Protein Structure and Folding
- The Cellulosome is an intricate macromolecular protein complex that centralizes the cellulolytic efforts of many anaerobic microorganisms through the promotion of enzyme synergy and protein stability. The assembly of numerous carbohydrate processing enzymes into a macromolecular multiprotein structure results from the interaction of enzyme-borne dockerin modules with repeated cohesin modules present in noncatalytic scaffold proteins, termed scaffoldins. Cohesin–dockerin (Coh-Doc) modules are typically classified into different types, depending on structural conformation and cellulosome role.
- Deconstruction of cellulose, the most abundant plant cell wall polysaccharide, requires the cooperative activity of a large repertoire of microbial enzymes. Modular cellulases contain non-catalytic type A carbohydrate-binding modules (CBMs) that specifically bind to the crystalline regions of cellulose, thus promoting enzyme efficacy through proximity and targeting effects. Although type A CBMs play a critical role in cellulose recycling, their mechanism of action remains poorly understood. Here we produced a library of recombinant CBMs representative of the known diversity of type A modules.
- The assembly of one of Nature's most elaborate multienzyme complexes, the cellulosome, results from the binding of enzyme-borne dockerins to reiterated cohesin domains located in a non-catalytic primary scaffoldin. Generally, dockerins present two similar cohesin-binding interfaces that support a dual binding mode. The dynamic integration of enzymes in cellulosomes, afforded by the dual binding mode, is believed to incorporate additional flexibility in highly populated multienzyme complexes. Ruminococcus flavefaciens, the primary degrader of plant structural carbohydrates in the rumen of mammals, uses a portfolio of more than 220 different dockerins to assemble the most intricate cellulosome known to date.
- Background: Cellulosomal cohesin-dockerin types are reversed in Bacteroides cellulosolvens.Results: Combined crystallographic and computational approaches of a lone cohesin yielded a structural model of the cohesin-dockerin complex that was verified experimentally.Conclusion: The dockerin dual-binding mode is not exclusive to enzyme integration into cellulosomes; it also characterizes cell-surface attachment.Significance: This combined approach provides a platform for generating testable hypotheses of the high affinity cohesin-dockerin interaction.
- Protein-protein interactions play a pivotal role in the assembly of the cellulosome, one of nature's most intricate nanomachines dedicated to the depolymerization of complex carbohydrates. The integration of cellulosomal components usually occurs through the binding of type I dockerin modules located at the C terminus of the enzymes to cohesin modules located in the primary scaffoldin subunit. Cellulosomes are typically recruited to the cell surface via type II cohesin-dockerin interactions established between primary and cell-surface anchoring scaffoldin subunits.
- Background: CBMs are, generally, functionally and structurally autonomous from their associated catalytic domains.Results: The structure of a novel cellulase, BhCel5B, reveals that the appended carbohydrate-binding module, CBM46, extends the enzyme catalytic cleft.Conclusion: CBM46 targets BhCel5B to xyloglucan and is part of the catalytic cleft required for the hydrolysis of β-1,3–1,4-glucans.Significance: CBM46 has a dual role in the hydrolysis of complex carbohydrates by BhCel5B.