Competing Plant Cell Wall Digestion Recalcitrance by Using Fungal Substrate – Adapted Enzyme Cocktails

Abstract

General needs for energyGeneral needs for energy are still increasing. In 2000, the energy provided worldwide was10 Gt of oil equivalent (Gtoe) and the demand is forecasted to be around 15 Gtoe for 2020(source: Energy Information Administration [EIA], 2002, as cited in Scragg, 2005). Duringthe 20th century, coal proportion in energy supply decreased whereas oil and gas increaseddrastically. First after the 1973 oil crisis and afterwards periodically depending on oil prices,developments for producing energy by new ways were considered. In the last decade, thedepletion of fossil energy sources appeared as a reality although exhaustion time remainshighly controversial. Currently, it is clear that considerable efforts to promote alternativesources of energy are driven by both environmental concern (limiting fuel by-productsemissions) and economic necessity linked to the fossil fuel depletion.1.2 Bioethanol among other alternative sourcesEthanol from biomass (Bioethanol) is one of these alternative sources. Despite polemics forbiomass uses i.e. biofuels vs food and some alarmist politicians’ declarations, this alternativeis really promising considering (i) the ability to satisfy a significant part of the demand forenergy and (ii) biomass renewability. Polemics and limitations could have been almostrational when first generation of biofuels was concerned, “noble parts” of plants, the sameused for food, being transformed. It is not the case anymore for any modern project.Another problem raised is the part of the cultivated surfaces to be reserved to biofuels, butin fact, realistic scenario is not to replace all fossil fuels volumes, but only part of them byusing wastes preferentially.1.3 “Biomass to ethanol” process and review of improvementsThe general scheme of “Biomass to ethanol” process is presented elsewhere in this book.Our purpose in this section is to highlight the numerous and various ways to optimize thewhole process from biomass to ethanol at different steps: choice of the biomass,pretreatment, enzyme productions, enzymatic hydrolysis, and ethanol fermentation. Firstof all, as discussed earlier, in the second generation of biofuels, biomass collection shouldnot compete with food plants. Biomass should be abundant and cultural practices as sustainable as possible. Interest was recently focused on plants providing good yields ofbiomass for a given surface as the tall Miscanthus. Reduction of lignin cell wall content isanother interesting approach to enhance sugar recovery from biomass, lignin being anabundant and resistant polymer limiting the digestion of biomass in biofuels processes.With anti-sense technology, tobacco plants lines were obtained with 20% lower lignincontent (Kavousi et al., 2010). The modified lines displayed a threefold increase ofsaccharification efficiency compared to wild type. Of course, the application of suchstudies in larger scales depends on the acceptance of transgenic plants by the society.Decision to use these plants has to be supported by studies of environmental risks andpotential benefits (Talukder, 2006). Literature about pretreatment is very abundant,describing various methods: physical, chemical or combination of both (Soccol et al. 2010).Fine optimization of conditions should be performed individually depending on biomass.Among innovative method proposed, dry wheat straw has been treated successfully withsupercritical CO2. After treatment, 1kg biomass yields to 149 g sugars (Alinia et al., 2010).Another currently emerging feature for bioethanol process amelioration is proteinengineering. For instance, a cellulase from the filamentous bacterium Thermobifida fuscahas been modified both in its catalytic domain and in its carbohydrate binding module (Liet al., 2010). A mutant enzyme displays a two fold increase activity, and a better synergywith other enzymes, leading it to be very useful for biomass digestion. At the next step,i.e. sugar fermentation to ethanol, many efforts have been run to allow yeast to performboth hexoses and pentoses fermentations. Industrial yeast Saccharomyces cerevisiae strains,fermenting only hexoses have been modified by addition of xylose degradation enzymes(Hector et al., 2010). Finally the outcome of engineering could be the use of syntheticbiology, which is creating cell systems able to convert biomass to sugars and also toferment them to ethanol. This strategy needs better fundamental knowledge to bedeveloped (Elkins et al., 2010).As discussed above, the step following the pretreatment of the biomass could beperformed via the enzymatic hydrolysis of the cell wall polysaccharides intofermentescible, monomeric sugars. Unfortunately, it is well known that recalcitrance ofplant cell wall to enzymatic digestion impairs the process. The behavior and the efficiencyof the cell wall degrading enzymes (CWDE) in situ and in vitro with isolatedpolysaccharides are completely different. The properties of the CWDE, as conformation,hydrophobicity, capacity of adsorption onto the cell wall, interaction with the lignins, andcatalytic efficiency in heterogeneous catalysis, are major parameters which should beconsidered and studied.This chapter focuses on biomass degradation enzymes. What is the best strategy to producethe most efficient enzymes? What is the best choice depending of up-and downstreamsteps: commercial enzyme cocktail, enzymes produced by a given microorganism orheterologous production of individual enzymes? Efficiencies and cost of enzymes, twobottlenecks in the process, will be discussed. For some authors, the improvements of theconversion of biomass to sugar offer larger cost-saving potential than those concerning thestep from sugar to biofuels (Lynd et al., 2008). These authors evaluated two scenarios; thefirst based on current technology and the second one including advanced nonbiologicalsteps. In both cases, conversion of polysaccharides from biomass could be improved byincreasing polysaccharides hydrolysis yields combined by lowered enzyme inputs. On-siteenzyme production was also identified as beneficial for cost of the whole ethanol productionprocess.