Synergistic interaction of an endo-β-1,4-glucanase and a β-glucohydrolase leads to more efficient hydrolysis of cellulose-like polymers in the gecarcinid land crab, Gecarcoidea natalis

This study investigated synergism between endo-β-1,4-glucanase and β-glucohydrolase enzymes from Gecarcoidea natalis. Together, these enzymes efficiently hydrolyse the cellulose-like polymer, carboxymethyl cellulose, to glucose. Endo-β-1,4-glucanase and β-glucohydrolase, isolated previously from G. natalis, were incubated in vitro using a ratio of the measured activities that matches that found in their digestive juice (5.4 : 1). Their combined activity, measured as the release of glucose from carboxymethyl cellulose, was greater than the sum of their separate activities. Hence they synergistically released glucose from carboxymethyl cellulose (degree of synergy: 1.27). This may be due to the complementary nature of the products of endo-β-1,4-glucanase activity and the preferred substrates of the β-glucohydrolase. β-glucohydrolase may also enhance cellulose hydrolysis by removing cellobiose, a potential competitive inhibitor of endo-β-1,4-glucanase. The synergistic interaction of these two enzymes further supports the previous suggestion that this species possesses a novel two-enzyme cellulase system that differs from the traditional three-enzyme fungal model.

Characterisation of cellulose and hemicellulose digestion in land crabs with special reference to Gecarcoidea natalis

This article reviews the current knowledge of cellulose and hemicellulose digestion by herbivorous land crabs using the gecarcinid Gecarcoidea natalis as a model species for this group. Cellulose digestion in the gecarcinids is hypothesised to require mechanical fragmentation and enzymatic hydrolysis. Mechanical fragmentation is achieved by the chelae, mandibles and gastric mill, which reduce the material to particles less than 53 µm. The gastric mill shows adaptations towards a plant diet; in particular, there are transverse ridges on the medial and lateral teeth and ventral cusps on the lateral teeth that complement and interlock to provide efficient cutting surfaces. Enzymatic hydrolysis of cellulose and hemicellulose is achieved through cellulase and hemicellulase enzymes. In the gecarcinids, 2–3 endo-β-1,4-glucanases, one β-glucohydrolase and a laminarinase have been identified. The endo-β-1,4-glucanases are multifunctional, with both endo-β-1,4-glucanase and lichenase activity. Complete cellulose hydrolysis is achieved through the synergistic action of the endo-β-1,4-glucanase and β-glucohydrolase. The evidence for the endogenous production of the cellulase and hemicellulase enzymes, their evolutionary origin and possible evolution in invertebrates as they colonised land is also discussed.

042. THE EFFECT OF THE INSECTICIDE PYRIPROXYFEN ON OVARY SYNTHESIS IN THE CHRISTMAS ISLAND RED CRAB, GECARCOIDEA NATALIS; A POSSIBLE CASE OF ENDOCRINE DISRUPTION?

The yellow crazy ant, Anopholepis gracilipes is an invasive species on Christmas Island, Indian Ocean whose population needs to be controlled before there is irrevocable environmental damage. The insecticide, pyriproxyfen, a compound which mimics juvenile hormone of insects, has been proposed to do this. Before it can be used in the field, its effects on non target species such as the endemic red crab, Gecarcoidea natalis, need to be investigated. Land crabs, like all decapods, may utilise a similar hormone called methyl farnesoate which is thought to be involved in controlling early ovary development. Pyriproxyfen may also mimic methyl farnesoate and thus disrupt this process. The effect of pyriproxyfen on early ovary synthesis in G. natalis was investigated by feeding crabs a mixture of leaf litter and bait containing 0.5% pyriproxyfen (experimental groups) or a mixture of leaf litter and bait containing no pyriproxyfen (control groups) at simulated baiting doses (2 kg ha–1 and 4 kg ha–1). Two additional groups of crabs were fed ad libitum, either bait containing 0.5% pyriproxyfen or the control bait. Experiments were conducted from July to September of 2007. Red crabs synthesise their ovaries annually over two months (July to September) in the dry season. This situation of high nitrogen demand is funded from small excesses of nitrogen assimilated from a mainly leaf litter diet. Pyriproxyfen affected early ovary development. Ovaries from crabs in the experimental groups at all baiting levels had a higher total nitrogen content and dry mass than that of the control animals. The ovaries from the experimental animals were also more mature; they contained more previtellogenic and early vitellogenic oocytes, of a larger diameter, than ovaries of the control animals. Significant amounts of pyriproxyfen were accumulated in the target tissues, the midgut gland and ovary. Minor amounts of pyriproxyfen were accumulated in muscle, a non-target tissue. By mimicking methyl farnesoate, pyriproxyfen may have caused endocrine disruption in G. natalis. In particular it may have stimulated early ovary development and synthesis of yolk protein.