Digestion Of Carbohydrates

Initial and intermediate digestion of starch (or glycogen) is accomplished by a-amylase. This enzyme cleaves internal bonds of the polysaccharide until it is reduced to small oligosaccharides or disaccharides (Fig. 1B). The amylases are not very active in digesting intact starch granules, making mastication prior to ingestion important. Insect amylases depend on calcium ions for activity or stability, they are activated by chloride ions (amylases in Lepidoptera are exceptions), their molecular masses are found in the range 48 to 68 kDa, and their pH optima vary widely (4.8-9.8) depending on the insect taxon. As described for trypsin, insects feeding on amylase inhibitor-containing food express new amylase molecules insensitive to the inhibitors.

The final digestion of starch chains occurs under a-glucosidases, enzymes that sequentially remove glucosyl residues from the nonreducing ends of short oligomaltosac-charides. If the saccharide is a disaccharide, it is named maltose (Fig. 1B). Because of that, a-glucosidase is also called maltase. As a rule, sucrose (glucose a1,P2-fructose) is hydrolyzed by a-glucosidase. If an enzyme is able to hydrolyze sucrose, but not maltose, it is likely a P-fructosidase, an enzyme attacking sucrose by the fructosyl residue. Sucrose is found in large amounts in nectar and phloem sap and in lesser amounts in some fruits and leaves.

The important insect hemolymph and fungal sugar trehalose (glucose a1,a1-glucose) is hydrolyzed only by the specific enzyme trehalase. This digestive enzyme occurs in luminal contents or immobilized at the surface of midgut cells and also as an enzyme at the midgut basal cell membrane, making available glucose from hemolymph trehalose.

Although cellulose is abundant in plants, most plant-feeding insects, such as caterpillars and grasshoppers, do not use it. Cellulose is a nonramified chain of glucose units linked by P-1,4 bonds (Fig. 1C) arranged in a crystalline structure that is difficult to disrupt. Thus, cellulose digestion is unlikely to be advantageous to an insect that can meet its dietary requirements using more easily digested food constituents. The cellulase activity found in some plant feeders facilitates the access of digestive enzymes to the plant cells ingested by the insects. True cellulose digestion is restricted to insects that have, as a rule, nutritionally poor diets, as exemplified by termites, woodroaches, and cerambycid and scarabaeid beetles. There is growing evidence that insects secrete enzymes able to hydrolyze crystalline cellulose, challenging the longstanding belief that microbial symbionts are necessary for cellulose digestion. The end products of cellulase action are glucose and cellobiose (Fig. 1C); the latter is hydrolyzed by a P-glucosidase.

Hemicellulose is a mixture of polysaccharides associated with cellulose in plant cell walls. They are P-1,4- and/or P-1,3-linked glycan chains made up mainly of glucose (glucans), xylose (xylans), and other monosaccharides. The polysaccharides are hydrolyzed by a variety of enzymes from which xylanases, laminarinases, and lichenases are the best known. The end products of the actions of these enzymes are monosaccharides and P-linked oligosaccharides. The final digestion of those chains occurs under the actions of P-glycosidases that sequentially remove glycosyl (usually glucosyl, galactosyl, or xylosyl) residues from the nonreducing end of the P-linked oligosaccharides. As these may be cellobiose, P-glycosidase is frequently also named cellobiase. Thus, P-glycosidases complete the digestion of cellulose and hemicelluloses.

A special P-glycosidase (aryl P-glycosidase) acts on glycolipids and in vivo probably removes a galactose from monogalactosyldiacylglycerol that together with digalactosyl-diacylglycerol is a major lipid of photosynthetic tissues. Digalactosyldiacylglycerol is converted into monogalactosyldiacylglycerol by the action of an a-galactosidase. The aryl p-glycosidase also acts on plant glycosides that are noxious after hydrolysis. Insects circumvent these problems by detoxifying the products of hydrolysis or by repressing the synthesis and secretion of this enzyme while maintaining constant the synthesis and secretion of the other p-glycosidases.

0 0

Post a comment