To produce bioethanol from cellulosic material, such as chipped or ground biomass, a common practice involves pretreating the biomass. This pretreatment aims to make long-chain carbohydrates, like cellulose, more accessible to enzymes, which then convert them into fermentable sugars for the production of ethanol.
Challenges of Lignocellulose
Due to the complex nature of lignocellulose, even with pretreatment, it remains highly resistant. Various methods have been developed to break down lignocellulose and expose polysaccharides, including soaking the material in acidic or alkaline solutions and using mechanical disruption. Special attention has been given to methods involving cellulolytic enzymes derived from microorganisms, which have proven to be highly efficient.
Unlocking the Power of Enzymes: Modular Features
Cellulolytic enzymes often exhibit modular features. Apart from their catalytic core, these enzymes possess noncatalytic domains, such as carbohydrate-binding modules (CBMs) and dockerin domains. CBMs facilitate interactions between enzymes and their carbohydrate substrates, enhancing enzymatic activity against tough substrates.
The Role of CBMs
Studies have shown that CBMs play a crucial role in improving enzymatic activity against challenging substrates. While the three-dimensional structures and specific modes of action may vary among CBMs from different species, they generally increase the likelihood of enzymatic action by facilitating binding between large enzymatic complexes and substrates.
Dockerin Domains and Cellulosomes
Dockerin domains found on cellulolytic enzymes mediate cohesin–dockerin interactions, forming larger macromolecular complexes known as cellulosomes. These complexes are naturally present in the cell membrane and cell wall structures of cellulolytic microorganisms. While the structure and function of cellulosomes may vary, a common feature is their ability to adapt the enzyme complement within the complex based on the available substrate in the environment.
Adaptable Microorganisms
Microorganisms expressing cohesin and dockerin-based complexes have characteristic repertoires of lignocellulolytic enzymes. These repertoires can be expressed at different levels and incorporated into cellulosomes depending on environmental conditions and metabolic needs.
Enzymatic Categories
Lignocellulolytic enzymes are categorized as cellulases, hemicellulases, ligninolytic enzymes, and pectinases. In the following section, we delve into a review of primary cellulolytic enzymes and their functions within natural cellulosomes.