future science group 61 ISSN 1759-7269 10.4155/BFS.11.150 © 2012 Future Science Ltd In 1950 Reese et al. proposed a mechanism for cel- lulose hydrolysis, which involved two general com- ponents, C 1 and C x , acting in sequence [1] . According to the model, the C 1 component first disrupted and swelled the crystalline cellulose, possibly releasing soluble oligosaccharides into solution. The C x compo- nent, which was shown to have endoglucanase activity, was then able to effectively hydrolyze the previously inaccessible substrate along with the soluble oligo- saccharides. Furthermore, the activity of the mixture was found to be higher than the activity of each com- ponent acting alone, indicating that the components were acting synergistically. In the following years, a number of groups began to identify and characterize the specific enzymes present in these components. It became clear that in addition to the endoglucanase activity demonstrated originally for the C x component, exoglucanase activity was also present in the filtrates of cellulolytic organisms and that the combination of exo- and endo- activities resulted in the synergistic hydrolysis of cellulose [2–5] . In 1979, Wood and McCrae summarized the findings and proposed what became the classical endo–exo model of enzymatic cellulose hydrolysis [6] . According to this model, endocellulases attack the bulk cellulose, creating new chain ends that are susceptible to exocellulase digestion. Exocellulases in turn create more substrate for endocellulases by disrupting the crystalline substrate and/or by expos- ing previously inaccessible less ordered substrate that is susceptible to attack by endocellulases. With time it became evident that some exocellulases are also able to act synergistically with each other [7–10] , giving rise to the suggestion that there are two classes of exocel- lulases that preferentially attack either the reducing or the nonreducing end of the cellulose chain. This was definitively demonstrated by Barr et al. in 1996 using 14 O- and 14 C-labeled cellooligosaccharides [11] . It seems likely that synergism occurs only when two cellulases attack different regions of the cellulose microfibril and that each enzyme creates new sites of attack for other enzymes in the mixture. However, this is an oversimplification of a complex process that is still not completely understood. There is no current evi- dence that synergism requires interactions between the synergizing cellulases, since cellulases from unrelated organisms, which would not have sites for binding to each other, often show cross synergism. However, it is not known if synergizing cellulases interact when they are bound to cellulose, and there could be interactions caused by two enzymes binding to a specific site on the cellulose. b-glucosidases, which cleave cellobiose and, to lesser extent, other oligosaccharides [12] to glucose, Biofuels (2012) 3(1), 61–70 Synergistic interactions in cellulose hydrolysis Maxim Kostylev & David Wilson* Cellulases acting on crystalline cellulose in synergistic mixtures have higher combined activities than the sum of their individual activities. The mechanisms by which diferent types of cellulases enhance each other’s activities are complex and not completely understood, and the published data is often inconsistent. We discuss the role of the substrate and experimental factors in synergism experiments and consider the various modes of synergism that have been proposed. New oxidoreductase enzymes that participate in novel synergistic reactions are also discussed. Finally, we propose a general encompassing synergism model, based on the observations described in the review. REVIEW Department of Molecular Biology and Genetics, Cornell University, 458 Biotechnology Building, Ithaca, NY 14853, USA *Author for correspondence: Tel.: +1 607 255 5706; Fax: +1 607 255 5706; E-mail: dbw3@cornell.edu