Three-dimensional structures of three engineered cellulose-binding domains of cellobiohydrolase I from Trichoderma reesei

Dynamic interaction of Trichoderma reesei cellobiohydrolases Cel6A and Cel7A and cellulose at equilibrium and during hydrolysis

The binding of cellobiohydrolases to cellulose is a crucial initial step in cellulose hydrolysis. In the search for a detailed understanding of the function of cellobiohydrolases, much information concerning how the enzymes and their constituent catalytic and cellulose-binding domains interact with cellulose and with each other and how binding changes during hydrolysis is still needed. In this study we used tritium labeling by reductive methylation to monitor binding of the two Trichoderma reesei cellobiohydrolases, Cel6A and Cel7A (formerly CBHII and CBHI), and their catalytic domains. Measuring hydrolysis by high-performance liquid chromatography and measuring binding by scintillation counting allowed us to correlate activity and binding as a function of the extent of degradation. These experiments showed that the density of bound protein increased with both Cel6A and Cel7A as hydrolysis proceeded, in such a way that the adsorption points moved off the initial binding isotherms. We also compared the affinities of the cellulose-binding domains and the catalytic domains to the affinities of the intact proteins and found that in each case the affinity of the enzyme was determined by the linkage between the catalytic and cellulose-binding domains. Desorption of Cel6A by dilution of the sample showed hysteresis (60 to 70% reversible); in contrast, desorption of Cel7A did not show hysteresis and was more than 90% reversible. These findings showed that the two enzymes differ with respect to the reversibility of binding.

Functional analysis of the carbohydrate-binding domains of Erwinia chrysanthemi Cel5 (Endoglucanase Z) and an Escherichia coli putative chitinase

The Cel5 cellulase (formerly known as endoglucanase Z) from Erwinia chrysanthemi is a multidomain enzyme consisting of a catalytic domain, a linker region, and a cellulose binding domain (CBD). A three-dimensional structure of the CBD(Cel5) has previously been obtained by nuclear magnetic resonance. In order to define the role of individual residues in cellulose binding, site-directed mutagenesis was performed. The role of three aromatic residues (Trp18, Trp43, and Tyr44) in cellulose binding was demonstrated. The exposed potential hydrogen bond donors, residues Gln22 and Glu27, appeared not to play a role in cellulose binding, whereas residue Asp17 was found to be important for the stability of Cel5. A deletion mutant lacking the residues Asp17 to Pro23 bound only weakly to cellulose. The sequence of CBD(Cel5) exhibits homology to a series of five repeating domains of a putative large protein, referred to as Yheb, from Escherichia coli. One of the repeating domains (Yheb1), consisting of 67 amino acids, was cloned from the E. coli chromosome and purified by metal chelating chromatography. While CBD(Cel5) bound to both cellulose and chitin, Yheb1 bound well to chitin, but only very poorly to cellulose. The Yheb protein contains a region that exhibits sequence homology with the catalytic domain of a chitinase, which is consistent with the hypothesis that the Yheb protein is a chitinase.

Widely different off rates of two closely related cellulose-binding domains from Trichoderma reesei

The filamentous fungus Trichoderma reesei produces two cellobiohydrolases (CBHI and CBHII). These, like most other cellulose-degrading enzymes, have a modular structure consisting of a catalytic domain linked to a cellulose-binding domain (CBD). The isolated catalytic domains bind poorly to cellulose and have a much lower activity towards cellulose than the intact enzymes. For the CBDs, no function other than binding to cellulose has been found. We have previously described the reversibility and exchange rate for the binding of the CBD of CBHI to cellulose. In this work, we studied the binding of the CBD of CBHII and showed that it differs markedly from the behaviour of that of CBHI. The apparent binding affinities were similar, but the CBD of CBHII could not be dissociated from cellulose by buffer dilution and did not show a measurable exchange rate. However, desorption could be triggered by shifting the temperature. The CBD of CBHII bound reversibly to chitin. Two variants of the CBHII CBD were made, in which point mutations increased its similarity to the CBD of CBHI. Both variants were found to bind reversibly to cellulose.

