Cell-Surface Enzyme Systems

In contrast to the free enzyme systems, some enzymes are attached directly to the cell wall. This is frequently accomplished in Gram-positive bacteria via a specialized type of module, the S-layer homology (SLH) module, previously shown to be associated with the cell surface of Gram-positive bacteria (Lupas et al. 1994). Attachment of enzymes to the cell wall may have evolved to provide a more efficient assimilation of the soluble sugars produced due to their proximity to the cell surface. This arrangement would serve to reduce competition with other bacteria for the soluble products.

Examples of putative cell surface enzymes that contain an SLH module include a GH5 cellulase and GH13 amylase-pullulanase from Bacillus, a GH10 xylanase from Caldicellulosiruptor (Saul et al. 1990), a GH5 endoglucanase from Clostridium josui, a GH16 lichenase and GH10 xylanase from Clostridium thermocellum (Jung et al. 1998), and a variety of enzymes (GH10 xylanases, a GH5 mannanase, and a GH13 amylase-pullulanase) from different species of Thermoanaerobacter (Matuschek et al. 1996). The modular architecture of these enzymes may be particularly intricate, containing numerous different modules in a single polypeptide chain, thus forming extremely large enzymes.