Green Production and Biotechnological Applications of Cell Wall Lytic Enzymes

Abstract

Energy demand is constantly growing, and, nowadays, fossil fuels still play a dominant role in global energy production, despite their negative effects on air pollution and the emission of greenhouse gases, which are the main contributors to global warming. An alternative clean source of energy is represented by the lignocellulose fraction of plant cell walls, the most abundant carbon source on Earth. To obtain biofuels, lignocellulose must be efficiently converted into fermentable sugars. In this regard, the exploitation of cell wall lytic enzymes (CWLEs) produced by lignocellulolytic fungi and bacteria may be considered as an eco-friendly alternative. These organisms evolved to produce a variety of highly specific CWLEs, even if in low amounts. For an industrial use, both the identification of novel CWLEs and the optimization of sustainable CWLE-expressing biofactories are crucial. In this review, we focus on recently reported advances in the heterologous expression of CWLEs from microbial and plant expression systems as well as some of their industrial applications, including the production of biofuels from agricultural feedstock and of value-added compounds from waste materials. Moreover, since heterologous expression of CWLEs may be toxic to plant hosts, genetic strategies aimed in converting such a deleterious effect into a beneficial trait are discussed.

A microalgal‐based preparation with synergistic cellulolytic and detoxifying action towards chemical‐treated lignocellulose

“…Plant expression of CWDEs is an interesting alternative to microbial-based biofactories because of their high productivity/ production cost ratio and carbon-neutral process. However, expression of CWDEs in plants could have side effects as CWDEs from lignocellulolytic fungi and bacteria are well-known pathogenic factors that could have detrimental effects for plant growth (Benedetti et al 2019b;Choi and Klessig, 2016;Ma et al 2015;Poinssot et al 2003). To prevent potential side effects of the expression of CWDEs in plants, different strategies have been proposed such as compartmentalized expression/accumulation (Park et al 2016), inducible gene expression (Tomassetti et al 2015) and expression of CWDEs with inducible activity, for example hyperthermophilic enzymes (Mir et al 2017).…”

“…However, expression of CWDEs in plants could have side effects as CWDEs from lignocellulolytic fungi and bacteria are well‐known pathogenic factors that could have detrimental effects for plant growth (Benedetti et al . 2019b; Choi and Klessig, 2016; Ma et al . 2015; Poinssot et al .…”

Bioprospecting of microbial strains for biofuel production: metabolic engineering, applications, and challenges

“…Apart from cellulose and hemicellulose, lignocellulosic biomass also contains little proportion of polysaccharides called pectin, which accounts for about 5% of total dry weight and is often found as a major component of agricultural wastes [ 47 ]. Pectin is composed of α-1,4- d -galacturonic acid linkages.…”

“…Endopolygalacturonase (EC 3.2.1.15) hydrolyzes homogalacturonan in pectic acid and oligomers by releasing digalacturonic and galacturonic acid units from their reducing ends, while exopolygalacturonase (EC 3.2.1.67) acts on the reducing end of galacturonyl-oligomers produced by endopolygalacturonase, cleaving the α1,4-glycosidic bonds and subsequently releasing galacturonic acid from the non-reducing end [ 47 , 49 ]. Esterase (EC 3.1.1.11) on the other hand, catalyzes the degradation of the methyl ester bonds in pectin by a de-esterification process, resulting in the production of pectic acid [ 48 ].…”

Expression of a Hyperthermophilic Cellobiohydrolase in Transgenic Nicotiana tabacum by Protein Storage Vacuole Targeting

“…Biological conversion into fermentable sugars requires production of large amounts of microbial Cell Wall Degrading Enzymes (CWDEs), whose expression in planta might provide a low-cost production platform together with a direct contact with their natural substrates—i.e., plant cell wall polysaccharides. The in muro targeting of CWDEs does enhance the hydrolysis of cell wall polysaccharides into fermentable sugars [ 1 ] and yet often leads to undesired side effects [ 2 ]. Indeed, CWDEs are produced by phytopathogens to dismantle the cell wall integrity, thus supporting the infection process [ 3 ] and providing carbon sources for the microbe [ 4 ].…”