Design of a pH-dependent cellulose-binding domain

Protein-carbohydrate interactions typically rely on aromatic stacking interactions of tyrosine, phenylalanine and tryptophan side chains with the sugar rings whereas histidine residues are rarely involved. The small cellulose-binding domain of the Cel7A cellobiohydrolase (formerly CBHI) from Trichoderma reesei binds to crystalline cellulose primarily using a planar strip of three tyrosine side chains. Binding of the wild-type Cel7A CBD is practically insensitive to pH. Here we have investigated how histidine residues mediate the binding interaction and whether the protonation of a histidine side chain makes the binding sensitive to pH. Protein engineering of the Cel7A CBD was thus used to replace the tyrosine residues in two different positions with histidine residues. All of the mutants exhibited a clear pH-dependency of the binding, in clear contrast to the wild-type. Although the binding of the mutants at optimal pH was less than for the wild-type, in one case, Y31H, this binding almost reached the wild-type level.

Homo-dimeric spherulin 3a: a single-domain member of the beta gamma-crystallin superfamily

The beta gamma-crystallin superfamily of eye lens proteins comprises a class of structurally related members with a wide variety of different functions. Common features of these proteins are 1. the Greek-key motif of antiparallel beta-sheets, called the crystallin fold, and 2. the high intrinsic long-term stability. Spherulin 3a (S3a), a dormant protein from the spherules of Physarum polycephalum, is the only known single-domain protein within the beta gamma-crystallin family. Based on sequence homology and 'domain swapping', it has been proposed to represent an evolutionary ancestor of present-day eye lens crystallins. Since S3a is highly expressed in spherulating plasmodia of P. polycephalum under a variety of stress conditions, it can be assumed that the protein may serve as a compatible solute in the cytosol of the slime mold. In order to investigate the stability and other physicochemical properties of a single-domain all-beta protein, we isolated natural S3a. For the large-scale purification, the recombinant protein was cloned and expressed in Escherichia coli. The detailed spectral and biochemical analysis proved the recombinant protein to be authentic. In its native form, S3a is dimeric. Due to its exposed cysteine residues (Cys4), in the absence of reducing agents intermolecular disulfide cross-linking leads to the formation of higher oligomers. In order to preserve the native quaternary structure without aggregation artifacts in denaturation/renaturation experiments, the Cys4-->Ser mutant (S3a C4S) was produced. Both the wild-type protein and its mutant are indistinguishable in their physicochemical properties. At pH 3 - 4, both proteins form a stable compact intermediate (A-state). Concentration-dependent thermal and chemical denaturation showed that the equilibrium unfolding of S3a obeys the simple two-state model with no significant occurrence of folding intermediates.

Small binding proteins selected from a combinatorial repertoire of knottins displayed on phage

Knottins are a group of small, disulphide-bonded proteins that bind with high specificity to their target molecules. These proteins appear to use different faces of the protein for their interactions with different targets. Here, we attempted to create knottins with novel binding activities based on the cellulose-binding domain of the fungal enzyme cellobiohydrolase I. Variation was introduced to the face of the protein that binds cellulose. Seven residues, which are located in two regions of the polypeptide chain and form a patch of about 400 A2 on the protein surface, were simultaneously varied by random mutation of the gene. The repertoire was cloned for display on filamentous bacteriophage (5.5 x 10(8) clones), and selected for binding to cellulose or to one of three enzymes (alpha-amylase, alkaline phosphatase and beta-glucuronidase). We thereby isolated variant knottins against cellulose (differing in sequence from the parent knottin) and also against alkaline phosphatase. The binding to (glycosylated) alkaline phosphatase was highly specific with an affinity of about 10 microM, required the presence of disulphide bonds and was mediated through protein (rather than carbohydrate) contacts. Knottin scaffolds therefore appear to be a promising architecture for the creation of small folded proteins with binding activities, with the potential for improvement of binding affinities by mutation, or of using other faces of the protein to provide greater structural diversity in the primary repertoire.