5.3a The Reaction of Cellulose: Cellulolysis
5.3a The Reaction of Cellulose: Cellulolysis djn12Cellulolysis is essentially the hydrolysis of cellulose. In low and high pH conditions, hydrolysis is a reaction that takes place with water, with the acid or base providing H+ or OH- to precipitate the reaction. Hydrolysis will break the β-1,4-glucosidic bonds, with water and enzymes to catalyze the reaction. Before discussing the reaction in more detail, let’s look at the types of intermediate units that are made from cellulose. The main monomer that composes cellulose is glucose. When two glucose molecules are connected, it is known as cellobiose – one example of a cellobiose is maltose. When three glucose units are connected, it is called cellotriose – one example is β -D pyranose form. And four glucose units connected together are called cellotetraose. Each of these is shown below.
We’ve seen the types of intermediates, so now let’s see the reaction types that are catalyzed by cellulose enzymes. The steps are shown below.
- Breaking of the noncovalent interactions present in the structure of the cellulose, breaking down the crystallinity in the cellulose to an amorphous strand. These types of enzymes are called endocellulases.
- The next step is hydrolysis of the chain ends to break the polymer into smaller sugars. These types of enzymes are called exocellulases, and the products are typically cellobiose and cellotetraose.
- Finally, the disaccharides and tetrasaccharides (cellobiose and cellotetraose) are hydrolyzed to form glucose, which are known as β-glucosidases.
Okay, now we have an idea of how the reaction proceeds. However, there are two types of cellulase systems: noncomplexed and complexed. A noncomplexed cellulase system is the aerobic degradation of cellulose (in oxygen). It is a mixture of extracellular cooperative enzymes. A complexed cellulase system is an anaerobic degradation (without oxygen) using a “cellulosome.” The enzyme is a multiprotein complex anchored on the surface of the bacterium by non-catalytic proteins that serve to function like individual noncomplexed cellulases but are in one unit. The figure below shows how the two different systems act. However, before going into more detail, we are now going to discuss what the enzymes themselves are composed of. The reading by Lynd provides some explanation of how the noncomplexed versus the complexed systems work.
This image is a comparative illustration of two distinct mechanisms for cellulose degradation, presented in two labeled panels: A and B. Panel A focuses on the free enzyme system, where individual enzymes act independently to break down cellulose. At the top, the structure of cellulose is shown, highlighting both crystalline and amorphous regions. Below, various enzymes—including endoglucanase, exoglucanase (CBHI and CBHII), and β-glucosidase—are illustrated interacting with the cellulose fibers. These enzymes work in concert to hydrolyze the cellulose into simpler sugars such as glucose, cellobiose, and cello-oligosaccharides.
Panel B illustrates the cellulosome-mediated degradation pathway, a more structured and synergistic approach. At the top, bacterial cell walls are shown with scaffoldin proteins anchored to them. These scaffoldins contain cohesin moieties that specifically bind to dockerin-containing enzymes like endoglucanase (CelF/CelS) and exoglucanase (CelE). The bottom part of this panel shows these enzymes forming a multi-enzyme complex, working together to efficiently degrade both crystalline and amorphous cellulose into simpler sugars.
A legend at the bottom of the image provides symbolic representations for the various components involved, including different enzymes, sugar products, and structural modules like carbohydrate-binding modules (CBMs) and phosphorylases. Overall, the image serves as a detailed visual comparison of the free enzyme system versus the cellulosome system in the biochemical breakdown of cellulose.