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An Overview of Microbial Indigo-Forming Enzymes Fabara, Andrea N.; Fraaije, Marco W

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An Overview of Microbial Indigo-Forming Enzymes Fabara, Andrea N.; Fraaije, Marco W University of Groningen An overview of microbial indigo-forming enzymes Fabara, Andrea N.; Fraaije, Marco W. Published in: Applied Microbiology and Biotechnology DOI: 10.1007/s00253-019-10292-5 IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite from it. Please check the document version below. Document Version Publisher's PDF, also known as Version of record Publication date: 2020 Link to publication in University of Groningen/UMCG research database Citation for published version (APA): Fabara, A. N., & Fraaije, M. W. (2020). An overview of microbial indigo-forming enzymes. Applied Microbiology and Biotechnology, 104(3), 925-933. https://doi.org/10.1007/s00253-019-10292-5 Copyright Other than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons). The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license. More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne- amendment. Take-down policy If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim. Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons the number of authors shown on this cover page is limited to 10 maximum. Download date: 01-10-2021 Applied Microbiology and Biotechnology (2020) 104:925–933 https://doi.org/10.1007/s00253-019-10292-5 MINI-REVIEW An overview of microbial indigo-forming enzymes Andrea N. Fabara1 & Marco W. Fraaije1 Received: 11 October 2019 /Revised: 23 November 2019 /Accepted: 28 November 2019 /Published online: 13 December 2019 # The Author(s) 2019 Abstract Indigo is one of the oldest textile dyes and was originally prepared from plant material. Nowadays, indigo is chemically synthesized at a large scale to satisfy the demand for dyeing jeans. The current indigo production processes are based on fossil feedstocks; therefore, it is highly attractive to develop a more sustainable and environmentally friendly biotechnological process for the production of this popular dye. In the past decades, a number of natural and engineered enzymes have been identified that can be used for the synthesis of indigo. This mini-review provides an overview of the various microbial enzymes which are able to produce indigo and discusses the advantages and disadvantages of each biocatalytic system. Keywords Indigo . Indole . Naphthalene dioxygenase . Styrene monoxygenase . P450 monoxygenase . Peroxygenase . Flavoprotein monooxygenase Introduction vats, hence the classification as a vat-dye. The deposition of the insoluble indigo on the fabric results in a special decora- Humans have decorated textiles with dyes and pigments, often tion of the fabric material as the fabric is not penetrated by the derived from plant material, since ancient times (Aino et al. dye. This gives denim and other indigo-dyed garments their 2018). Indigo blue (in short: indigo) is one of the oldest dyes special appearance, where wear or abrasion exposes the white to be used for textile dyeing (Fig. 1). In 2009 at Huaca Prieta, interior of the fiber. Peru, the oldest indigo-dyed cotton fabric was discovered, The most effective plant-based process of producing dating back to 6000 years ago (Splitstoser et al. 2016). the indigo dye involved the plant Indigofera tinctoria, Indigo-dying was also known in ancient Egypt; the funerary which is distributed throughout the tropical and subtrop- wardrobe of Tutankhamun included an indigo-dyed state robe. ical regions of the world and particularly in India. In Clearly, the use of indigo as dye was widespread already early fact, the Greeks referred to the dye as “indikon” which in human history (Clark et al. 1993). means “from India.” Yet, also in more moderate climate Traditionally, indigo has been prepared from various regions, indigo has been produced and used from alter- plants. This is done by fermenting plant material, which will native plants. In Europe, woad (Isatis tinctoria)was liberate indoxyl from indigo precursors, typically glycosylated traditionally used as the source of indigo. In fact, for forms of indoxyl such as indican and isatan B (Fig. 1). centuries, there were many regions where woad cultiva- Through air-mediated oxidation, indoxyl then spontaneously tion and processing was a dominant industry. However, dimerizes to form indigo. To dye textiles, indigo is first chem- through increasing trade between Europe and other con- ically reduced which results in the soluble reduced form of tinents, the import of indigo from India fully eradicated indigo, leucoindigo. The indigo-dying process, in which indi- woad-based industries starting in the 16th century. go is chemically treated, was traditionally done in buckets or While Indigofera contains relatively high concentration of indican, woad contains significantly lower amounts of two indigo precursors: indican and isatan B (Fig. 1) * Marco W. Fraaije (Gilbert et al. 2004). [email protected] At the advent of the 20th century, a petroleum-based chem- ical process to synthesize indigo was introduced. This rapidly 1 Molecular Enzymology group, University of Groningen, Nijenborgh evolved into an effective industrial process and replaced most 4, 9747 AG Groningen, The Netherlands of the plant-based production of indigo. 