Enzymes in Biorefinery

Biorefineries integrate various processes to obtain different products from a single waste material. The complex nature of waste materials necessitates the employment of different physical, chemical, and biological methods to degrade the substrates. Biological methods use enzymes, mainly hydrolases and oxidoreductases, to convert substrates into valuable products. The specificity of enzymes, the high reaction rate under biorefinery conditions, and the ability to modify their properties through genetic manipulation render them a useful and environmentally friendly tool in large-scale processes [1].

Considering the composition of lignocellulosic wastes, cellulases, hemicellulases, and laccases are the most important enzymes used in biorefineries. Cellulases and hemicellulases break down their respective polymeric substrates into oligosaccharides and monosaccharides, which can be utilized further to produce different valuable products [2]. Lignin is mainly degraded by oxidative enzymes such as laccases that also find applications in environmental remediation.

Cellulose is the most abundant polymer on this planet; hence, cellulases are the most extensively studied enzymes in lignocellulose biorefineries. Most aerobic cellulolytic organisms produce free cellulases comprising endoglucanases that cleave β-1,4 glycosidic bonds internally, releasing oligosaccharides, whereas exoglucanases act on the terminal to release cellobiose. β-glucosidases catalyze the hydrolysis of cellobiose and other short-chain oligosaccharides to release glucose [2]. In the production of cellulosic ethanol, glucose is fermented by yeast into bioethanol [3]. The presence of lignin and the crystalline nature of cellulose make cellulose degradation difficult; hence, pretreatment technologies have been developed to improve the saccharification efficiency of cellulase systems [3].

Hemicellulose, a heterogeneous polymer, is attacked by different enzymes, mainly xylanases and mannanases. The former degrades a prevalent hemicellulose, xylan, and releases xylose and oligosaccharides [4]. Hemicellulases exhibit synergy with cellulases and are used to completely degrade lignocellulosic materials.

Lignin is a polyaromatic compound that is mainly degraded by laccases and manganese peroxidases. These enzymes break the aromatic ring and, hence, are utilized in the degradation of other polyphenolics to alleviate environmental pollution [5].

In addition to lignocellulose-degrading enzymes, lipases are also important from a biorefinery perspective. Lipases can undergo transesterification and are therefore used for the production of biodiesel. Researchers have investigated the use of different crude or waste oil sources for biodiesel production using lipases that can withstand different environmental conditions.

Naturally, enzymes are sensitive to environmental conditions; therefore, novel microbial strains that thrive in harsh environments have been reported as a promising source of stable biocatalysts. Moreover, advances in protein engineering techniques have enabled tailor-made biocatalysts with desirable properties. For example, lignocellulose-degrading enzymes that can withstand acidic conditions, saline environments, or high solid loading are of particular interest from a biorefinery perspective [6]. Likewise, solvent resistance is a prerequisite for lipases to be employed in biorefineries for biodiesel production. Genetic engineering, along with protein engineering, can be used to develop resistant catalysts. The emergence of technologies for synthetic biology will certainly provide new and improved enzymes that will boost the landscape of biorefinery products.

The significant development in biorefineries is challenged by various factors, mainly the high cost of enzyme production. This factor impedes the viability of biorefineries; therefore, future research in this direction will provide financial viability to biorefinery processes. The instability of the enzyme is another impeding factor that has been taken up by emerging fields including nanotechnology, synthetic biology, proteomics, and metagenomics. The proportion of enzymes in biorefineries is expected to follow an ever-expanding trend.