Carbohydrate binding modules (CBMs) are a super-family of ancillary modules, which are widely distributed in nature and diversely support the functions of enzymes that degrade sugar polymers, particularly cellulose. The CBMs classified in family 3 (CBM3) are among the most distinctive, diverse and robust proteins in nature, with over 480 known various representatives from more than 70 bacteria. The CBM3 molecules comprise ~150 amino-acid residues creating β-sandwich fold composed of 9 β-strands. They share conserved features such as a cellulose-binding planar strip and a Ca2+-binding site usually composed of five conserved residues.
CBM3s appear to play three different roles: (i) comprising the major substrate targeting entity in various free cellulases and cellulosomes, (ii) functioning as helper modules in multi-modular cellulases by converting the endo mode of action to a voracious progressive type, (iii) participating presumably as sensors in the recently discovered biomass-sensing system. In view of the multiplicity of apparent functions that are reflected in characteristic modifications of key residues upon the various known CBM3 molecules, our working hypothesis states that the CBM3 fold serves as a general and versatile molecular scaffold that nature uses to produce a broad range of different functions.
In this research, we have determined nine new high-resolution 3D structures of family 3 CBMs from scaffoldin subunits, cellulosomal enzymes and the recently discovered carbohydrate-sensing system. The diversity that was revealed from the determined structures help to characterize the significant functional diversity in the family that is conveyed in fine structural variations. This research is only a step towards a better understanding of the different functions that should correlate with the structural variations that we have determined.
