EGEE 439
Alternative Fuels from Biomass Sources

6.3a The Reaction of Cellulose: Cellulolysis

6.3a The Reaction of Cellulose: Cellulolysis

Cellulolysis is essentially the hydrolysis of cellulose. If you recall from Lesson 5 (see Figure 5.16), in the 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 (Lesson 5, Figure 5.9a). When two glucose molecules are connected, it is known as cellobiose – one example of a cellobiose is maltose (Lesson 5, Figure 5.10b). 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 in Figure 6.11.

Glucose structure with carbons numbered.
Figure 6.11a: Glucose structure with carbons numbered.
Cellobiose, or maltose (glucose + glucose) chemical structure
Figure 6.11b: Cellobiose, or maltose (glucose + glucose) chemical structure.
Cellotriose in the β-D-pyranose form
Figure 6.11c: Cellotriose in the β-D-pyranose form.
Cellotetraose chemical structure
Figure 6.11d: Cellotetraose.
Credit: Green Chem., 2012,14, 1284-1288

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 in Figure 6.12.

  1. 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.
  2. 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.
  3. Finally, the disaccharides and tetrasaccharides (cellobiose and cellotetraose) are hydrolyzed to form glucose, which are known as β-glucosidases.
Crystallized cellulose with endocellulase produces cellulose. Add exocellulase it produces cellobiose. Add Cellobiase & it produces glucose
Figure 6.12: Reaction sequence of cellulose with 1) endocellulase, 2) exocellulase, and 3) β-glucosidase.

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 the individual noncomplexed cellulases but is in one unit. Figure 6.13 shows a figure of 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.

Hydrolysis schematic representation - see caption and text above
Figure 6.13: Schematic representation of the hydrolysis of amorphous and microcrystalline cellulose in A) noncomplexed and B) complexed cellulase systems.
Credit: Lynd et al., 2002