Predominant Nonproductive Substrate Binding by Fungal Cellobiohydrolase I and Implications for Activity Improvement

Abstract

Enzymatic conversion of the most abundant renewable source of organic compounds, cellulose to fermentable sugars is attractive for production of green fuels and chemicals. The major component of industrial enzyme systems, cellobiohydrolase I from Hypocrea jecorina (Trichoderma reesei) (HjCel7A) processively splits disaccharide units from the reducing ends of tightly packed cellulose chains. HjCel7A consists of a catalytic domain (CD) and a carbohydrate-binding module (CBM) separated by a linker peptide. A tunnel-shaped substrate-binding site in the CD includes nine subsites for β-d-glucose units, seven of which (-7 to -1) precede the catalytic center. Low catalytic activity of Cel7A is the bottleneck and the primary target for improvement. Here it is shown for the first time that, in spite of much lower apparent k of HjCel7A at the hydrolysis of β-1,4-glucosidic linkages in the fluorogenic cellotetra- and -pentaose compared to the structurally related endoglucanase I (HjCel7B), the specificity constants (catalytic efficiency) k /K for both enzymes are almost equal in these reactions. The observed activity difference appears from strong nonproductive substrate binding by HjCel7A, particularly significant for MU-β-cellotetraose (MUG ). Interaction of substrates with the subsites -6 and -5 proximal to the nonconserved Gln101 residue in HjCel7A decreases K by >1500 times. HjCel7A can be nonproductively bound onto cellulose surface with K ≈2-9 nM via CBM and CD that captures six terminal glucose units of cellulose chain. Decomposition of this nonproductive complex can determine the rate of cellulose conversion. MUG is a promising substrate to select active cellobiohydrolase I variants with reduced nonproductive substrate binding.

The dissociation mechanism of processive cellulases

“…It is thus tempting to speculate that formation of unidentified but exceptionally long-lived, nonproductive complexes is responsible for the low k off values measured by biochemical methods. The existence of strong nonproductive complexes [which has been proposed in earlier studies between cellulases and cellooligosaccharides (49-51)] has been suggested to include those with an α conformation of the leading glucose moiety (46), an incorrectly oriented "zigzag" pattern of glycosidic bonds (46), or partially processed chains (51). SEE experiments produce an average dissociation constant for all molecules, including moving CBHs [as those seen in HS-AFM experiments (18,31), which would have a higher k off ] as well as more stable nonproductive subpopulations.…”

Enzyme kinetics by GH7 cellobiohydrolases on chromogenic substrates is dictated by non‐productive binding: insights from crystal structures and MD simulation

“…The inhibition of nitrophenyl glycoside hydrolysis is often employed to measure equilibrium binding of nondegradable ligands. A yet more sensitive detection of glycosidase activity is achieved by the use of fluorogenic substrates such as methylumbelliferyl glycosides [ 8 , 10 , 11 ]. However, the use of low molecular weight model substrates for polymer degrading enzymes with long arrays of substrate binding subsites is often complicated, since the model substrates may occupy, or even prefer non‐productive positions, sometimes to such an extent that the non‐productive binding modes are dominating, meaning that these substances can be characterized as reversibly binding inhibitors [ 12 , 13 ].…”