Recent advances in microbial production of phenolic compounds

Highlights

• The biosynthetic pathways for the representative phenolic compounds are summarized.

• The strategies used to improve the biosynthetic efficiency are reviewed.

• Current challenges and future perspectives are discussed.

Abstract

Phenolic compounds (PCs) are a group of compounds with various applications in nutraceutical, pharmaceutical and cosmetic industries. Their supply by plant extraction and chemical synthesis is often limited by low yield and high cost. Microbial production represents as a promising alternative for efficient and sustainable production of PCs. In this review, we summarize recent advances in this field, which include enzyme mining and engineering to construct artificial pathways, balance of enzyme expression to improve pathway efficiency, coculture engineering to alleviate metabolic burden and side-reactions, and the use of genetic circuits for dynamic regulation and high throughput screening. Finally, current challenges and future perspectives for efficient production of PCs are also discussed.

Introduction

Phenolic compounds (PCs) comprise a diverse group of plant secondary metabolites, and can be categorized into polyphenols, phenolic acids, phenylpropanoids, flavonoids, stilbenes, coumarins and their derivatives (e.g. esters, glycosides and polymers) [1]. PCs possess the aromatic ring(s) bearing one or more hydroxyl groups, and exhibit multifaceted activities (antioxidation, anti-inflammation, anti-tumor, antiviral, anti-bacteria and so on) [2,3]. Due to these health-beneficial properties, PCs are widely used in nutraceutical, pharmaceutical and cosmetic industries.

Currently, PCs are mainly manufactured by either plant extraction or chemical synthesis. However, the plant extraction process often encounters bottlenecks such as low yield, fluctuation of active ingredients caused by seasonal/climatic variations, and over-exploitation of plant resources, and thus is difficult to fulfill the growing demands. Chemical synthesis also faces problems such as expensive precursors, inadequate regio- and stereo-selectivity, use of toxic catalysts and harsh reaction conditions. Therefore, engineering microbial cell factories has become a promising alternative approach to produce these high-value PCs.

PCs like other aromatic compounds are generally biosynthesized through the shikimate pathway. This pathway consists of seven enzymatic steps that convert the precursors erythrose-4-phosphate (E4P) and phosphoenolpyruvate (PEP) to chorismate. The involved enzymes and the underlying regulation mechanism are well elucidated, and numerous studies have been conducted to boost the shikimate pathway for the production of various aromatic compounds by metabolic strategies like increasing the supply of PEP and E4P, alleviating feedback inhibition of end-products on upstream enzymes, and blocking/weakening the competing pathways. These strategies are well summarized already [[4], [5], [6]] and thus will not be the emphasis of this review.

The rapid expansion of available information on plant genomes and transcriptomes has accelerated enzyme mining and identification of the biosynthetic pathways from the native species. However, so far there are many PCs whose native pathways are still elusive. Fortunately, artificial pathways can be designed by mining genes from different species based on the bioinformatics databases. To meet the industrial application requirements, the titer, yield and productivity of the recombinant microorganisms need to be improved. To this end, the advancements in protein engineering, metabolic engineering and synthetic biology provide powerful tools to remove obstacles in the construction of microbial cell factories to produce PCs. In this review, we summarize the recent progress on the design of artificial pathways for the biosynthesis of PCs, and the strategies used to improve the production efficiency, which include enzyme mining and engineering, modular pathway assembly and optimization, coculture engineering, and dynamic regulation of carbon flux (Table 1).