Cellulose-degrading
enzyme systems are of significant interest from both a scientific and technological perspective due to the diversity of
cellulase families, their unique assembly and substrate binding mechanisms, and their potential applications in several key industrial sectors, notably
cellulose hydrolysis for second-generation
biofuel production. Particularly fascinating are cellulosomes, the multimodular extracellular complexes produced by numerous anaerobic bacteria. Using single-molecule force spectroscopy, we analyzed the mechanical stability of the intermolecular interfaces between the
cohesin and the dockerin modules responsible for self-assembly of the cellulosomal components into the multienzyme complex. The observed
cohesin-dockerin
rupture forces (>120 pN) are among the highest reported for a receptor-
ligand system to date. Using an atomic force microscope protocol that quantified single-molecule binding activity, we observed force-induced dissociation of
calcium ions from the duplicated loop-helix F-hand motif located within the dockerin module, which in the presence of
EDTA resulted in loss of affinity to the
cohesin partner. A
cohesin amino acid mutation (D39A) that eliminated hydrogen bonding with the dockerin's critically conserved
serine residues reduced the observed
rupture forces. Consequently, no
calcium loss occurred and dockerin activity was maintained throughout multiple forced dissociation events. These results offer insights at the single-molecule level into the stability and folding of an exquisite class of high-affinity
protein-
protein interactions that dictate fabrication and architecture of
cellulose-degrading molecular machines.