926 Appl Microbiol Biotechnol (2020) 104:925– 933 Fig. 1 Structural formulas of indigo, indoxyl, and some of its plant precursors, indican and isatan B Chemical production of indigo friendly, and leads to problems in the disposal of the dye waste. Because these factors are not priced in, this is a com- About 150 years ago, Adolf von Baeyer succeeded in eluci- mercially viable process. To prevent the formation of large dating the structural formula of indigo. He was also the first in waste streams and to be able to use a renewable feedstock, it developing a method for the chemical synthesis of indigo; in is desirable to develop a more sustainable process. For this, 1870, he described the synthesis of indigo starting from isatin. biotechnological approaches are being explored. In subsequent years, Baeyer and others developed alternative Indigo production based on plant material, reviving and synthetic routes for the synthesis of indigo, but most of them optimizing the traditional plant-based processes, could be an started from expensive starting material and were not commer- option. This would involve the breeding of plants with high cially viable. Yet, within a few decades, several effective in- indican content and other beneficial traits. Recently, obtained digo production processes were developed (Scheme 1), and genetic insights in the natural indigo synthesis pathway in indigo production at tonne scale was established. Industrial plants could be exploited for this (Jin et al. 2016; Inoue et al. production of indigo started around 1900 and developed into 2017), and it has already been shown that through genetic the current large scale industrial processes, replacing virtually engineering, plants can be bred that produce various indigo all former plant-based production sites. Current large scale dyes (Fräbel et al. 2018). However, production of indigo using production processes typically rely on aniline as a cheap plants would compete with food and feed production. fossil-based feedstock. As an alternative and promising approach, the production of indigo by fermentation has been explored since the early 1980’s when the first bacterial strains capable of indigo syn- Alternative indigo production processes thesis were identified. Among the discovered microbial indigo producers, the majority are aromatic hydrocarbon-degrading Indigo has been the world’s most important and popular dye- bacteria (Bhushan et al. 2000). Although various bacterial stuff, and the demand is still increasing, especially in the den- strains and enzymes have been identified to be able to produce im industry. Plant material, mainly leaves, contains indican as indigo, there is still no large-scale industrial biotechnological a colorless compound. By fermentation, indican is hydrolysed process for producing indigo. into β-D-glucose and indoxyl, while in a next step, using air With the current realization that production processes as oxidant, indigo is obtained as solid material (Fig. 1). should be (more) sustainable, the demand for a biotechnolog- Current chemical production processes do not start from re- ical indigo production process becomes more urgent, and newable material but instead depend on relatively cheap pet- modern techniques for enzyme and strain engineering make rochemical precursors. The chemistry used for the industrial it more feasible. In the next paragraphs, details of the micro- production of indigo is quite harsh, not environmentally bial enzymes known to form indigo are discussed. In all cases, Scheme 1 Timeline of indigo indigo produced synthec biotech indigo derived from various plants production processes from Indigofera indigo indigo 4000 BC 1500 AD 1900 AD 2030 AD? Appl Microbiol Biotechnol (2020) 104:925– 933 927 these enzymes oxidize indole to form indoxyl, which sponta- the medium. This suggested that the oxygenase was capable to neously dimerizes into indigo. Besides naturally occurring convert indole into indoxyl, leading to indigo formation. Yet, enzymes, enzymes engineered towards indigo production will the initial (labile) product of action of NDO is indoline-2,3- also be discussed. diol which quickly decays to form indoxyl, which subsequent- ly condenses into indigo (route A in Fig. 2). NDO (EC 1.14.13.8) has a broad substrate range and cata- Microbial enzymes producing indigo lyzes several types of reactions including cis-dihydroxylation, monooxygenation, and desaturation reactions (Parales et al. In the last few decades, a large number of indigo-producing 2000).
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