Cellulose degradation: a therapeutic strategy in the improved treatment of Acanthamoeba infections

Abstract

Acanthamoeba is an opportunistic free-living amoeba that can cause blinding keratitis and fatal brain infection. Early diagnosis, followed by aggressive treatment is a pre-requisite in the successful treatment but even then the prognosis remains poor. A major drawback during the course of treatment is the ability of the amoeba to enclose itself within a shell (a process known as encystment), making it resistant to chemotherapeutic agents. As the cyst wall is partly made of cellulose, thus cellulose degradation offers a potential therapeutic strategy in the effective targeting of trophozoite encased within the cyst walls. Here, we present a comprehensive report on the structure of cellulose and cellulases, as well as known cellulose degradation mechanisms with an eye to target the Acanthamoeba cyst wall. The disruption of the cyst wall will make amoeba (concealed within) susceptible to chemotherapeutic agents, and at the very least inhibition of the excystment process will impede infection recurrence, as we bring these promising drug targets into focus so that they can be explored to their fullest.

Figure 1

Beta-D-glucose is the basic subunit of cellulose, which is an important component of the cyst wall of Acanthamoeba .

Figure 2

Classes of enzymes involved in cellulose breakdown. Endoglucanase produce random cuts at an internal position within cellulose fiber releasing cellooligosaccharides. Exoglucanases/cellobiohydrolase act on chain ends releasing cellobiose which are then acted up on by β-glucosidase to release glucose (adopted from Xie et. al., 2007).

Figure 3

Schematic representation of multi-domain organization shown by non-complex cellulases. Cellulose binding domain exist as single, double or triple domain and could be located on either C- or N terminal of the protein. They are connected to catalytic domain via a linker sequence rich in proline and threonine. CBD is carbohydrate binding domain, EXG is exoglucanase, CBHII is cellobiohydrolase II, EGB is endoglucanase B.

Figure 4

Hydrolysis of cellulose by non-complexed cellulase system. The enzymes are secreted extracellularly resulting in the release of sugars which are eventually taken up by the cells and metabolized.

Figure 5

Cellulose degradation by Trichoderma reesei . Cellulose is attacked by endoglucanases releasing cellodextrins which is further broken down by exoglucanases into cellobiose. The hydrolysis of cellobiose is then mediated by β – glucosidases releasing glucose.

Figure 6

Cellulose degradation by Cellulomonas fimi . Cellulose is attacked by endoglucanases releasing cellodextrins which is further broken down by exoglucanases into cellobiose. The hydrolysis of cellobiose is then mediated by β – glucosidases releasing glucose.

Figure 7

Hydrolysis of cellulose by complexed cellulase system. The cellulase components are associated tightly forming multiprotein complexes called “cellulosome” and are found attached to bacterial cells. Components of hydrolysis are released in the vicinity of cells which are immediately taken up by the cells and metabolized.

Figure 8

Cellulose degradation by Clostridium thermocellum . Cellulose is attacked by various endoglucanases present in cellulosome releasing cellotetraose and/or cellodextrins. These cellooligosaccharides are then attacked by exoglucanases within cellulosome releasing cellobiose which is eventually converted to glucose by β – glucosidase.