A relevant feature of CWDE activities is the synergy observed between them. Typically an endo-enzyme acts randomly on the polysaccharide to yield oligomers. These oligomers are numerous for a single starting polysaccharide and their extremities are hydrolyzed to di — or monosaccharides by exo-acting enzymes. Accessory enzymes (debranching or desubstituting) proceed if necessary and all three kinds of enzymes work together. Synergy leads to the concept of "Minimal enzyme cocktail", i. e. a few selected enzymes (Sorensen et al., 2007), supposed to be sufficient for the complete digestion of plant cell wall. This paper described efforts performed to digest the more efficiently wheat arabinoxylan to arabinose and xylose. Starting from an "enzymatic base" (a P-xylosidase, and three endo-P-xylanases), they screened three different arabinofuranosidases to enhance substrates digestion. AbfIII was found to be the best enzyme when used alone (Fig. 5). Addition of Abfl significantly increased hydrolysis yields, although in different extent depending on the substrate used. Addition of the three arabinofuranosidases together did not improve the yields further. The strategy used and the results obtained clearly support the view of the necessity of a rational design of the process leading to ethanol. In other words, such a study has to be repeated when another biomass is concerned. Would the same enzyme be as efficient with the other substrate? Only experiments could address this question. Nevertheless, this study indicates that the starting biomass influence the choice of the enzymes to be used. For the complete

Fig. 5. Yields of arabinose and xylose released from different substrates (substrates are indicated below each pyramid). Base mix of enzyme (top of each pyramid) is constituted of a xylosidase and three endo-P-xylanases. ArfI, AfrI and ArfIII are three arabinofuranosidases belonging to GH (glycosyl hydrolase) families 51, 43 and 51, respectively. As described in the top left pyramid, the more different enzymes were added, the more the result is shown at the bottom of the pyramid. Yield superior to 100% was due to pretreatment. Adapted from Sorensen et al., 2007.

digestion of a plant biomass, minimal enzyme cocktails for all kind of polysaccharide are required. Note that Section2 of this chapter concludes to a great diversity of cell wall fine structures leading to design of much larger and much diverse cocktails.