Abstract
Cellulose waste biomass is the most abundant and attractive substrate for “biorefinery strategies” that are aimed to produce high-value products (e.g. solvents, fuels, building blocks) by economically and environmentally sustainable fermentation processes. However, cellulose is highly recalcitrant to biodegradation and its conversion by biotechnological strategies currently requires economically inefficient multistep industrial processes. The need for dedicated cellulase production continues to be a major constraint to cost-effective processing of cellulosic biomass.Research efforts have been aimed at developing recombinant microorganisms with suitable characteristics for single step biomass fermentation (consolidated bioprocessing, CBP). Two paradigms have been applied for such, so far unsuccessful, attempts: a) “native cellulolytic strategies”, aimed at conferring high-value product properties to natural cellulolytic microorganisms; b) “recombinant cellulolytic strategies”, aimed to confer cellulolytic ability to microorganisms exhibiting high product yields and titers.By starting from the description of natural enzyme systems for plant biomass degradation and natural metabolic pathways for some of the most valuable product (i.e. butanol, ethanol, and hydrogen) biosynthesis, this review describes state-of-the-art bottlenecks and solutions for the development of recombinant microbial strains for cellulosic biofuel CBP by metabolic engineering. Complexed cellulases (i.e. cellulosomes) benefit from stronger proximity effects and show enhanced synergy on insoluble substrates (i.e. crystalline cellulose) with respect to free enzymes. For this reason, special attention was held on strategies involving cellulosome/designer cellulosome-bearing recombinant microorganisms.
Highlights
Research efforts have been aimed at developing recombinant microorganisms with suitable characteristics for single step biomass fermentation
By starting from the description of natural enzyme systems for plant biomass degradation and natural metabolic pathways for some of the most valuable product biosynthesis, this review describes state-of-the-art bottlenecks and solutions for the development of recombinant microbial strains for cellulosic biofuel CBP by metabolic engineering
Rational metabolic engineering to increase ethanol yield and pathway, i.e. thl, crt, bcd, etfB, etfA, bcd, and adhE genes (Figure 2), purity has been performed by two main strategies: introduce was introduced in well-established and/or solvent tolerant heterologous gene, and disrupt genes involved in by-product formation that compete with ethanol synthesis [34]
Summary
Research efforts have been aimed at developing recombinant microorganisms with suitable characteristics for single step biomass fermentation (consolidated bioprocessing, CBP). Rational metabolic engineering to increase ethanol yield and pathway, i.e. thl, crt, bcd, etfB, etfA, bcd, and adhE genes (Figure 2), purity has been performed by two main strategies: introduce was introduced in well-established and/or solvent tolerant heterologous gene, and disrupt genes involved in by-product formation that compete with ethanol synthesis [34].
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