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The family 1 CBMs are found mainly in fungal enzymes [1, 2]. Early work showed that family 1 CBMs bind to cellulose [3] and that some, but not all, family 1 CBMs bind to chitin as well [4]. There is also a contribution of CBMs in binding to lignin, but this binding was shown to be non-specific as it was easily blocked by surfactants [5]. The site of binding of a CBM1 to crystalline cellulose was determined by transmission electron microscopy of gold labelled CBM1 fused to modified Protein A. The data showed that the CBM1 bound to the 110 face of Valonia cellulose [6] Based on NMR measurements it was shown that family 1 CBMs could bind to cellohexaose, but not to shorter cellotriose and cellobiose [7].
Structural Features
Figure 1.Top. A backbone trace of the T.reesei Cel7A CBM showing residues that have been identified as important for the interaction with cellulose. Below. The structure flipped 90 degrees to show how the aromatic residues align with pyronose rings in a cellulose molecule.
Structurally the family 1 CBMs are distinct from other families. They are relatively small, only about 35 amino acids and have two or three disulphide bridges that stabilize their fold [8]. This type of fold is called a cystine knot and is also found in a family of toxins, called conotoxins produced by cone shells [9]. This structure is rigid and on the CBM there are three aromatic residues (tyrosines or tryptophans) that align so that their spacing is the same as every second pyranose ring on cellulose. Together with some hydrogen bond forming side chains this triad of aromatic residues form a binding face that docks onto the cellulose surface. It has also been shown using synthesised peptides that carbohydrates added to the CBMs affect their binding properties [10].
Functionalities
The CBMs affect enzyme activity by bringing the enzymes close to the cellulose surface [11], but there are reports that family 1 CBMs can disrupt the crystalline structure of cellulose as well [12]. Family 1 CBMs are found widely in fungal cellulases, also in several enzymes that are not active on cellulose such as mannanase [13] and acetyl xylan esterase [14]. Also swollenins have been found to contain family 1 CBMs [15].
Family 1 CBMs have been used in different types of applications such stabilizing colloid dispersions of drugs by mediating binding to nanocellulose [16].
Family Firsts
First Identified
Family 1 CBMs were found first in studies on the Trichoderma reesei Cel7A enzyme (then called cellobiohydrolase I, CBHI) using papain for fragmentation. These studies revealed that Cel7A had a “bifunctional” organization with one part binding strongly to cellulose and the other part containing the catalytic machinery [17]. It was noted that sequences homologous to the smaller cellulose binding polypeptide sequence was found in many fungal cellulases and that a synthetic analogue functioned identically to the native fragments produced by proteolysis [3].
First Structural Characterization
The synthetic version of the Trichoderma reesei Cel7A cellulose binding domain was analysed by NMR and it was the first CBM1 structure determined [8]. With the structure determined the research then led to a number of structure-function studies identifying the amino acids responsible for binding [18] and changing of binding properties by protein engineering [19].
Varjonen, S.; Laaksonen, P.; Paananen, A.; Valo, H.; Hähl, H.; Laaksonen, T.; Linder, M. Ben. Self-Assembly of Cellulose Nanofibrils by Genetically Engineered Fusion Proteins. Soft Matter 2011, 7, 2402–2411. DOI:10.1039/C0SM01114B[Varjonen2011]
van Tilbeurgh, H.; Tomme, P.; Claeyssens, M.; Bhikhabhai, R.; Pettersson, G. Limited Proteolysis of the cellobiohydrolase I from Trichoderma Reesei Separation of Functional Domains. FEBS Lett. 1986; 204, 223–227. DOI:10.1016/0014-5793(86)80816-X[vantilbeurgh1986]
