolysis of chromophoric glycosides from cello-oligosaccharides and lactose by the cellobiohydrolases (CBH I and CBH II) purified from Trichoderma reesei and Penicillium pinophilum were determined. They coincide with those found for the parent unsubstituted sugars. CBH I enzyme from both organisms attacks these substrates in a random manner. Turnover numbers are, however, low and do not increase appreciably as a function of the degree of polymerization of the substrates. The active-site topology of the CBH I from T. reesei was further probed by equilibrium binding experiments with cellobiose, cellotriose, lactose and some of their derivatives. These point to a single interaction site (ABC), spatially restricted as deduced from the apparent independency of the thermodynamic parameters. It appears that the putative subsite A can accommodate a galactopyranosyl or glucopyranosyl group, and subsite B a glucopyranosyl group, whereas in subsite C either a glucopyranosyl or a chromophoric group can be bound, scission occurring between subsites B and C. The apparent kinetic parameters (turnover numbers) for the hydrolysis of cello-oligosaccharides (and their derivatives) by the CBH II type enzyme increase as a function of chain length, indicative of an extended binding site (A-F). Its architecture allows for specific binding of beta-(1----4)-glucopyranosyl groups in subsites A, B and C. Binding of a chromophore in subsite C produces a non-hydrolysable complex. The thermodynamic interaction parameters of some ligands common to both type of enzyme were compared: these substantiate the conclusions reached above.
Claeyssens
M
M
Laboratorium voor Bichemie, Rijksuniversiteit Gent, Belgium.
Van Tilbeurgh
H
H
Tomme
P
P
Wood
T M
TM
McRae
S I
SI
D043366:Cellulose 1,4-beta-Cellobiosidase; D006026:Glycoside Hydrolases / Q000378:metabolism*; D008024:Ligands; D003904:Mitosporic Fungi / Q000201:enzymology*; D010407:Penicillium / Q000201:enzymology*; D013379:Substrate Specificity; D013816:Thermodynamics; D014242:Trichoderma / Q000201:enzymology*
D008024:Ligands; D006026:Glycoside Hydrolases; D043366:Cellulose 1,4-beta-Cellobiosidase