In the intricate landscape of biomass degradation, the cellulosome emerges as a key player, residing on bacterial and fungal cell surfaces and orchestrating the deconstruction of plant cell wall lignocellulose (Lamed et al., 1983b). This complex extracellular protein assembly has captivated researchers with its modular architecture and multifaceted components.
Cellulosome Composition
At its core, the cellulosome is a composite structure comprising several subunits, each distinguished by its modular design. The structural foundation, known as scaffoldin, provides the framework for the entire assembly, enabling the attachment of other subunits (Tokatlidis et al., 1991). Among these subunits are both structural and catalytic elements, with the latter featuring noncatalytic dockerin domains (Hall et al., 1988).
Dockerin–Cohesin Dynamics
The interaction between dockerin and cohesin is pivotal for cellulosome functionality. Dockerin modules, typically located at the C-terminus of cellulosomal enzymes, engage in high-affinity interactions with cohesins present in scaffoldins. This calcium-dependent interaction is crucial for the stability and folding of dockerin, thereby influencing the overall structural stability of the cellulosome (Yaron et al., 1995; Choi and Ljungdahl, 1996).
Flexibility in Binding Motifs
Notably, dockerin exhibits a degree of flexibility in its binding motifs, allowing for two proposed configurations in dockerin–cohesin interactions. This flexibility contributes to diverse states in cellulosome assembly, accommodating various enzyme activities and interactions within the complex (Carvalho et al., 2007).
Cohesin Homology and Specificity
Despite the structural plasticity observed in cellulosomes, cohesins maintain a high degree of amino acid sequence homology across species. Intriguingly, the three-dimensional structure of cohesins remains consistent, yet cohesin–dockerin interactions appear to be species-specific. Hydrophobic interactions between beta-sheet domains in cohesins and dockerin helices mediate this specificity (Pagès et al., 1997; Spinelli et al., 2000; Carvalho et al., 2007).
Scaffoldin’s Role in Cellulosome Attachment
In certain cellulosome systems, scaffoldins are tethered to the cell surface through a distinct type of cohesin, serving as an anchoring protein. This mechanism ensures the association of the cellulosome with the extracellular surface while binding to the substrate (Leibovitz and Béguin, 1996).
Catalytic Components and Carbohydrate-Binding Modules (CBMs)
Most cellulosomal enzymes lack carbohydrate-binding modules (CBMs), emphasizing the role of CBM on scaffoldin in mediating the attachment of cells to polysaccharide substrates. While some reports suggest CBMs disrupt substrate crystalline structure, their primary function is acknowledged to target and bring catalytic domains into proximity of substrates (Knowles et al., 1987; Hervé et al., 2010).
Diversity in CBMs
CBMs, located at the N-terminus or C-terminus of scaffoldin, exhibit considerable diversity, with classification into 67 families based on amino acid sequence. Alternatively, a three-fold classification based on binding specificity categorizes Type A, Type B, and Type C CBMs (Boraston et al., 2004; Lombard et al., 2014).
Conclusion
In unraveling the intricacies of cellulosomes, this exploration of structure, function, and mechanisms reveals a complex orchestration of interactions. From dockerin–cohesin dynamics to the diverse roles of CBMs, the cellulosome stands as a testament to nature’s ingenuity in biomass degradation. As researchers delve deeper, the multifaceted nature of cellulosomes continues to unveil new dimensions, promising insights into sustainable bioconversion processes.