Synthetic Biology towards Lignin Utilization

Abstract

Lignin, a component of plant cell walls, is the world's most abundant sustainable resource of aromatic polymers. The aromatic structure of lignin allows it to be used in applications that traditionally rely on fossil resources. The paper and pulp industry produce large lignin-rich side streams. Lignin is commonly used in these industries for energy and heat production. This leaves the value of lignin at a fraction of what it would be if it were used to produce higher value-added products such as materials, fuels or chemicals. This literature review will explore the whole process chain that enables lignin utilization. The focus is on biological methods and the impact of synthetic biology on the efficiency of lignin utilization is explored as well. Lignin has a heterogeneous, branched and cross-linked structure, which raises challenges in its utilization. Other challenges of lignin utilization include its insolubility in water, toxicity to microorganisms and poor chemical reactivity. The properties of lignins also vary depending on their source: lignins from industrial side streams may need chemical pretreatments to improve their characteristics. Nevertheless, there are several different thermochemical and biological methods to break down lignin into monomers, which is called depolymerization. Fungi are efficient biological depolymerizers in nature. On the other hand, bacteria are also capable of breaking down lignin and they have the advantage of rapid growth rates and ease of genetic modifications. After depolymerization, the lignin-derived products are a mixture of different aromatic monomers. The most efficient method for their separation and further processing is biological conversion. Microbes use the degradation products as a source of carbon and convert them towards some intermediates such as catechol and protocatechuate. The microbes can form e.g. cis,cis-muconic acid through a central metabolic pathway. From here on, the downstream processing methods depend on the application. Nylon and polyethylene terephthalate (PET) can be formed from cis,cis-muconic acid. Lignin can also be used as a material for 3D printing, as a filler for epoxies and adhesives, and in medicine for drug delivery or as pharmaceuticals. Synthetic biology is used to enhance biological processes. For example, a microbe can be engineered to express several lignin-degrading enzymes simultaneously. Effective biological utilization of lignin can only be achieved through a deep understanding of microbial lignin metabolism. In addition, the use of synthetic biology tools should be mastered in order to achieve the desired properties for microbes. Research into lignin utilization has increased in recent years and new opportunities are constantly being discovered. Synthetic biology enhances lignin utilization processes, which may eventually allow lignin to be utilized on an industrial scale.