The free enzymes comprise a catalytic module alone with no accessory modules or with a CBM. The simplest enzymes often specialize on degrading soluble oligosaccharide break­down products or bind to the polysaccharide substrate such as cellulose or xylan via the intrinsic affinity of its active site. In contrast, the polypeptide chain of many free enzymes includes both a catalytic module together with a CBM. This basic bi-modular arrangement can be further extended by the inclusion of additional types of modules or repeating units of the same module, all of which serve to modify the activity of the catalytic module on the polysaccharide substrate. The free enzyme, however, remains unattached to other enzymes and can work in an independent manner on a given substrate. The enzyme systems of aerobic fungi and bacteria usually contain numerous enzymes that are basically in the free state (Knowles et al. 1987; Wilson 1992; Warren 1993, 1996; Teeri 1997; Teeri et al. 1998; Wilson 2004; Viikari et al. 2007; Kumar et al. 2008).