Probing pH-Dependent Functional Elements in Proteins: Modification of Carboxylic Acid Pairs in Trichoderma reesei Cellobiohydrolase Cel6A

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Abstract
Two carboxylic acid side chains can, depending on their geometry and environment, share a proton in a hydrogen bond and form a carboxyl−carboxylate pair. In the Trichoderma reesei cellobiohydrolase Cel6A structure, five carboxyl−carboxylate pairs are observed. One of these pairs (D175-D221) is involved in catalysis, and three other pairs are found in, or close to the two surface loops covering the active site tunnel of the catalytic domain. To stabilize Cel6A at alkaline pH values, where deprotonation of the carboxylic acids leads to repulsion of their side chains, we designed two mutant enzymes. In the first mutant, one carboxyl−carboxylate pair (E107-E399) was replaced by a corresponding amide-carboxylate pair (Q107-E399), and in the second mutant, all three carboxyl−carboxylate pairs (E107-E399, D170-E184, and D366-D419) were mutated in a similar manner. The unfolding studies using both intrinsic tryptophan fluorescence and far-ultraviolet circular dichroism spectroscopy at different pH values demonstrate that the unfolding temperature (Tm) of both mutants has changed, resulting in destabilization of the mutant enzymes at acidic pH and stabilization at alkaline pH. The effect of stabilization seems additive, as a Cel6A triple mutant is the most stable enzyme variant. This increased stability is also reflected in the 2- or 4-fold increased half-life of the two mutants at alkaline pH, while the catalytic rate on cellotetraose (at t = 0) has not changed. Increased operational stability at alkaline pH was also observed on insoluble cellulosic substrates. Local conformational changes are suggested to take place in the active site loops of Cel6A wild-type enzyme at elevated pHs (pH 7), affecting to the end-product spectrum on insoluble cellulose. The triple mutant does not show such pH-dependent behavior. Overall, our results demonstrate that carboxyl−carboxylate pair engineering is a useful tool to alter pH-dependent protein behavior.