Metabolic engineering strategies for consolidated production of lactic acid from lignocellulosic biomass

Abstract

Lactic acid (LA) is one of the most desired molecules by the chemical industry. Current expansion of LA market is mainly driven by its application as building block for the synthesis of polylactide (PLA), that is, a family of biodegradable and biocompatible plastic polymers. PLA can potentially replace oil‐derived polymers as general purpose plastic, but current LA prices fails to make PLA cost‐competitive with traditional plastics. Nowadays, LA is mainly produced by fermentation of expensive starchy biomass. Hopefully, cheaper lignocellulosic feedstock could be used in future second‐generation biorefinery processes. However, most efficient natural LA producers cannot ferment lignocellulose without prior biomass saccharification. Metabolic engineering may develop improved microorganisms that feature both efficient biomass hydrolysis and LA production, thus supporting consolidated bioprocessing (CBP), that is, one‐pot fermentation, of lignocellulose to LA. CBP could dramatically reduce LA production cost, thus contributing to the expansion of more environmental sustainable plastics and commodity chemicals. This review presents an overview of “recombinant cellulolytic strategies”, mainly consisting in introducing cellulase systems in native producers of LA, and “native cellulolytic strategies” aimed at improving LA production in natural cellulolytic microorganisms. Issues and perspectives of these approaches will be discussed.

Lactic Acid Production from Lignocellulose – A Review of Major Challenges and Selected Solutions

“…These include exo-glucanase, endoglucanase, and b-glucosidase ( Fig. 11) [28,29]. Engineered L. plantarum with Cel8A endoglucanase obtained from C. thermocellum was able to grow on cellooligosaccharides up to 5-6 glucose residues long [30].…”

“…This has limited the number of cellulases secreted by the recombinant LAB strains developed until this date. They only enabled them to metabolize short cellodextrins with a maximum of 8-9 cellulose units or partially break down cellulose as it is not sufficient to fully metabolize complex LIB [29]. This is also seen in their low cellulolytic…”

“…-Recombinant LAB strains that can grow on LIB without the need for complex nutritional requirements -Significant reduction in LA production costs by eliminating the need for a pretreatment and enzyme hydrolysis reaction -Uncovering mechanism of cellulase secretion by the native cellulolytic microorganisms and the way they synergize to breakdown cellulose to glucose. -More investigation on cellulose active proteins such as lytic polysaccharide monooxygenases (LPMOs) -Further studies on genetic engineering tools for unlabeled gene integration into LAB genomic DNA such as homologous recombination and single-stranded DNA recombineering [28,29] Microwave-assisted DES pretreatment of LIB -High delignification rate on LIB -Producing pretreatment products with little or no harmful by-products -Reduce pretreatment costs -Investigating this new pretreatment method in LA production [14] CSSF with enzyme and cell recycling -Reduces enzyme costs by recycling -Eliminates feedback, substrate and end product inhibition together with separation problems -Produces higher LA productivity, yield and concentration compared to batch and fed-batch fermentations. -Reduces LA production cost -Investigating this process configuration in LA production [10,38] Figure 11.…”

“…The focus should be on native LA producers with lower nutritional requirements, such as the Bacillus genus, including B. coagualns, B. subtilis, and B. licheniformis strains, targeting their growth on LIB without the addition of expensive yeast extract and other nutrient sources[28][29]. 2.…”

Construction of lactic acid overproducing Clostridium thermocellum through enhancement of lactate dehydrogenase expression

“…However, the most efficient natural producers of LA, i.e. lactic acid bacteria (LAB), bacteria belonging to the Bacillus genus and fungi belonging to the Rhizopus genus, cannot ferment lignocellulosic material without prior biomass saccharification [8,9].…”