Digestion of crystalline cellulose substrates by the Clostridium thermocellum cellulosome: structural and morphological aspects

The action of cellulosomes from Clostridium thermocellum on model cellulose microfibrils from Acetobacter xylinum and cellulose microcrystals from Von ia ventricosa was investigated. The biodegradation of these substrates was followed by transmission electron microscopy, Fourier-transform IR spectros copy and Xray diffraction analysis, as a function of the extent of degradat ion. The cellulosomes were very effective in catalysing the complete digest ion of bacterial cellulose, but the total degradation of Valonia microcryst als was achieved more slowly. Ultrastructural observations during the diges tion process suggested that the rapid degradation of bacterial cellulose wa s the result of a very efficient synergistic action of the various enzymic components that are attached to the scaffolding protein of the cellulosomes . The degraded Valonia sample assumed various shapes, ranging from thinned- down microcrystals to crystals where one end was pointed and the other inta ct. This complexity may be correlated with the multi-enzyme content of the cellulosomes and possibly to a diversity of the cellulosome composition wit hin a given batch. Another aspect of the digestion of model celluloses by c ellulosomes is the relative invariability of their crystallinity, together with their I-alpha/I-beta composition throughout the degradation process. C omparison of the action of cellulosomes with that of fungal enzymes indicat ed that the degradation of cellulose crystals by cellulosomes occurred with only limited levels of processivity, in contrast with the observations rep orted for fungal enzymes. The findings were consistent with a mechanism whe reby initial attack by a cellulosome of an individual cellulose crystal res ults in its 'commitment' towards complete degradation.