Dextranase

Dextran is a chemically and physically complex polymer, breakdown of which is carried out by a variety of endo- and exodextranases. Enzymes in many groups can be classified as dextranases according to function: such enzymes include dextranhydrolases, glucodextranases, exoisomaltohydrolases, exoisomaltotriohydrases, and branched-dextran exo-1,2- -glucosidases. Cycloisomalto-oligosaccharide glucanotransferase does not formally belong to the dextranases even though its side reaction produces hydrolyzed dextrans. A new classification system for glycosylhydrolases and glycosyltransferases, which is based on amino acid sequence similarities, divides the dextranases into five families.

However, this classification is still incomplete since sequence information is missing for many of the enzymes that have been biochemically characterized as dextranases. Dextran-degrading enzymes have been isolated from a wide range of microorganisms. The major characteristics of these enzymes, the methods for analyzing their activities and biological roles, analysis of primary sequence data, and three-dimensional structures of dextranases have been dealt with in this review. Dextranases are promising for future use in various scientific and biotechnological applications.

Initial interest in the enzymes hydrolyzing dextran arose from studies that aimed to elucidate the structure of dextran and to obtain partially hydrolyzed dextran polymers produced by Leuconostoc mesenteroides for infusion purposes (80). Dextranases also have other important industrial applications since these enzymes can depolymerize various troublesome microbial dextran deposits. The presence of dextran in harvested sugar canes and dextran formation by microbes in sugar factories lead to lowered sucrose yield. The fact that dextran is a component of dental plaque, which is considered to contribute to the development of dental caries, has been one of the main driving forces to investigate dextran-hydrolyzing enzymes. Dextran can be modified by dextranases to be used in many biotechnological applications.
Since the first reports on Cellvibrio fulva dextranase in the 1940s, more than 1,500 scientific papers and more than 100 patents have been issued on dextran-hydrolyzing enzymes found in a number of microbial groups, fungi being the most important commercial source of dextranase. Higher organisms also possess dextran-hydrolyzing activities, but relatively few studies focusing on such enzymes have been published.

APPLICATIONS OF DEXTRANASES

The dextrans themselves are polydisperse and as such mostly not suitable for technological applications. However, enzymatically processed fractionated dextrans possess a significant commercial interest in cosmetics, drug formulations, and vaccines, as cryoprotectants, and as stabilizers in the food industry. Selected dextran fractions in combination with polyethylene glycol solutions form a two-phase system. In addition to using dextranases for processing dextrans, the enzymes themselves are increasingly important in the food, dental, and detergent industries. Finally, dextran-hydrolyzing enzymes are important for elucidating the fine structure of dextran and certain other polysaccharides.

Clinical Applications of Dextran and Dextranases

Initial interest in dextranases was raised in regard to their possible application in commercial production of clinical dextran, i.e., a sterile solution of dextran of a specific molecular weight to be used to restore blood volume in patients suffering shock as a result of blood loss. Relatively low-molecular-weight clinical dextrans have previously been produced from dextran by controlled acid hydrolysis followed by organic solvent fractionation. However, the yields are low (10 to 12%) due to losses during hydrolysis and fractionation. The enzymatic method seemed to have potential for replacing the acid hydrolysis for clinical dextran production and such processes were patented in the 1950s. The enzymatic method needs less energy and simpler equipment and results in a more uniform product with a 25% to 52% yield.

Dextranase can be used as universal targeting method for therapeutic agents. In the case of cancer, for example, a bispecific antibody has been created against cancer antigen and dextranase. The bispecific antibody is first injected into the blood circulation, where it binds to cancer cells. Dextranase is then injected and subsequently captured by the antibody-bound cancer cells. Finally, a cytotoxic therapeutic agent conjugated to dextran is injected into the bloodstream, and the conjugate is cleaved by the action of dextranase to release the cytotoxic drug selectively into the cancer cells.

In endocarditis, an exopolysaccharide product from viridans streptococci (glycocalyx, composed predominantly of dextran) has been associated with a delayed antimicrobial efficacy in cardiac vegetations. Enzymatic digestion of the glycocalyx by dextranase has been shown to enhance the antibiotic activity of penicillin and temafloxacin.

Dextrans also contribute to human health since they are resistant to mammalian digestive enzymes in the small intestine but are readily fermented in the large intestine, particularly by probiotic bacteria belonging to the genera Lactobacillus and Bifidobacterium. Prebiotic oligosaccharides, including isomalto-oligosaccharides, are believed to promote the growth and proliferation of these microbes most efficiently. Immobilized dextransucrase with soluble dextranase has been used for synthesis of prebiotic oligosaccharides.

Applications of Dextranases in Treatment of Dental Plaque

Dental plaque, the bacterial film adhering to tooth surfaces, is composed of closely packed bacteria and noncellular material. Roughly 20% of the dry weight of dental plaque is water-insoluble glucans. Degradation and removal of these glucans have been suggested to prevent oral diseases such as dental caries. Dextranase can inhibit the synthesis of insoluble glucans as well as the adherence of streptococci. Simultaneous use of several enzymes, such as dextranase and mutanase, could be advantageous. A novel glucanhydrolase, DXAMase from Lipomyces starkeyi, appears to be effective in reducing synthesis of insoluble glucans, inhibiting sucrose-dependent adhesion to glass, and removing bacterial films previously formed in the presence of sucrose. These in vitro properties of DXMase are considered propitious for dental plaque agent. For the treatment of dental plaque, various compositions that comprise enzymes hydrolyzing or inhibiting glucans have been proposed.

Use of Dextranases in the Sugar Industry

One of the major industrial applications of dextranases is the reduction of sliming in sugar production processes. The growth of Leuconostoc and Lactobacillus spp. is the most important factor in contributing to the postharvest deterioration of cane sugar and frost-damaged beet sugar. Problems caused by dextran in raw sugar include sucrose loss, increased viscosity of process syrups, and poor recovery of sucrose due to inhibition of crystallization. Dextranases are used in various analytical methods for measuring glucan content in sugar juices and in raw sugar.

3D Structure of Dextranase